Cranial cruciate ligament (CrCL) disease is a common orthopedic condition of stifle in dogs and is considered one of the leading causes of secondary degenerative joint disease.1–3 Tibial plateau leveling osteotomy (TPLO) is a surgical treatment option that dynamically stabilizes the CrCL-deficient stifle during loadbearing.1–5
The kinematics and contact mechanisms of the femorotibial and patellofemoral joints are altered with the rupture of the CrCL.6 The kinematics and contact mechanics of the stifle joint have also been investigated in dogs with CrCL-deficient stifles treated surgically by TPLO.7 It was found that TPLO eliminated craniocaudal stifle instability during weight bearing but failed to restore the femorotibial contact mechanics to normal.7 In an in vitro experiment, Pozzi et al8 demonstrated that TPLO disrupted the normal patellofemoral alignment. They subsequently concluded that the change in patellofemoral joint biomechanics is due to the reduced patellar tilt angle following TPLO.8 A subtle increase in cranial patellar translation relative to the trochlear groove, predominately in the stance phase, was noted on horizontal fluoroscopic images during treadmill walking.9 A study by Saytha et al10 elucidated the relationship between post-TPLO tibial plateau angle (TPA) adjustments and the patellar tendon angle in CrCL-deficient stifles. The study revealed a patellar tendon angle measurement of 91.4 ± 5.5°, juxtaposed with a post-TPLO TPA of 7.6 ± 3.3°.10
The authors of this study speculated that these changes could increase the strain in the patellar ligament, which could be responsible for the patellar desmopathies seen postoperatively. Studies have indicated that although translational forces (in the form of cranial tibial thrust) are neutralized during the stance phase of gait, the pressure dynamics in the joint are altered.7,8 Additionally, dogs undergoing TPLO have a shorter patellar ligament length at the postoperative and follow-up evaluations (36 to 364 days [mean, 88 days; median, 63 days]) than at the preoperative evaluation.11
Patellar desmopathies, including patellar ligament desmitis, tibial tuberosity fractures, and patella fractures, are a group of conditions seen either in combination or as a single entity following TPLO.12–15 Three different studies12,13,16 reported patellar ligament thickening in post-TPLO patients. The size of the ligament increased by 93%,12 80%,13 and 87%.16 Distal patellar ligament thickening has been assessed radiographically by first measuring the proximal thickness and then the distal thickness of the patellar ligament on a mediolateral radiograph. Thereafter, the proximal-to-distal thickness ratio was calculated.13 Mattern et al13 demonstrated that the reduced TPAs seen post-TPLO were associated with patellar ligament desmitis. The incidence of patellar ligament thickness increased with a post-TPLO TPA of < 6°, as evidenced by increased distal patellar ligament longitudinal and transverse thickness measurement using ultrasonographic transverse area measurements.13 The supposition was that excessive loading of the patellar ligament, secondary to altered biomechanics after TPLO, was the underlying cause.12
To the authors’ knowledge, patellar ligament strain following TPLO in the CrCL-deficient stifle has not been investigated.17 Determining patellar ligament strain can deepen the understanding of the development of patellar desmopathies.
The aim of our ex vivo study was to investigate the strain of the patellar ligament following TPLO in an experimentally induced CrCL-deficient stifle under compression force to mimic weight bearing during the stance phase of gait. We hypothesized that there would be no difference in the patellar ligament strain at different TPAs and compared to CrCL-intact stifle under conditions that mimic weight bearing at a normal standing angle.
Methods
Study specimens
Twelve pelvic limbs were collected at postmortem from 7 adult nonchondrodystrophic large breed dogs (> 20 kg) with no evidence of clinical and radiographic stifle or hip joint pathology. The owners of all animals gave signed consent for the cadavers to be used for research purposes, and the dogs were euthanized for reasons unrelated to this study. The Animal Ethics Committee, Faculty of Veterinary Science, University of Pretoria, Pretoria, South Africa, approved the ex vivo prospective observational study (REC 158-21). The justification for the number of cadaver limbs required was made from previous studies as no real power analysis could be performed on the descriptive cohort study design of this study. Kim et al9 used 10 dogs (5 pelvic limbs), Warzee et al18 utilized 15 cadaver canine pelvic limbs, and Pozzi et al8 used 4 dogs (7 pelvic limbs). The study sample of Reif et al5 was 3 dogs (6 pelvic limbs), and the study by Zann et al19 had a study size of 10 dogs (10 pelvic limbs). Based on these studies, the sample size of 6 dogs (12 cadaveric pelvic limbs) was selected for the study.
Postmortem radiographs included a craniocaudal radiograph and a mediolateral 90° flexed radiograph of each intact CrCL stifle to determine the pre-TPLO TPAs. Radiographs were taken with an x-ray machine (Baccara 90/25 HV; Apelum-DMS Group) using an electronic cassette radiography system (Fujifilm Corp).
The TPA for each cadaver pelvic limb was determined pre-TPLO as described previously.5 The principal investigator made all the measurements using orthopedic surgical planning software (Siemens Healthcare Veterinary Orthopedic Solution; mediCAD Hectec GmbH).
Study specimen preparation
Both pelvic limbs of each cadaver were dissected en bloc with a proximal femoral amputation; thereafter, they were stripped of all muscular tissues. The passive stabilizers of the stifle (cruciate and collateral ligaments), patellar ligament (including the patella), and the soft tissues distal to the talocrural joint were preserved during dissection.
The specimens were wrapped in saline-soaked (0.9% NaCl; Adcock Ingram) abdominal swabs, placed in plastic bags marked with a specific identifier number, and frozen at −20 °C until testing. The night before testing, the specimens were placed in a saline bath to thaw at room temperature (22 °C). Soft tissues were kept moist throughout the experiment by spraying the specimens with 0.9% saline. A speckled pattern with black, nontoxic alcohol-based ink (ShowOffs Body Art LC) was applied to the surface of the dissected-out patellar ligament using an airbrush (GAV Mini 200). The proximal femur was secured with a custom-made adjustable fixation to allow normal motion of the stifle joint. Distal fixation included securing the metatarsal bones distally in a position that mimicked a weight-bearing stance. The stance was achieved using an adjustable paw boot, which was secured to the base of the Instron Model 34TT-5 Universal Testing Machine.
Adjustable turnbuckles secured the proximal femur attachment to the Instron Model 34TT-5 Universal Testing Machine. The turnbuckles were attached to loops made from high-modulus polyethylene leader line fiber (JDB 8X Camo Covert Braid 180LB) and secured on both sides of the frame. The setup mimicked the origin of the medial quadriceps femoris vastus lateralis, medial quadriceps femoris vastus intermedius, and medial quadriceps femoris vastus medialis. Distally, the turnbuckles were attached to the patella to mimic the insertion of the quadriceps muscle group (Figure 1).
Application of the variable angle tibial plateau leveling osteotomy (V-TPLO) plate onto each specimen. A—Specimen with intact patellar ligament and collateral ligaments of the stifle joint placed into the baseplate for the V-TPLO plate and secured with a 1.6-mm Kirschner wire, which was placed at the intercondylar eminence. B—Jig based on the 24-mm biradial TPLO saw blade placed onto base and secured through the same 1.6-mm pin in A and an additional 1.6-mm Kirschner wire placed into distal segment to secure the jig onto the specimen. C—Radial osteotomy that was seen with point of rotation around proximal 1.6-mm Kirschner wire cutting the proximal tibia via an osteotomy glide hole. D—Custom-made V-TPLO plate based on the template of the 3.5-mm left/right TPLO plate. The V-TPLO plate was placed over the 2 wires placed and secured to the specimen limb via 3.5-mm cortical screws proximally and distally to simulate TPLO plating. The protractor attached to the V-TPLO was divided into 1° increments. E—Specimen with V-TPLO plate attached to proximal tibia secured placed into the frame of the Instron Model 34TT-5 Universal Testing Machine. F—Adjustable turnbuckles attached on both sides of the frame securing the proximal femur attachment to the machine, mimicking the origin of the medial quadriceps femoris vastus lateralis, medial quadriceps femoris vastus intermedius, and medial quadriceps femoris vastus medialis (*). The gastrocnemius muscle was simulated by securing an adjustable turnbuckle to two 3.5-mm cortical screws caudal distal on the femur proximally and distally to the calcaneus bone via a bolt screw placed through a bone tunnel (white arrow).
Citation: American Journal of Veterinary Research 2025; 10.2460/ajvr.24.08.0245
The gastrocnemius muscle was simulated by securing an adjustable turnbuckle to two 3.5-mm cortical screws (Hebu Medical). Each screw was inserted at the point of origin of the medial gastrocnemius as described by Warzee et al.18 Thereafter, the adjustable turnbuckle was secured distally to the calcaneus bone using a bolt screw placed through a bone tunnel.
The pre-TPLO measurement for the location of the radial osteotomy was determined as described by Slocum and Slocum.20 When performing the tibial radial osteotomy, specific landmarks were established. The proximal landmark was positioned at a 90° angle to the tibial cortex and located at least 10 mm caudal to the cranial margin of the tibial tuberosity. The caudal distal landmark of the osteotomy exited the caudal tibial cortex at a 90° angle.20 The center point of the radial osteotomy was located over the intercondylar eminences.
A variable angle TPLO (V-TPLO) plate was designed (open source FreeCAD under Lesser General Public License) and then laser cut from a 5-mm aluminum plate (Figure 1). A drill and alignment jig accompanied the plate to ensure the accuracy and repeatability of the osteotomy and drill hole locations. The authors determined the location of the V-TPLO screw hole based on the template of the 3.5-mm left or right (depending on limb) locking TPLO plates (DePuy Synthes).
The V-TPLO procedure, as described by Warzee et al,18 involved mounting the proximal aspect of the jig to the proximal tibia. A transcondylar 1.6-mm Steinmann pin (Hebu Medical GmbH) was inserted 5 mm below the surface of the tibial plateau behind the medial collateral ligament and through the jig. The pin was passed perpendicularly to the sagittal plane in the proximal tibia at the level of the intercondylar eminence.18 The pin functioned as the center point for the radial osteotomy saw to rotate around. A 24-mm biradial TPLO saw blade (Veterinary Instrumentation) was used to cut the proximal tibia through an osteotomy glide hole in the jig.
The jig was attached distally to the proposed osteotomy on the tibia using two 3.5-mm cortical bone screws. This was to mimic bone plating in a TPLO procedure. The V-TPLO plate was placed over the Steinmann pin previously placed at the intercondylar eminence. The V-TPLO contained an adjustable protractor attached to the distal portion of the proximal tibia segment below the osteotomy line. The protractor was used to rotate the tibial plateau as defined for each angle (Figure 1).
The authors adjusted the TPA for each stifle using an adjustable protractor. The protractor was attached to the distal aspect of the osteotomized segment. The TPA was adjusted to 5 different angles: −5°, 0°, 5°, 10°, and 15°. To maintain the rotation at each angle, the jig attached to the protractor was secured using a 2.5-mm bolt and wing nut.
Mechanical testing
During the midpoint of the stance phase at a walk, the normal position of the pelvic limb has previously been reported as the femoral longitudinal axis at a 70° angle with the horizontal plane, stifle flexion angle at 135°, and hock flexion angle at 145°.21 The turnbuckles were used to adjust the length of the gastrocnemius to ensure the stifle angle remained at 135° and the hock angle at 145°.
The limbs were preloaded to 20% of the mean body weight (60 N) to simulate the normal weight distribution to each pelvic limb in a normal stance.22 The turnbuckles were adjusted to achieve the correct angles, mimicking the physiological weight-bearing stance in a normal standing position. Each limb was subsequently loaded to 40% of the mean body weight (120 N) to simulate the full weight bearing of the limb during walking. The grip vertical displacement, measuring the displacement of the apparatus, was recorded at the preset force point on the Instron Model 34TT-5 Universal Testing Machine when each limb was loaded by the 120 N at the different TPAs. The grip vertical displacement was measured to ensure we standardized the experiment for each experimental group.
Measurement of patellar ligament deformation
The deformation of the patellar ligament was evaluated using a 3-D method that involved using 2 digital image correlation (DIC) cameras (IDT Vision NX8-S2). Digital image correlation employs correlation-based full-field displacement measurements of surface speckles during testing to compute local surface strain.23 The 3-D DIC stereo setup was designed for a 600-mm stand-off distance and a 166 X 124-mm field of view. The setup used 50-mm fixed focal length lenses with an intercamera angle of 20°, and it ensured optimal imaging conditions (Figure 2). A rig was machined on a CNC milling machine to rigidly fix the cameras to ensure the setup adhered to these design specifications. The cameras were synchronized and triggered with an external switch and captured images synchronously at 5 frames per second. Reference images were captured daily before testing to calibrate the stereo setup for lens distortion effects and camera orientation.
Digital image correlation camera set up with specimen setup in the Instron Model 34TT-5 Universal Testing Machine.
Citation: American Journal of Veterinary Research 2025; 10.2460/ajvr.24.08.0245
The patellar ligament deformation was measured and recorded for the following conditions: pre-TPLO intact CrCL, pre-TPLO transected CrCL, and each of the respective post-TPLO TPAs (−5°, 0°, 5°, 10°, and 15°). The deformation in the patellar ligament was measured in triplicate under lighting levels that maximized gray level distribution in the dynamic range, and the mean measurement was used for analysis.
Three-dimensional Green-Lagrange principal deformation maps were calculated during processing with specific software (Image Systems TEMA T2021a-64). Thereafter, the average deformation value for each time step was calculated during postprocessing in Matlab (R2021b under Academic License; MathWorks). The 3-D DIC configuration and the middle substance region of interest (ROI) selected for DIC processing were employed to accommodate for cranial translation and internal rotation of the tibia relative to the femur during stifle loading in mechanical testing. This was done to ensure that the measurements were not compromised by nonuniform strain fields at the attachment sites of the patellar ligament, both proximally to the patella and distally to the tibial tuberosity24 (Figure 3).
Region of interest of the patella ligament used for digital image correlation analysis to generate the 3-D Green-Lagrange deformation maps of the patellar ligament.
Citation: American Journal of Veterinary Research 2025; 10.2460/ajvr.24.08.0245
Statistical analysis
The strain data (unitless deformation/axial strain [e33] measures) were recorded in triplicate on a spreadsheet (Microsoft Excel workbook) for each tested TPA. Then, the triplicate strain values were averaged using the arithmetic mean, and these values were used in further calculation. The change in strain was calculated by subtracting the pre-TPLO intact CrCL values from all subsequent measured values. The percent change in strain was calculated by dividing the change in strain by the pre-TPLO intact CrCL value and converting this number to a percentage. The processed data set was then transferred to commercially available statistical software for analysis (MiniTab 18.1; Minitab Inc). Data distribution was assessed by plotting histograms, inspecting descriptive statistics, and applying the Anderson-Darling test for normality. Data are nonparametric and reported as median (IQR).
For hypothesis testing, the strain measures at each TPA were compared using a general linear mixed model (random factor, limb; fixed factor, degree rotation). The residual plots were inspected for linearity and randomness to determine how well the model fit the data. Multiple comparisons were performed by applying the Dunnett method using the strain measurements for pre-TPLO intact CrCL values as the control. The grip vertical displacement was compared using the same general linear mixed model that was used for comparing the strain. Significance was interpreted as P < .05.
Results
The 6 dogs that met the inclusion had a median (IQR) body mass of 30.0 kg (28.7 to 32.0 kg). The study population included 3 Rottweilers, 2 Labrador Retrievers, 3 German Shepherd Dogs, and 2 cross breeds. Data from 9 of 12 the pelvic limbs were analyzed, as 3 of the specimens were lost to breakage of the construct (Supplementary Table S1).
The pre-TPLO TPA median was 25.0° (22.0° to 27.5°). The grip vertical displacement was 5.9 mm (4.6 to 7.9 mm) for the pre-TPLO intact CrCL and differed from the 7.2 mm (5.8 to 13.7 mm) for the pre-TPLO transected CrCL (P = .014). The remainder of the grip vertical displacements were not significantly different from the pre-TPLO intact CrCL, and the following measurements were determined for the respective TPAs: −5° was 7.1 mm (6.2 to 8.3 mm); 0° was 5.8 mm (5.3 to 9.7 mm); 5° was 6.8 mm (5.8 to 9.0 mm); 10° was 6.3 mm (5.6 to 2 mm); and 15° was 6.3 mm (5.1 to 7.8 mm).
The patellar ligament strain in the pre-TPLO intact CrCL stifle was 0.012 (0.010 to 0.020) and significantly lower than 0.030 (0.020 to 0.041) for 0° (P = .019) and 0.025 (0.020 to 0.040) for 5° (P = .031). The remaining TPAs were not different from pre-TPLO intact CrCL stifle and were 0.020 (0.011 to 0.031) for pre-TPLO transected CrCL, 0.021 (0.012 to 0.034) for −5°, 0.020 (0.014 to 0.024) for 10°, and 0.019 (0.009 to 0.027) for 15° (Figure 4). The change in patellar ligament strain was 0.005 (0.004 to 0.029) for 0° and different from the pre-TPLO intact CrCL stifle (P = .047). The changes in patellar ligament strain for the remaining TPAs were not different and were −0.001 (−0.020 to 0.003) for pre-TPLO transected CrCL, −0.005 (−0.023 to 0.004) for −5°, −0.008 (−0.025 to −0.004) for 5°, −0.001 (−0.010 to 0.0007) for 10°, and −0.003 (−0.014 to 0.002) for 15°. The percent change in patellar ligament strain was different at −5° (P = .041), 0° (P = .013), and 5° (P = .026) TPAs compared to pre-TPLO intact CrCL construct. The median magnitude of percent change in strain was 35.1% (−26.2% to 217.2%), 37.0% (22.1% to 250.3%), and 79.0% (24.3% to 180.9%) for −5°, 0° and 5°, respectively. Furthermore, the median magnitude of percent change in strain was −6.1% (−169.7% to 17.6%), −7.1% (−92.1% to 6.6%), and −21.1% (−124.2% to 19.2%) for pre-TPLO transected CrCL, 10°, and 15°, respectively.
A to C—Boxplot diagrams demonstrating the descriptive statistics for the correlation between the patellar ligament strain in the pre-TPLO intact cranial cruciate ligament stifle (A), change in the patellar ligament (B), and percent change in patellar strain (C) against the various specimens tested. *Outliers. #Significant values in the data set.
Citation: American Journal of Veterinary Research 2025; 10.2460/ajvr.24.08.0245
The 3-D Green-Lagrange principal deformation maps demonstrated an increase in the deformation in the patellar ligament at the distal attachment of the patellar ligament onto the proximal tibial tuberosity in 6 of the 12 limbs at a post-TPLO TPA of 5° (Figure 5).
Digital image correlation (DIC) camera images depict the increase in deformation on the patellar ligament seen in the distal third of the patellar ligament with increasing axial loading. A—Deformation that was seen illustrated by the DIC in the post-TPLO 0° group, indicating increased strain mostly in the distal third of the patellar ligament. B—Deformation that was seen illustrated by the DIC in the post-TPLO 5° group, indicating increased strain mostly in the length of the patellar ligament. C—Deformation that was seen illustrated by the DIC in the post-TPLO 10° group, once again indicating increased strain in the patellar ligament, mostly in the distal third of the patellar ligament.
Citation: American Journal of Veterinary Research 2025; 10.2460/ajvr.24.08.0245
Discussion
An increase in median deformation/axial strain in the patellar ligament, as measured using DIC, in the CrCL-deficient stifle compared to a CrCL-intact stifle was observed in this study. Additionally, an increase in patellar ligament strain with post-TPLO TPAs of 0° and 5° was observed compared to the post-TPLO TPAs of −5°, 10°, and 15°.
Digital image correlation has emerged as a pivotal tool in the mechanical evaluation of biological materials. Digital image correlation operates by monitoring the displacement and deformation of random speckle pattern subsets within the region of interest on a target specimen.25 Consequently, it enables the acquisition of full-field strain data. When employing a single camera for DIC, known as 2-D DIC, measurements are confined to in-plane displacement and deformations, particularly on flat objects. However, in the presence of more complex geometries and out-of-plane motion, multiple cameras are necessary to execute 3-D DIC, facilitating the measurement of resultant full-field 3-D deformation and strain.
During the stance phase of gait, the craniocaudal stability of the stifle is dependent on the CrCL; therefore, cranial tibial translation is absent in an intact stifle.26 Cranial tibial subluxation was observed in a CrCL-deficient stifle during the stance phase of gait.26 The subluxation is believed to be due to the unopposed cranial force, known as the cranial tibial thrust, being present in a CrCL-deficient stifle.26 A study by Pennasilico et al27 demonstrated a patellar ligament thickening in the CrCL-deficient stifle from the time of onset of lameness to the time of diagnosis of CrCL disease, suggesting that the cranial tibial thrust plays some role in the patellar desmopathy seen in these patients. The increase in the patellar ligament strain seen in the pre-TPLO transected CrCL stifle compared to the pre-TPLO intact CrCL stifle in this study could potentially support these hypotheses. However, this study did not take all the soft tissue components and normal physiological forces that would act on the CrCL-deficient stifle in vivo into consideration, which could contribute to the patellar ligament thickening seen in CrCL-deficient patients.
The authors selected the post-TPLO TPA angles of 0°, 5°, 10°, and 15° based on previous studies5,21,28 that demonstrated good clinical outcomes with post-TPLO TPA range between 0° and 14°. The post-TPLO TPA of −5° was selected to investigate the effect of overrotation following TPLO. The authors however elected to not include post-TPLO TPAs of > 15°, as the purpose of the study was to investigate the effect of the recommended post-TPLO TPA angles and overrotation on patellar ligament strain.
In this study, the median grip vertical displacement during the stance phase of walking of the CrCL-intact stifle was less than observed when the same force was applied to the CrCL-deficient stifle but no different from all other post-TPLO TPAs. This suggests that the strain measures were obtained under similar conditions.
Warzee et al18 demonstrated that by performing TPLO, the joint surface should become perpendicular to the tibial functional axis during weight bearing in the stance phase of gait. After TPLO, the original cranial tibial thrust becomes increasingly more caudally orientated as the rotation increases and TPA decreases, ultimately resulting in a caudal tibial thrust.18 Warzee et al18 investigated the effect of TPLO on tibial subluxation and tibial axial rotation. They18 showed that overrotating the TPLO resulted in significant caudal tibial subluxation and decreased tibial axial internal rotation after TPLO. The axial displacement seen in our study for the transected CrCL and all the post-TPLO TPAs could be due to subluxation seen in CrCL-deficient stifles. However, the causal relationship between grip vertical displacement and femorotibial translation was not specifically investigated.
In this study, an increase in the median patellar ligament strain was observed in the post-TPLO TPAs of the 0° and 5°. The post-TPLO TPAs of 0° and 5° eliminate the cranial tibial thrust; however, the increase in rotation would increase caudal tibial thrust translation due to overrotation.18 Excessive rotation could heighten the risk of caudal cruciate ligament strain, tibial tuberosity fracture, fibular fracture, and patellar fracture and cause abnormal femorotibial contact mechanics.7,14,15,18,29–32 Geier et al14 evaluated the risk of patellar fracture following TPLO and showed that the odds of patellar fractures increased by 21.7% for every 1° decrease in the post-TPLO TPA, and the median TPA for the patellar fracture group was 1.4°. Another study by Rutherford et al15 investigated the occurrence of patella fractures post-TPLO and found the incidence to be 2%, with all dogs experiencing a patellar fracture having a post-TPLO TPA of ≤ 5°. Although these conditions are potential complications believed to be a result of overrotation, the forces acting on these anatomical structures were not evaluated in this study. The authors believe this increase in the caudal tibial translation and overrotation could be responsible for the increase seen in the median patellar ligament strain. The change in strain and magnitude of percent change in strain for post-TPLO TPAs of 0° and 5° supports the finding that the patellar ligament strain for these 2 angles is indeed increased compared to the pre-TPLO intact CrCL stifle. Therefore, we amassed enough evidence to reject our hypothesis. This finding could support the notion that tibial plateau rotation for a target TPA of 5° may be excessive.33
A decrease in median patellar ligament strain was observed in the post-TPLO TPA 10°, which approached the strain level observed in the original intact CrCL stifle. Other studies4,5,8,18,28 have demonstrated that a post-TPLO TPA of 0° to 14° is sufficient for creating a dynamically stable joint and reducing the femorotibial subluxation in CrCL-deficient stifles without any adverse clinical implications.
The authors postulate that the absence of an increase in patellar ligament strain in the underrotated postoperative TPA of 15° could be attributed to the continued cranial tibial thrust seen in CrCL-deficient stifles.26
The authors were unable to explain the rationale behind the absence of a significant increase in the outlier post-TPLO TPA of −5°. However, it is plausible to speculate on the potential impact of patellar length and patellar angle in altering the biomechanical forces acting on the patellar ligament, thereby influencing the observed outcomes.
The findings in this study contradict the findings by Drew et al17 who demonstrated no significant change in stifle extensor mechanism load following TPLO. We speculate that this contradiction could be attributed to Drew et al17 not focusing on isolating the strain solely to the patellar ligament when investigating the strain experienced by the ligament following TPLO. Another limitation of the ex vivo study conducted by Drew et al17 was the cruciate-deficient stifle not being investigated.
This study model differs from previous studies17,34 investigating patellar load as strain gauges were not used to determine the strain of the patellar ligament. The rationale was that securing the gauges onto the patellar ligament would influence the integrity of the patellar ligament and only test that specific portion of the ligament. Due to its noncontact nature, DIC effectively mitigates superficial hysteresis effects arising from compliance during strain calculation from grip-to-grip displacement.24 Unlike contact sensors like strain gauges and linear variable differential transformer sensors, DIC does not disrupt the integrity of the test specimen.24 Three-dimensional DIC is limited to measuring surface strain. Digital volume correlation is a technique that can measure 3-D internal strain but requires the test to be conducted in an MRI scanner.25 The experiment did pose the limit that this could not be performed in the MRI due to the risk posed to equipment and personnel due to the metal composition of the implants and the mechanical testing system.
This study, the 3-D Green-Lagrange deformation maps led to an incidental observation that there was an increase in the deformation measured in some specimens at the attachment of the patellar ligament at the tibial tuberosity compared to the rest of the patellar ligament. The increase in deformation at this location is suspected to play a role in the presence of patellar ligament desmitis pathophysiology. Mattern et al13 showed radiographically that the preferential site of patellar ligament thickening is the distal aspect of the patellar ligament. The authors propose that this observation could be explained by the alteration of the extracellular matrix in the distal third of the patellar ligament, leading to a transition toward a fibrocartilaginous composition near its insertion on the tibial tuberosity. The most fibrocartilaginous tendons are heavily loaded and permanently bent around their pulleys.31 This transition may involve changes in the arrangement and distribution of collagen fibers and the presence of specialized cells and matrix components characteristic of fibrocartilage. An increase in type II collagen has been suggested in heavily loaded tendons and ligaments.34 Further studies35 utilizing histological techniques (Masson's trichrome stain and sirius red) can provide more detailed insight into the histological changes in the patellar ligament based on anatomical locations.
There were notable limitations to our study. The model did not take all the musculoskeletal structures exerting forces on the stifle joint into consideration. Specifically, the authors did not consider the muscular deformation of the stifle extensor mechanism. The rationale was to eliminate the influence of the stifle extensor mechanism lever arm, which could have played a role in reducing the strain in the patellar ligament as an entity. Another limitation was the lack of measurement of the cranial and caudal tibial translation that occurred at the various angles of rotation of TPLO. However, the Matlab software used to analyze the DIC images accounted for the cranial displacement of the markers as the stifle bends, and therefore, all efforts were made to eliminate possible distortion to the deformation measurements obtained. The behavior of the polyethylene leader line fiber was not taken into consideration when mechanical testing was performed; the authors did however obtain the deformation values for the strain equation from the middle substance ROI, and therefore, it is believed that the leader line fiber elasticity would not influence the ROI as this was fixed for each specimen regardless of the testing group. The study did not assess patellar lengths, patellar angles, or clinical assessment of meniscal pathology at the various post-TPLO TPAs. Another limitation of this study is the lack of assessing the damage caused by a TPLO to the blood supply of the patellar ligament as previously demonstrated in an ex vivo study, as a possible underlying pathology for the described patellar tendinopathies.36 Additional limitations to the study include the lack of recording the freezing time, thawing time, and dog body mass to account for random factors to the model. The authors can therefore not pass comment on whether these random variables would have influenced the outcome of this study.
In conclusion, the authors of the study have demonstrated that the angle of rotation of the TPLO plays a role in patellar ligament strain. Significant increases in patellar ligament strain were at TPAs of 0° and 5°. This increased strain could contribute to the development of postoperative patellar ligament desmitis. The patellar ligament strain measured at post-TPLO TPA of 10° closely resembled that of the pre-TPLO intact CrCL stifle; therefore, we postulate that a 10° TPA will result in sufficient rotation to eliminate femorotibial subluxation/translation and decrease the strain in the patellar ligament but warrants further research.
Supplementary Materials
Supplementary materials are posted online at the journal website: avmajournals.avma.org.
Acknowledgments
The authors thank A. J. De Villiers, MEng, Tshwane University of Technology, for assisting in the design of the V-TPLO plate.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
The Arbeitsgemeinschaft für Osteosynthesefragen (AO) Foundation, AO Vet Clinical Division, funded this study through a seed grant (AOVETS-II-22-02B to Elizabeth G. Bester).
References
- 1.↑
Pacchiana PD, Morris E, Gillings SL, Jessen CR, Lipowitz AJ. Surgical and postoperative complications associated with tibial plateau leveling osteotomy in dogs with cranial cruciate ligament rupture: 397 cases (1998–2001). J Am Vet Med Assoc. 2003;222(2):184–193. doi:10.2460/javma.2003.222.184
- 2.
Paatsama S. Ligament Injuries of the Canine Stifle Joint: a Clinical and Experimental Study. Dissertation. College of Veterinary Medicine; 1992.
- 3.↑
Marshall JL, Olsson SE. Instability of the knee. A long-term experimental study in dogs. J Bone Joint Surg. 1971;53(8):1561–1570. doi:10.2106/00004623-197153080-00008
- 4.↑
Priddy NH, Tomlinson JL, Dodam JR, Hornbostel JE. Complication with and owner assessment of the outcome of tibial plateau leveling osteotomy for treatment of cranial cruciate ligament rupture in dogs: 193 cases (1997–2001). J Am Vet Med Assoc. 2003;222(12):1726–1731. doi:10.2460/javma.2003.222.1726
- 5.↑
Reif U, Hulse DA, Hauptman JG. Effect of tibial plateau leveling on stability of the canine cruciate-deficient stifle joint: an in vitro study. Vet Surg. 2002;31(2):147–154. doi:org/10.1053/jvet.2002.31041
- 6.↑
Guerrero TG, Pozzi A, Dunbar N, et al. Effect of tibial tuberosity advancement on the contact mechanics and the alignment of the patellofemoral and femorotibial joints. Vet Surg. 2011;40(7):839–848. doi:10.1053/jvet.2002.31041
- 7.↑
Kim SE, Pozzi A, Banks SA, Conrad BP, Lewis DD. Effect of tibial plateau leveling osteotomy on femorotibial contact mechanics and stifle kinematics. Vet Surg. 2009;38(1):23–32. doi:10.1111/j.1532-950X.2008.00470.x
- 8.↑
Pozzi A, Dunbar NJ, Kim SE. Effect of tibial plateau leveling osteotomy on patellofemoral alignment: a study using canine cadavers. Vet J. 2013;198(1):98–102. doi:10.1016/j.tvjl.2013.06.029
- 9.↑
Kim SE, Zann GJ, Tinga S, Moore EJ, Pozzi A, Banks SA. Patellofemoral kinematics in dogs with cranial cruciate ligament insufficiency: an in-vivo fluoroscopic analysis during walking. BMC Vet Res. 2017;13(1):250. doi:10.1186/s12917-017-1186-1
- 10.↑
Saytha S, Gilbert P, Sharma A, Hendrick S. Effect of tibial plateau levelling osteotomy on patellar tendon angle: a prospective clinical study. Vet Comp Orthop Traumatol. 2014;18(5):235–242.
- 11.↑
Jay MR, Mattoon JS, Gilbert PJ, Tanaka TT, Beaty BL. Radiographic evaluation of patellar ligament length after tibial plateau leveling osteotomy in dogs. Am J Vet Res. 2019;80(6):607–612. doi:10.2460/ajvr.80.6.607
- 12.↑
Carey K, Aiken SW, DiResta GR, Herr LG, Monette S. Radiographic and clinical changes of the patellar tendon after tibial plateau leveling osteotomy 94 cases (2000–2003). Vet Comp Orthop Traumatol. 2005;18(4):235–242. doi:10.1055/s-0038-1632960
- 13.↑
Mattern KL, Berry CR, Peck JN, De Haan JJ. Radiographic and ultrasonographic evaluation of the patellar ligament following tibial plateau leveling osteotomy. Vet Radiol Ultrasound. 2006;47(2):185–191. doi:10.1111/j.1740-8261.2006.00126.x
- 14.↑
Geier CM, Frederick SW, Cross AR. Evaluation of the risk of patella fracture as the result of decreasing tibial plateau angle following tibial plateau leveling osteotomy. Vet Surg. 2021;50(5):984–989. doi:10.1111/vsu.13640
- 15.↑
Rutherford S, Bell JC, Ness MG. Fracture of the patella after TPLO in 6 dogs. Vet Surg. 2012;41(7):869–875. doi:10.111/j.1532-950X.2012.01018.x
- 16.↑
Gallagher A, Cross AR, Sepulveda G. The effect of shock wave therapy on patellar ligament desmitis after tibial plateau leveling osteotomy. Vet Surg. 2012;41(4):482–485. doi:10.1111/j.1532-950X.2012.00958.x
- 17.↑
Drew JO, Glyde M, Hosgood GL, Hayes AJ. The effect of tibial plateau levelling osteotomy on stifle extensor mechanism load: a canine ex vivo study. Vet Comp Orthop Traumatol. 2018;31(2):131–136. doi:10.1055/s-0038-1627476
- 18.↑
Warzee CC, Dejardin LM, Arnoczky SP, Perry RL. Effect of tibial plateau leveling on cranial and caudal tibial thrusts in canine cranial cruciate-deficient stifles: an in vitro experimental study. Vet Surg. 2001;30(3):278–286. doi:10.1053/jvet.2001.21400
- 19.↑
Zann GJ, Kim SE, Tinga S, Pozi A, Banks SA. The effect of tibial plateau leveling osteotomy on patellofemoral kinematics in dogs: an in vivo study. Vet Surg. 2020;49(1):207–213. doi:10.1111/vsu.13300
- 20.↑
Slocum B, Slocum TD. Tibial plateau leveling osteotomy for repair of cranial cruciate ligament rupture in the canine. Vet Clin North Am Small Anim Pract. 1993;23(4):777–795. doi:10.1016/S0195-5616(93)50082-7
- 21.↑
Hottinger HA, DeCamp CE, Olivier NB, Hauptman JG, Soutas-Little RW. Noninvasive kinematic analysis of the walk in healthy large-breed dogs. Am J Vet Res. 1996;57(3):381–388. doi:10.2460/ajvr.1996.57.03.381
- 22.↑
Goldner B, Fuchs A, Nolte I, Schilling N. Kinematic adaptations to tripedal locomotion in dogs. Vet J. 2015;204(2):192–200. doi:10.1016/j.tvjl.2015.03.003
- 23.↑
Pearson T, Glyde MR, Day RE, Hosgood GL. The effect of intra-medullary pin size and plate working length on plate strain in locking compression plate-rod constructs under axial load. Vet Comp Orthop Traumatol. 2016;29(6):451–458. doi:10.3415/VCOT-16-01-0008
- 24.↑
Mallett KF, Arruda EM. Digital image correlation-aided mechanical characterization of the anteromedial and posterolateral bundles of the anterior cruciate ligament. Acta Biomater. 2017;56:44–57. doi:10.1016/j.actbio.2017.03.045
- 25.↑
Govender S, Cronjé JY, Keough N, et al. Emerging imaging techniques in anatomy: for teaching, research and clinical practice. Adv Exp Med Biol. 2023:1392:19–42. doi:10.1007/978-3-031-13021-2_2
- 26.↑
Korvick DL, Pijanowski DL, Schaeffer DJ. Three-dimensional kinematics of the intact and cranial cruciate ligament deficient stifle of dogs. J Biomech. 1994;27(1):77–87. doi:10.1016/0021-9290(94)90034-5
- 27.↑
Pennasilico L, Volta A, Sassaroli S, et al. Ultrasound and elastosonographic features of the patellar ligament in dogs affected by cranial cruciate ligament disease. Vet Sci. 2024;11(3):126. doi:10.3390/vetsci11030126
- 28.↑
Robinson DA, Mason DR, Evans R, Conzemius MG. The effect of tibial plateau angle on ground reaction forces 4–17 months after tibial plateau leveling osteotomy in Labrador Retrievers. Vet Surg. 2006;35(3):294–299. doi:10.1111/j.1532-950X.2006.00147.x
- 29.↑
Kim SE, Pozzi A, Banks SA, Conrad BP, Lewis DD. Effect of cranial cruciate ligament deficiency, tibial plateau levelling osteotomy, and tibial tuberosity advancement on contact mechanics and alignment of the stifle in flexion. Vet Surg. 2010;39(3):363–370. doi:10.1111/j.1532-950X.2010.00655.x
- 30.
Windolf M, Leitner M, Schwieger K, et al. Accuracy of fragment positioning after TPLO and effect on biomechanical stability. Vet Surg. 2008;37(4):366–373. doi:10.1111/j.1532-950X.2008.00390.x
- 31.↑
Benjamin M, Ralphs JR. Fibrocartilage in tendons and ligaments—an adaptation to compressive load. J Anat. 1998;193(4):481–494. doi:10.1046/j.1469-7580.1998.19340481.x
- 32.↑
Tuttle TA, Manley PA. Risk factors associated with fibular fracture after tibial plateau levelling osteotomy. Vet Surg. 2009;38(3):355–360. doi:10.1111/j.1532-950X.2009.00504.x
- 33.↑
Drygas KA, Pozzi A, Goring RL, Horodyski M, Lewis DD. Effect of tibial plateau levelling osteotomy on patellar tendon angle: a radiographic cadaveric study. Vet Surg. 2010;39(4):418–424. doi:10.1111/j.1532-950X.2010.00665.x
- 34.↑
Stapleton JP. MicroStrain Pattern Analysis of the Canine Patellar Tendon Following a Tibial Plateau Leveling Osteotomy. Dissertation. University of Tennessee; 2004.
- 35.↑
Marcod-Garces V, Harvat M, Molina Aguilar P, Ferrandez Izquierdo A, Ruiz-Sauri A. Comparative measurement of collagen bundle orientation by Fourier analysis and semiquantitative evaluation: reliability and agreement in Masson's trichrome, Picrosirius red and confocal microscopy techniques. J Microsc. 2017;267(2):103–142. doi:10.1111/jmi.12553
- 36.↑
Johnson MD, Shmon CL, Linn KA, Singh B. Ex vivo evaluation of the effect of various surgical procedures on blood delivery to the patellar tendon of dogs. Am J Vet Res. 2016;77(5):548–555. doi:10.2460/ajvr.77.5.548.
