Cranial cruciate ligament rupture is common in dogs and leads to instability of the stifle joint, lameness, and development of osteoarthritis.1–5 Tibial tuberosity advancement is one of the most commonly used techniques for the dynamic repair of cranial cruciate ligament defciency.6–10,a,b The goal of TTA is to reduce the PTA to ≤ 90°.6,7,11–13,a,b This approach is a technically challenging procedure that involves multiple implants.a
To simplify the traditional technique, a modification of TTA was developed and named the MMT.14,15 The MMT is based on the concept of biological repair with maximal preservation of soft tissue integrity and vascularity and minimal use of implants for fracture healing. Because it preserves the attachment of the distal portion of the tibial crest, the MMT is faster and less invasive and requires fewer implants to achieve fixation of the tibial crest than does traditional TTA. Indeed, the only implant used to advance and achieve fixation of the tibial tuberosity with the MMT is an advancement cage.14–16 Recently, specific cage implants were designed for the MMT to replace the traditional TTA titanium cage and improve tibial crest fixation.16–18 Initial clinical studies19–23 revealed promising results for the MMT15,16 that were similar to those obtained with traditional TTA.
Preoperative planning for TTA and MMT procedures requires use of a mediolateral radiograph of the affected stifle joint at 135° of extension for PTA measurement.20,24 The PTA landmarks are used to determine the required advancement and select the optimal cage size.a Preoperative measurements are performed to reduce the PTA to the intended 90° at stance.6,a,b Failure to reduce the PTA to 90° could lead to the persistence of femorotibial shear forces and lameness.6
Several factors can influence measurement of the required advancement distance for the tibial tuberosity in dogs: stifle joint extension angle,24–27 method of stifle joint extension angle measurement,28 femorotibial subluxation,29 method of PTA measurement,30,31 and method of advancement measurement.30–32 The impact of these factors on advancement measurement was previously investigated in several studies24–32 involving traditional TTA. To the authors' knowledge, the impact of these factors on the required advancement distance when the MMT is used has not been evaluated. However, the impact of all these factors (with the exception of advancement measurement method) could be expected to be the same for both procedures.
Radiographic preoperative planning methods have been described for determining the distance by which the tibial tuberosity needs to be advanced, including the conventional methoda and the correction method.32 Because the nature of the tibial crest osteotomy differs between traditional TTA and the MMT, the nature of the displacement of the tibial tuberosity differs between the procedures. In fact, no proximal displacement of the tibial crest is observed when the MMT is used.32,a Therefore, the required advancement distance calculated for the traditional TTA method cannot be applied to the MMT. Conversely, the correction method as first described was based on a schematic TTA model similar to the MMT (the tibial crest was not translated proximally).32 Consequently, the correction method could be more appropriate than the conventional method for use with the MMT.
Advancement measurement is used to select a commercially available cage size. Because cages are not available in all sizes, the larger cage size is generally selected when the required advancement distance is larger than one cage size but smaller than the next larger available size; however, this choice can lead to differences in cage size recommendations between preoperative planning methods. In a recent study,31 use of different PTA landmarks (tibial plateau vs common tangent) and various techniques to assess the required advancement distance (traditional TTA vs virtual TTA approach) led to different cage size recommendation for 86% of dogs.
The true impact of commonly used methods for TTA measurement32,a on the PTA reduction achieved via the MMT has not been evaluated in dogs. The purpose of the study reported here was to compare the PTA reduction achieved in dogs via the MMT when both radiographic methods of advancement measurement were used, determine the true advancement distance required if neither method resulted in a postoperative PTA of approximately 90°, and determine the impact of the results on cage size recommendations. We hypothesized that the reduction in PTA achieved would be less than the intended 90° when the conventional radiographic method for advancement measurement was used. We also hypothesized that, on the basis of findings of a theoretical study,32 the correction method for advancement measurement would lead to a postoperative PTA closer to the intended 90° than would the conventional method.
Materials and Methods
Sample
Pelvic limbs (n = 24) were collected from 12 adult (> 1.5 years of age) medium- to large-breed dogs (> 15 kg) that had been euthanatized for reasons unrelated to this investigation. The limbs were harvested by hemipelvectomy to conserve all muscular attachments to the patellar tendon and the hamstring muscles.33 Stifle joints were included in the study when palpation and manipulation of the joints revealed no abnormality and radiography revealed no signs of open physes or osteoarthritis. Specimens were wrapped in towels soaked in saline (0.9% NaCl) solution and frozen at −20°C prior to testing. Frozen specimens were thawed for 24 hours at room temperature (approx 20°C) before testing began.
Preoperative preparation
The same investigator (TC) prepared all stifle joints by removing the skin and subcutaneous tissues. A spring and a turnbuckle were used to simulate the quadriceps mechanism and induce tension in the patellar tendon.6 A cable (1-mm wire) was inserted through the distal quadriceps tendon and connected to the pelvis at the proximal insertion of the rectus femoris by use of a spring and turnbuckle inserted through the muscles (Figure 1).6 The adjustable turnbuckle was turned as necessary to straighten the patellar tendon.6 This mechanism allowed the patellar tendon to remain straight while advancing the tibial tuberosity.
Preoperative mediolateral radiograph of an orthopedically normal pelvic limb from a canine cadaver, with the stifle joint fixed in the true lateral position26,34 at an angle of extension of 135 ± 1° by use of so-called eminence landmarks.28 The position of the joint was maintained with a custom-made external skeletal fixator. The quadriceps mechanism was simulated with an adjustable turnbuckle turned as necessary to create tension through the spring to straighten the patellar tendon.6
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Preoperative mediolateral radiograph of an orthopedically normal pelvic limb from a canine cadaver, with the stifle joint fixed in the true lateral position26,34 at an angle of extension of 135 ± 1° by use of so-called eminence landmarks.28 The position of the joint was maintained with a custom-made external skeletal fixator. The quadriceps mechanism was simulated with an adjustable turnbuckle turned as necessary to create tension through the spring to straighten the patellar tendon.6
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Preoperative mediolateral radiograph of an orthopedically normal pelvic limb from a canine cadaver, with the stifle joint fixed in the true lateral position26,34 at an angle of extension of 135 ± 1° by use of so-called eminence landmarks.28 The position of the joint was maintained with a custom-made external skeletal fixator. The quadriceps mechanism was simulated with an adjustable turnbuckle turned as necessary to create tension through the spring to straighten the patellar tendon.6
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Stifle joints were positioned at an angle of extension to within 1° of 135° by use of so-called eminence landmarks.28 The relative position of the tibia to the femur was maintained with a custom-made external skeletal fixator to allow flexion and rotation during positioning; however, the position could be locked to neutralize any movement of the stifle joint (Figure 1).33 A computed radiography systemc was used to obtain a mediolateral radiograph including the femur from the greater trochanter and the tibia to the talus to confirm that the extension angle was within 1° of 135°. Superimposition (< 2 mm discrepancy) of the tibial condyles and the femoral condyles (true lateral positioning)26,34 was obtained with fluoroscopic guidance, and the custom-made external skeletal fixator was locked to maintain the stifle joint in the true lateral position at within 1° of 135°.
A preoperative mediolateral radiograph was acquired to measure the PTA by use of the tibial plateau method.9,a,b The cranial tibial plateau landmark was identified as the proximal aspect of the cranial extent of the medial portion of the tibial plateau, and the caudal landmark was identified as the caudal extent of the medial portion of the tibial plateau. The slope of the tibial plateau was digitally marked with a line by a board-certified surgeon (TC). The PTA was defined by the intersection of a line drawn immediately parallel to the cranial aspect of the patellar tendon and the line marking the tibial plateau slope.6 The TPA was also measured.9
The required advancement distance was calculated by use of the 2 radiographic methods for each limb consecutively (Figure 2). To determine the advancement distance by use of the conventional method,a a line perpendicular to the tibial plateau slope was drawn on the radiograph at the proximal site of insertion of the patellar tendon and was extended distally beyond the cranialmost point of the tibial tuberosity. The distance between the proximal portion of the tibial tuberosity and the line drawn on the same radiograph perpendicular to the tibial plateau slope was determined along a line parallel to the tibial plateau. To determine the required advancement distance by use of the correction method,32 the value obtained with the conventional method was divided by the cosine of the TPA.
Preoperative mediolateral radiographic view of an orthopedically normal stifle joint from a canine cadaver showing measurements of the desired advancement angle made by means of the conventional method (AM) and the correction method (AE). The joint has been immobilized at 135° in the true lateral position. The circle to the left of the joint represents a 25-mm-diameter radiographic reference positioned at the same level as the joint to allow measurement of the anatomic structures and calculation of the radiographic magnification ratio.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Preoperative mediolateral radiographic view of an orthopedically normal stifle joint from a canine cadaver showing measurements of the desired advancement angle made by means of the conventional method (AM) and the correction method (AE). The joint has been immobilized at 135° in the true lateral position. The circle to the left of the joint represents a 25-mm-diameter radiographic reference positioned at the same level as the joint to allow measurement of the anatomic structures and calculation of the radiographic magnification ratio.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Preoperative mediolateral radiographic view of an orthopedically normal stifle joint from a canine cadaver showing measurements of the desired advancement angle made by means of the conventional method (AM) and the correction method (AE). The joint has been immobilized at 135° in the true lateral position. The circle to the left of the joint represents a 25-mm-diameter radiographic reference positioned at the same level as the joint to allow measurement of the anatomic structures and calculation of the radiographic magnification ratio.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
The position of the Maquet hole was determined on the same radiograph. The planned osteotomy site was oriented from a point immediately cranial to the medial meniscus to a point over a distance equivalent to 150% of the length of the tibial crest,35 which was defined as point O (Maquet hole). Two distances were determined relative to point O: the distance from the most proximal part of the tibial crest to point O (distance 1) and the thickness of the cranial cortex of the tibia at the level of point O (distance 2).
All measurements were performed with the aid of a digital radiographic viewing program.d Calibration was performed by use of a 25-mm-diameter radiographic reference positioned at the same level as the joint to allow measurement of the anatomic structures and calculation of the radiographic magnification ratio.
Surgical procedure
The surgical procedure was performed by a board-certified surgeon (TC). The tibiae were prepared for the MMT by drilling a 3-mm Maquet hole in a mediolateral direction at point O by use of distances 1 and 2. An MMT osteotomy was performed as described elsewhere14 by use of a motorized oscillating saw.e The saw cut ended in the Maquet hole, thus leaving the distal portion of the cortex intact.
A bone mark was made 3 mm below the proximal aspect of the tibial bone margin on the caudal edge of the osteotomy site with an osteotome for consistent advancement measurement. A 3.5-mm cortical screw was inserted in a 2.5-mm craniocaudal hole drilled in the proximal portion of the tibial crest. At the proximal extent of the osteotomy site, a stainless steel plate was placed along the cut surface of the bone on the side of the tibial body.6 A screw was inserted into the tibial crest, with the tip of the screw in contact with the stopper plate within the osteotomy site. This allowed alteration of the distance between the tibia and the tibial tuberosity by turning of the screw (Figure 3).6 Advancement distances were measured perpendicular to the osteotomy site at the level of the previously made mark on the tibial bone margin by use of a digital caliper (precision, 0.01 mm). The tibial crest was successively advanced to the positions derived from the conventional and corrected radiographic methods. An additional 0.6-mm advancement was made relative to these positions to take into account the blade width.
Superimposed pre- and postoperative mediolateral radiographic views of a stifle joint from a canine cadaver as achieved with a radiographic viewing program.d The MMT (screw and plate) has been used to achieve advancement of the tibial tuberosity. The line through the joint represents the tibial plateau slope.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Superimposed pre- and postoperative mediolateral radiographic views of a stifle joint from a canine cadaver as achieved with a radiographic viewing program.d The MMT (screw and plate) has been used to achieve advancement of the tibial tuberosity. The line through the joint represents the tibial plateau slope.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Superimposed pre- and postoperative mediolateral radiographic views of a stifle joint from a canine cadaver as achieved with a radiographic viewing program.d The MMT (screw and plate) has been used to achieve advancement of the tibial tuberosity. The line through the joint represents the tibial plateau slope.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
The turnbuckle was turned as necessary to prevent application of excessive tension during advancement of the tibial tuberosity while maintaining the patellar tendon straight. For each advancement, a mediolateral radiograph was obtained with the stifle joint positioned exactly in the same position as the preoperative radiograph, as facilitated by the custom-made external fixator, so that the 2 radiographs could be superimposed and compared.
Postoperative PTA measurements
One observer (PP), who was unaware of TTA measurement method used, measured the postoperative PTA for each advancement measurement method in a random order (determined by use of a random number generator) on 3 occasions, with a minimum interval of 1 week separating measurements. To ensure use of the same tibial plateau landmarks, the tibial plateau slope marked on the preoperative radiograph was transferred through to the postoperative radiographs by digital superimposition (Figure 3).
True advancement determination
When neither of the 2 radiographic methods of advancement measurement led to a postoperative PTA of approximately 90°, the tibial tuberosity was advanced as necessary by turning the screw until the PTA was within 0.1° of 90° on the mediolateral radiographic view (Figure 4). Advancement distance was measured at the bone landmark by use of the digital caliper. To account for the blade width, 0.6 mm was subtracted from that measurement, and the true advancement distance was recorded. The same observer, blinded to advancement measurement method, measured the PTA with true advancement in a random order on 3 occasions, with a minimum interval of 1 week separating measurements.
Postoperative mediolateral radiographic view of a stifle joint from a canine cadaver in which the MMT was used to achieve advancement of the tibial tuberosity. The joint has been immobilized at 135° in the true lateral position, with the PTA at 90° (true advancement).
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Postoperative mediolateral radiographic view of a stifle joint from a canine cadaver in which the MMT was used to achieve advancement of the tibial tuberosity. The joint has been immobilized at 135° in the true lateral position, with the PTA at 90° (true advancement).
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Postoperative mediolateral radiographic view of a stifle joint from a canine cadaver in which the MMT was used to achieve advancement of the tibial tuberosity. The joint has been immobilized at 135° in the true lateral position, with the PTA at 90° (true advancement).
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Limb constructs were kept until completion of the study and confirmation of an acceptable PTA (within 0.1° of 90°). When the mean final PTA of the series of 3 measurements for each limb was not equal to that acceptable value, new radiographs were obtained to determine the true advancement distance.
Cage size recommendation
Advancement measurements for all 3 methods (2 radiographic methods and true advancement) were used to select 1 of 9 commercially available cage sizes (3, 4.5, 6, 7.5, 9, 10.5, 12, 15, and 16 mm). When the required advancement distance was between that achievable with 2 commercially available cage sizes, the larger of the 2 cages was selected. When the advancement exceeded 16 mm (the largest commercially available cage size), the cage size recommendation was recorded as not applicable, resulting in a tenth possible treatment recommendation.
Statistical analysis
Statistical softwaref was used for all analyses. Because some data were not normally distributed as indicated by the Shapiro-Wilk test, results (preoperative PTA, TPA, advancement distance, and postoperative PTA measurements) are reported as median, mean, IQR, and range.
Agreement among the conventional, corrected, and true advancement distance results was assessed by calculation of the Lin CCC.36–38,f,g The CCC can assume a value between −1 and 1; CCC values equaling 1, < 1 and ≥ 0.9, ≥ 0.7 and < 0.9, ≥ 0.5 and < 0.7, and < 0.5 indicate perfect repeatability or perfect agreement, excellent repeatability or excellent agreement, good repeatability or good agreement, moderate repeatability or moderate agreement, and poor repeatability or poor agreement, respectively. Disagreement can be further qualified by examining the precision (ρ) and accuracy (χa) factors, with perfect agreement characterized by values of ρ equaling 1 and χa equaling 1.
Concordance correlation coefficients were also calculated to assess the agreement among the postoperative PTAs achieved with each advancement measurement method. The Wilcoxon signed rank test was performed to compare each postoperative PTA with the ideal 90°. Values of P < 0.05 were considered significant. Repeatability of the postoperative PTA measurements was evaluated by calculation of an overall CCC.39
Proportions of identical cage choices and their 95% CIs were calculated for each pair of advancement measurement methods.h Confidence intervals are reported as Wilson intervals.40
Results
Preoperative measurements
Median preoperative PTA of the 24 canine stifle joints for the conventional radiographic method of advancement measurement was 102.7° (mean, 103.9°; IQR, 101.4° to 107.4°; range, 100.0° to 110.1°). Median preoperative TPA for the correction radiographic measurement method was 23.5° (mean, 23.4°; IQR, 22.0° to 24.8°; range, 19.8° to 26.8°).
Median advancement distance measured with the conventional method was 10.6 mm (mean, 10.9 mm; IQR, 8.9 to 12.4 mm; range, 6.5 to 15.6 mm), and that measured with the correction method was 11.5 mm (mean, 11.9 mm; IQR, 9.6 to 13.7 mm; range, 6.9 to 17.2 mm). Advancement distances obtained with the correction method were systematically (general observation) higher than the values obtained with the conventional method. Level of agreement between the 2 measurement methods was excellent (CCC = 0.92), with near perfect precision and very good accuracy achieved between methods (Table 1). The difference between measurement methods was variable and increased with increasing advancement distance. The data were somewhat close to the equality line (y = x) that represented perfect agreement, and the difference between the slope for the data and the slope of the equality line was small (Figure 5).
Scatterplots of relationships between the advancement distances for the tibial tuberosity in stifle joints (n = 24) of 12 canine cadavers determined by means of conventional (AM) and correction (AE; A) radiographic methods, the distance determined by means of the AM and the true advancement distance required to achieve a PTA reduction to 90° (TA; B), and values determined by means of AE and TA (C). The 45° diagonal line in each panel represents the equation y = x (ie, equality line or perfect agreement). The smaller the distance between the individual data points and the regression line, the more absolute the agreement is between methods. The smaller the difference between the slope of the regression line and the equality line, the more consistency there is between methods.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Scatterplots of relationships between the advancement distances for the tibial tuberosity in stifle joints (n = 24) of 12 canine cadavers determined by means of conventional (AM) and correction (AE; A) radiographic methods, the distance determined by means of the AM and the true advancement distance required to achieve a PTA reduction to 90° (TA; B), and values determined by means of AE and TA (C). The 45° diagonal line in each panel represents the equation y = x (ie, equality line or perfect agreement). The smaller the distance between the individual data points and the regression line, the more absolute the agreement is between methods. The smaller the difference between the slope of the regression line and the equality line, the more consistency there is between methods.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Scatterplots of relationships between the advancement distances for the tibial tuberosity in stifle joints (n = 24) of 12 canine cadavers determined by means of conventional (AM) and correction (AE; A) radiographic methods, the distance determined by means of the AM and the true advancement distance required to achieve a PTA reduction to 90° (TA; B), and values determined by means of AE and TA (C). The 45° diagonal line in each panel represents the equation y = x (ie, equality line or perfect agreement). The smaller the distance between the individual data points and the regression line, the more absolute the agreement is between methods. The smaller the difference between the slope of the regression line and the equality line, the more consistency there is between methods.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Concordance correlation coefficients and 95% CIs, precision (ρ), and accuracy (χa) for comparisons among advancement distances determined for the tibial tuberosity in stifle joints (n = 24) from 12 canine cadavers by means of conventional and correction radiographic methods and the true advancement distance required to achieve a PTA reduction to 90°.
| Comparison | CCC | 95% CI | ρ | χa |
|---|---|---|---|---|
| Conventional vs correction | 0.92 | 0.86–1.00 | 0.99 | 0.89 |
| Conventional vs true advancement | 0.59 | 0.43–0.99 | 0.99 | 0.62 |
| Correction vs true advancement | 0.79 | 0.67–0.99 | 0.99 | 0.81 |
Postoperative PTA and true advancement distances
Median PTA achieved after advancement of the tibial tuberosity by means of the MMT for the conventional radiographic method of advancement measurement was 93.4° (mean, 93.5°; IQR, 93.2° to 93.7°; range, 92.4° to 94.6°), and that for the correction method was 92.3° (mean, 92.4°; IQR, 92.2° to 92.6°; range, 91.9° to 93.4°). Postoperative PTAs obtained with conventional advancement measurement were systematically higher than postoperative PTAs obtained with correction advancement measurement, which were systematically higher than the ideal 90°. The level of agreement between both sets of postoperative PTA values was poor (Table 2). Despite good precision (ρ = 0.86), accuracy was low (χa = 0.29) between the 2 methods.
Concordance correlation coefficients and 95% CIs, precision (ρ), and accuracy (χa) for comparisons between PTAs achieved in the stifle joints (n = 24) of 12 canine cadavers following surgical advancement of the tibial tuberosity by the distance calculation methods in Table 1.
| Comparison | CCC | 95% CI | ρ | χa |
|---|---|---|---|---|
| Conventional vs correction | 0.25 | 0.12–0.37 | 0.86 | 0.29 |
| Conventional vs true advancement | 0.00 | 0.00–0.00 | −0.30 | 0.00 |
| Correction vs true advancement | 0.00 | 0.00–0.00 | −0.29 | 0.01 |
Three measurements were made for each method for each joint, and results were averaged for these comparisons.
Regression analysis revealed that data were far from the equality line, and the difference between the slope for the data and the slope of the equality line was low between postoperative PTA measurements (2 radiographic methods and true advancement; Figure 6). Both radiographic methods of advancement measurement led to a mean postoperative PTA that was significantly (P < 0.001) different from 90°. The overall CCC for the conventional method was high (0.96), with very good precision (ρ = 0.96) and accuracy (χa = 1), and the same was true for the correction method (overall CCC, 0.94; ρ = 0.95; χa = 0.99).
Scatterplots of the relationships between PTAs achieved following use of the MMT in stifle joints (n = 24) of 12 canine cadavers in which the advancement distance for the tibial tuberosity was determined by use of the methods in Figure 5. See Figure 5 for remainder of key.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Scatterplots of the relationships between PTAs achieved following use of the MMT in stifle joints (n = 24) of 12 canine cadavers in which the advancement distance for the tibial tuberosity was determined by use of the methods in Figure 5. See Figure 5 for remainder of key.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Scatterplots of the relationships between PTAs achieved following use of the MMT in stifle joints (n = 24) of 12 canine cadavers in which the advancement distance for the tibial tuberosity was determined by use of the methods in Figure 5. See Figure 5 for remainder of key.
Citation: American Journal of Veterinary Research 77, 12; 10.2460/ajvr.77.12.1401
Both radiographic methods of advancement measurement failed to result in the true advancement distance for all 24 stifle joints evaluated. Median true advancement distance was 13.5 mm (mean, 13.9 mm; IQR, 11.6 to 15.9 mm; range, 8.4 to 19.6 mm). Agreements between the true advancement measurements and the conventional and correction advancement measurements were moderate (CCC = 0.59) and good (CCC = 0.79), respectively (Table 1).
Despite very good precision (ρ = 0.99), the conventional method was moderately accurate for determining the true advancement distance (χa = 0.59). The difference between the true advancement distance and the advancement distance determined by use of the conventional method was variable and increased with increasing distance. Regression analysis revealed that the data were far from the equality line (y = x), and the difference between the slope for the data and the slope of the equality line was low (Figure 5).
The correction method also yielded very good precision (ρ = 0.99); however, its accuracy for approximating the true advancement distance (χa = 0.79) was greater than that of the conventional method. Differences between the true advancement distance and the advancement distance measured with the correction method were fairly constant. The data were slightly far from the equality line, and the difference between the slope for the data and the slope of the equality line was low (Figure 5).
The mean final PTA of the series of 3 measurements achieved with the true advancement distance was equal to 90° within 0.1° for each stifle joint; no limbs required new radiographs. The overall mean final PTA was not significantly (P = 0.79) different from 90°. There was no agreement (CCC = 0.00) between the true advancement PTA and postoperative PTAs achieved following use of both radiographic methods of advancement measurement, with a very low precision and low accuracy (Table 2). For the conventional and correction methods, the data were far from the equality line and the differences between the slopes for the data and the slope of the equality line were high (Figure 6).
Cage size recommendations
For the 24 stifle joints evaluated, the conventional and correction radiographic methods of advancement measurement led to the same cage size recommendations as determined with the true advancement measurements for only 1 (4%) and 4 (17%) stifle joints, respectively. The conventional and the correction methods led to the same cage size recommendations for 11 (46%) stifle joints. Proportions of stifle joints with identical cage size recommendations were moderate between radiographic methods (0.46; 95% CI, 0.28 to 0.65) and very low to low between true advancement measurements and conventional measurements (0.04; 95% CI, 0.00 to 0.20) or correction measurements (0.25; 95% CI, 0.12 to 0.45).
Discussion
Preoperative measurements of the distance required to advance the tibial tuberosity in dogs with cranial cruciate ligament rupture are one of the most important components of the MMT and traditional TTA.30,31,33 The goal of preoperative planning is to determine the advancement distance required to create a postoperative PTA of 90° at stance and to select the optimal cage size to achieve this. Results of the present study suggested that commonly used radiographic methods of advancement measurement failed to correctly yield the true advancement distance required for PTA reduction to the intended 90° when the MMT was used.
Initial clinical studies revealed promising results for the MMT15,16 that were similar to those obtained with traditional TTA19–23; however, the authors are unaware of any study conducted to evaluate postoperative PTAs achieved with the MMT. The study reported here revealed that both radiographic methods of advancement measurement failed to result in a reduction in PTA that was close to 90° for the 24 canine stifle joints evaluated. Postoperative PTAs achieved with use of the conventional radiographic method were systematically higher than those achieved with use of the correction method, which were also systematically higher than the intended 90°. In previous studies,6,7,11–13,a,b investigators demonstrated that femorotibial shear force neutralization was achieved by a PTA reduction to ≤ 90° at stance. Therefore, results of the present study suggested that the MMT would fail to neutralize femorotibial shear forces at stance when either radiographic method of advancement measurement is used.
Because cages are not available in all sizes, the discrepancy obtained between the planned and true advancement measurements should have been mitigated by the available cage size selection in the present study. However, the conventional and correction advancement measurements led to the same cage size recommendations as those applicable to the true advancement distance for only 4% and 17% of stifle joints evaluated, respectively. This finding suggested that neither radiographic method was able to correctly yield an optimal cage size recommendation. The conventional and correction advancement measurements resulted in the same cage size recommendations for only 46% of stifle joints. When cage size recommendations for both methods disagreed, the cage sizes selected with the correction method were systematically higher than those selected with the conventional method.
One of the main differences between traditional TTA and the MMT is the osteotomy shape. With the MMT, the tibial crest is left distally attached to the tibia and only advanced in a curvilinear fashion without proximal displacement.14 The displacement of the tibial tuberosity does not follow the same pattern as that with traditional TTA. Therefore, the lack of proximal translation of the tibial crest with the MMT could explain why the conventional radiographic method for measurement of advancement distance originally described for traditional TTA failed to yield the true advancement distance required in the present study. Indeed, when the conventional method is used, advancement distance is determined along a line parallel to the tibial plateau slope. However, as explained in the report of a geometric study,32 the plane of the tibial crest osteotomy site is not perpendicular to the tibial plateau. Because the tibial crest is not proximally translated and is advanced in only a curvilinear fashion when the MMT is used, the degree of underestimation of the advancement distance determined with the conventional method increases concomitantly with the TPA.32
On the other hand, the correction method of advancement measurement accounts for the TPA.32 Moreover, this method is valid only when the tibial crest is not translated proximally during advancement of the tibial tuberosity32 and is thus more appropriate for the MMT. Although the postoperative PTAs achieved with the correction method were closer to 90° than those achieved with the conventional method in the present study, the correction method also failed to yield the true advancement distance. Several factors could explain this result. For both radiographic methods, advancement measurements are made at the level of the proximal aspect of the tibial tuberosity. However, current recommendations are to place the cage at the proximal extent of the osteotomy site but 2 to 3 mm from the proximal tibial bone margin.20 Because the tibial crest is not proximally translated when the MMT is used, the cage is placed more proximally than the level of the advancement measurement. Therefore, the advancement distance at the level of the tibial tuberosity is less than expected, which could be the reason the correction method also failed to yield the true advancement distance in the present study. This failure could be mitigated by lowering the cage at the level of the tibial crest as described for a TTA procedure with high advancement distance in large- to giant-breed dogs.41 However, without a cancellous bone block placed proximal to the cage to provide buttress support, this approach would dramatically increase the risk of tibial crest fracture.41
In a canine cadaveric study,6 neutralization of femorotibial shear forces occurred at a mean ± SD PTA of 90.3 ± 9°. All the postoperative PTAs obtained in the present study were within this interval. Therefore, it is possible that the underestimation of advancement distance achieved with both radiographic measurement methods in the present study may not have a clinical impact. However, another in vitro study13 showed that a PTA < 90° was required to achieve stability of the stifle joint and that a PTA as low as 86° was required at higher limb loads (50% of body weight). The true cutoff value for under-reduction of the PTA is unknown. A PTA reduction close to the ideal 90° may be sufficient to adequately neutralize femorotibial shear forces in most situations, but a true 90° angle may be more critical in some dogs than others.41
A substantial margin of error has been suggested to exist in TTA regarding the final PTA,19–24,42 as observed with tibial plateau leveling osteotomy. However, we are convinced that an accurate and repeatable preoperative planning method is important.19,22,23 Indeed, an under-reduction of the PTA could lead to the persistence of instability, which could explain the high rate of late meniscal tears (5% to 28%) observed when traditional TTA is performed.20–23,43,44 In light of a study42 that showed the persistence of cranial tibial subluxation after traditional TTA in 70% of dogs during weight bearing, we recommend using the method that leads to the larger cage size recommendation and choosing the larger cage when a choice must be made between 2 sizes. We presumed that overestimation of the required cage size would counteract the underestimation of the advancement distance determined with the commonly used radiographic methods of advancement measurement, thereby resulting in a PTA as close as possible to the intended 90°.
In a recent study,31 advancement measurements for the tibial tuberosity were compared between the transparent overlay method and a virtual software method. The virtual software method led to greater values than did the transparent overlay method. However, the virtual software method was used to perform a virtual TTA, and the correction method of radiographic advancement measurement was not evaluated, thereby limiting an ability to compare those results with the results of the present study. Moreover, the true impact on PTA reduction was not evaluated in that study.31 The virtual software method was not available for the present study and therefore could not be compared with the 2 commonly used radiographic methods of advancement measurement.
A discrepancy between the line drawn parallel to the cranial aspect of the patellar tendon and the line drawn parallel to the cranial aspect of the bone landmarks (patella and tibial crest) was observed on each postoperative mediolateral radiograph of stifle joints in the present study. In fact, the distal portion of the patellar tendon appeared to lie on the proximal aspect of the tibial tuberosity after advancement, particularly when the advancement distance was high (Figure 4). Because the virtual method is based on transposition of bone landmarks (tibial crest), we presume that this method would lead to an overestimation of the required advancement distance and fail to yield the true advancement distance; however, this presumption remains to be confirmed.
The stifle joint extension angle was fixed and maintained with an external skeletal fixator during all testing in the present study because stifle joint flexion is a major contributor to the PTA (linear decrease of the PTA with increasing flexion).26 Cranial tibial subluxation can result in an underestimation of the PTA29; therefore, the cranial cruciate ligament was kept intact in the stifles joints evaluated in the present study. A spring and turnbuckle were used to simulate the quadriceps mechanism.6 The adjustable turnbuckle was turned as necessary to straighten the patellar tendon, which facilitated the identification of the patellar tendon axis (cranial aspect of the patellar tendon). Because the cranial and caudal cruciate ligaments were kept intact, the patellar tendon was indeed straightened subjectively without care taken to avoid applying excess tension so that the tension applied was not similar between dogs but was dependent on the stifle conformation.
Standardization of the tension applied could have predisposed some of the stifle joints to become overextended or could have prevented the cranial translation of the tibial crest. Only radiographic and photographic images obtained in the true lateral position were used to ensure accurate identification of the various landmarks and accurate determination of the PTA landmarks.34
Despite the small sample size in the study reported here, advancement measurements with both radiographic methods had low agreement with the true advancement measurements. Use of only 1 observer for postoperative PTA measurements was also a limitation of the study. However, because identification of tibial plateau landmarks can introduce variability, identical tibial plateau slopes were obtained for pre- and postoperative radiographs, as identified via digital superimposition. Indeed, reproducibility of the PTA measurements was very good owing to use of this methodology.
The PTA can be determined by use of the tibial plateau method9,a,b or the common tangent method.26 The impact of PTA measurement method on advancement measurements involving the tibial tuberosity has already been evaluated.30,31 Because the purpose of the present study was to evaluate the impact of advancement measurement method (conventional vs correction) on the PTA achieved with the MMT and not to evaluate PTA measurement methods (tibial plateau vs common tangent), tibial plateau slope was measured by use of only 1 method. In another study,33 the common tangent and tibial plateau methods of PTA measurement led to an underestimation and overestimation of the PTA, respectively, and the investigators recommended use of the method (tibial plateau) that resulted in overestimation. Moreover, this method is reportedly easier, quicker to use, and more reliable than the common tangent method.30,33
As a consequence of using superimposition of radiographic images in the present study, we were able to use the same tibial plateau landmarks for the determination of the desired radiographic advancement distances and the true advancement distance required to achieve a PTA reduction to 90°. Therefore, we believe that the method used for PTA measurement had no impact on the results obtained; the disparity between desired and true advancement distances would be the same.
Similar to any study involving cadavers, the anatomic variations and the lack of forces typically exerted by muscles are also limitations of the study reported here. In addition, other variables must also be considered for thorough assessment of the efficacy of the MMT in dogs, including proximodistal displacement of the patella, the translation of the femur relative to the tibia, and the angle between the tibia and the ground.32
In the study reported here, conventional and correction methods of radiographic measurement of advancement distance failed to yield the true advancement distance required for PTA reduction to the intended 90° following use of the MMT. The clinical importance of underestimating the required advancement distance is unknown. However, given the study findings, we recommend use of the correction method, which could be expected to lead to selection of a larger cage size than is necessary, therefore prioritizing the larger cage size when one is forced to choose between 2 sizes because the exact size needed is unavailable. Overestimation of the necessary cage size could be expected to counteract any underestimation of required advancement distance, thereby resulting in a PTA as close as possible to the intended 90°. Results suggested that neither radiographic method is ideal and that an alternative method of advancement measurement may be required to allow appropriate planning of MMT procedures in dogs.
Acknowledgments
The authors declare that there were no conflicts of interest.
The authors thank Dr. Sylvain Larrat for assistance with the statistical analyses.
ABBREVIATIONS
| CCC | Concordance correlation coefficient |
| CI | Confidence interval |
| IQR | Interquartile range |
| MMT | Modified Maquet technique |
| PTA | Patellar tendon–tibial plateau angle |
| TPA | Tibial plateau angle |
| TTA | Tibial tuberosity advancement |
Footnotes
Montavon PM, Damur DM, Tepic S. Advancement of the tibial tuberosity for the treatment of cranial cruciate deficient canine stifle (abstr), in Proceedings. 1st World Orthop Vet Congr 2002;152.
Tepic S, Damur DM, Montavon PM. Biomechanics of the stifle joint (abstr), in Proceedings. 1st World Orthop Vet Congr 2002;189–190.
Fujifilm FCR XG-1 computed radiography, Fujifilm, Düsseldorf, Germany.
Osirix (64 bit), Pixmeo, Geneva, Switzerland.
PMF (10.8 V, 180 lithium), Bosh, Gerligen, Germany.
R, version 3.2.0, R Foundation for Statistical Computing, Vienna, Austria. Available at: www.r-project.org. Accessed Apr 17, 2015.
epiR, R Foundation for Statistical Computing, Vienna, Austria. Available at: CRAN.R-project.org/package=epiR. Accessed Jul 16, 2015.
Hmisc (binconf), R Foundation for Statistical Computing, Vienna, Austria. Available at: CRAN.R-project.org/package=Hmisc. Accessed Nov 1, 2015.
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