Ground reaction forces and kinematic data in humans are routinely combined with morphometric measurements to calculate torque and power patterns across joints through an inverse dynamics approach.1 This approach characterizes a limb as a series of connected linkages and integrates morphometric data that provide information on inertial properties (eg, mass, location of COM, and mass moment of inertia) of each limb segment. Results of such analyses have greatly advanced knowledge about the pathogenesis of joint diseases in humans. For example, results from inverse dynamics analyses have revealed that differences in joint position, mobilization, and relative strength of quadriceps and caudal thigh muscles are factors that predispose women athletes to CCL disease.2–5
The use of inverse dynamics to compute joint forces, torques, and power in veterinary medicine has been limited because of a number of factors, including a lack of morphometric measures for dogs. Body segment parameters in humans were initially measured on cadavers,6–11 but more recently, they have been determined on live subjects via mechanical techniques12–14 or medical imaging technology.15–22 Currently, BSPs in humans are commonly calculated by use of regression equations derived from anthropometric measurements of cadavers, images, or geometric models of body segments.1,9,16,23–33
Reports34–36 of morphometric data in horses and dogs are limited by the number of samples included and number of parameters evaluated. For example, morphometric data on canine hind limb segments in 1 study36 were generated from cadavers of 3 Labrador Retrievers and 4 Greyhounds and included only mass and COM. Regression equations allow approximation of BSP from easily obtainable morphometric measurements but are only available for horses.34 Consequently, none of the noninvasive methods has been proposed to calculate BSP in dogs in a representative sample, thereby requiring direct measurements for each dog after it has been euthanatized. Investigators in 1 study36 extrapolated BSP data measured from a small number of cadavers to other dogs of the same breeds. However, their small number of samples may not accurately represent the BSP in these breeds. Obtaining BSP values from cadavers is a time-consuming, expensive process that may restrict the canine population being investigated.
The first objective of the study reported here was to determine BSPs for the hind limb segments in live Labrador Retrievers with or without CCL deficiency. The second objective was to develop regression equations on the basis of easily obtainable morphometric measurements to estimate those BSPs. We hypothesized that BSPs would differ between normal, CCL-deficient, and contralateral limbs. Consequently, we hypothesized that regression equations would differ for BSPs among these 3 groups of limbs in Labrador Retrievers.
Materials and Methods
Animals—Twenty-four client-owned purebred Labrador Retrievers were used in the study. Informed consent was obtained from owners prior to enrollment of dogs into the study. The Institutional Animal Care and Use Committee of the University of Illinois approved all study procedures.
Fourteen clinically normal dogs (28 normal hind limbs) were used in the study. Each dog was > 6 years old and did not have a history of orthopedic disease of the stifle joint and was considered currently free of orthopedic disease of the stifle joint on the basis of results of physical examination, radiography, and CT. An additional 10 adult dogs were examined at the Veterinary Teaching Hospital at the University of Illinois because of weight-bearing lameness attributable to nontraumatic unilateral CCL disease. The duration of lameness before examination was determined on the basis of the history in the medical records. The diagnosis of CCL disease was confirmed by arthroscopy or exploratory arthrotomy at the time of surgical correction of the CCL disease. For these dogs, the 10 CCL-deficient limbs comprised a group of limbs, whereas the 10 normal contralateral limbs of the CCL-deficient dogs comprised another group of limbs. Each limb in the study was analyzed separately.
Procedures—Signalment and body mass were recorded. Complete physical and orthopedic examinations were performed on each dog. Additional procedures were performed as part of another study. Joint angles with each dog in a standing position and range of motion (flexion and extension) of the hip, stifle, and tarsal joints were obtained. Radiographs of the pelvis, femur, and tibia and DEXAa of the entire body of each dog were obtained, which was followed by a gait analysis.b
Morphometric measurements were obtained in triplicate; mean values were calculated and used for analysis. Length of the thigh was defined as the distance between the greater trochanter and the most distal point of the lateral femoral epicondyle. Length of the crus was defined as the distance between the most distal point of the lateral femoral epicondyle and the lateral malleolus of the fibula. Length of the foot was defined as the distance between the lateral malleolus of the fibula and the fifth metatarsophalangeal joint. Lengths were measured by use of motion-capture softwarec as the distance between markers designed for kinematic studies that had been affixed to the skin at designated anatomic landmarks. Measurements were performed after kinematic data were recorded from each dog as part of another study. Each length measurement consisted of the mean length of the segment during a complete gait cycle. Circumference of the thigh (ie, thigh girth) was measured in the midshaft region of the femur at a point midway between the greater trochanter and lateral femoral epicondyle. Circumference of the crus (ie, crus girth) was measured at the level of the tibial crest. Circumferences were measured by use of a single standard plastic nonstretchable metric tape with the dogs positioned in lateral recumbency. Lateromedial widths of the stifle, tarsal, and metatarsophalangeal joints were measured with a caliper at the level of the lateral femoral epicondyle, lateral malleolus of the fibula, and fifth metatarsophalangeal joint, respectively.
Length of the foot was also measured on DEXA images. Total area, bone mineral content, bone mineral density, lean mass, and total mass of the foot were also determined by use of DEXA analysis.
Dogs were anesthetized, and the pelvis and both hind limbs of each anesthetized dog were imaged by use of a 3-D helical CT system.d Dogs were positioned in dorsal recumbency with the stifle and tarsal joints extended in both hind limbs. All CT scans were associated with a coordinate system in the x-, y-, and z-axes, with a common reference origin at the bottom of the field of view. Because the dogs were in dorsal recumbency with the hind limbs extended parallel to the z-axis, the x-, y-, and z-axes of the CT images were approximately aligned with the transverse, craniocaudal, and longitudinal directions, respectively, of the hind limb segments. Each image was subsequently analyzed by use of softwaree to yield data on the cross-sectional area of bone, muscle, and fat tissues. Pixel intensity values of every image were correlated with tissue densities by the software.e The operator (CAR) adjusted these ranges of pixel values for each tissue by selecting different points for each tissue and comparing the pixel intensity recorded with the range of pixel intensities provided by the software for the selected tissue (Figure 1). The range of pixel intensities for each tissue type was held constant throughout the study.
Three-dimensional CT images of the hip joint (transverse view [A]) and midfemur (frontal view [B] and parasagittal view [C]) obtained from a representative Labrador Retriever. Notice the outline of fat (yellow outline [yellow arrow]), muscle (red outline [red arrow]), and bone (dark gray outline [white arrow]) tissues as determined on the basis of their respective pixel values by use of software.e
Citation: American Journal of Veterinary Research 69, 9; 10.2460/ajvr.69.9.1188
Following 3-D reconstruction of CT images, each limb was divided into thigh, crus, and foot segments. A transverse plane was defined as parallel to the transverse and craniocaudal axes (x- and y-axes, respectively) and perpendicular to the longitudinal axis (z-axis) of the segment. Proximal boundary of the thigh was defined by the transverse plane that crossed through the proximal end of the greater trochanter. The pelvic bones and pelvic inlet, tail, and prepuce were excluded. Distal boundary of the thigh was defined by the transverse plane that crossed the most distal point of the lateral femoral condyle. The crus segment extended from the transverse plane that crossed the most distal point of the lateral femoral condyle to the transverse plane that crossed the distal point of the fibular malleolus. Tarsal elements proximal to the fibular malleolus (with the tarsal joint in extension) were included in the crus segment. The foot segment extended from the transverse plane that crossed the distal point of the fibular malleolus to the tip of the most distal phalanx among all digits.
To determine the inertial properties of each segment by use of 3-D CT scans, tissue densities of muscle, fat, and bone were fixed at values of 1.06, 0.95, and 1.8 g/cm3, respectively.37 The density of each tissue type within a segment was assumed to be uniform. Muscles, tendons, and ligaments were not delineated in this analysis because they have similar densities. Dermal tissues were treated as the layer in closest contact to muscle or fat. The 3-D CT volumes obtained by use of solid modeling softwaref for each tissue section were multiplied by the density value for each tissue to determine the mass of each tissue within each segment. The segment mass was then calculated by summing the values for each of the tissue masses and expressed as %BM. Location of the COM was represented along the longitudinal axis of each segment, and the distance of the estimated COM from the proximal joint of the segment was determined by use of solid modeling softwaref and expressed as %L. The mass moment of inertia of each segment about a transverse (mediolateral) axis through its estimated COM was calculated by use of solid modeling softwaref and adjusted on the basis of body mass (Figure 2).
Three-dimensional image of the left thigh of a representative Labrador Retriever. Volumes of fat (yellow), muscle (red), and bone (gray) tissues determined by use of solid modeling softwaref were used to calculate BSPs. The location of the center of mass (arrow) and the 3 axes (x-, y-, and z-axis, respectively) are indicated.
Citation: American Journal of Veterinary Research 69, 9; 10.2460/ajvr.69.9.1188
Statistical analysis—Statistical analysis was performed on the data obtained. Regression models were developed to estimate BSPs.
DEMOGRAPHICS AND BSPS
Clinically normal and CCL-deficient dogs were compared for sex distribution by use of an exact χ2 test.g Body mass was logarithmically transformed to achieve normality. Differences among groups with regard to age and logarithmically transformed body mass were determined by use of a 1-way ANOVA followed by use of the Tukey HSD test.
Analysis of BSP measurements for the thigh and crus required special methods because the CCL-deficient and contralateral limbs were paired within each dog, whereas normal limbs were paired within each clinically normal dog but were not related to limbs in the CCL-deficient or contralateral groups. Limbs were not independent; rather, they were considered repeated measurements on each dog. To account for within-dog pairing of limbs, a cluster-correlated ANOVA (mixed regression) followed by pairwise tests of group differences (Tukey HSD test) was performed.h Segment mass was expressed as %BM. Location of the COM was expressed as %L. Mass moment of inertia was standardized by dividing it by the body mass. Because data for the standardized mass moment of inertia were not normally distributed, inverse transformation was required. Differences among groups for adjusted segment mass (ie, %BM), COM location, and inversely transformed standardized moment of inertia were determined by use of a 1-way ANOVA followed by the Tukey HSD test. A paired t test was used when the comparison of BSP involved only right and left sides for crus and thigh in normal limbs.
Because of technical difficulties, the distal extremity of the foot segment was scanned in its entirety in only 20 limbs (18 normal, 1 CCL-deficient, and 1 contralateral limb). Morphometric measurements obtained during physical examination and DEXA of the 48 feet included in the study (28 normal, 10 CCL-deficient, and 10 contralateral limbs) were analyzed by use of mixed regression analysis and the Tukey HSD testh to validate extrapolation of BSPs of the foot in normal limbs to the BSPs of the foot in CCL-deficient and in contralateral limbs. Because the data of some variables of the foot were not normally distributed, logarithmic transformation was required for those variables. For all analyses, values of P < 0.05 were considered significant.
REGRESSION MODELS
Estimation equations were constructed from morphometric dimensions to predict mass, location of the COM, and mass moment of inertia for the thigh, crus, and foot of normal, CCL-deficient, and contralateral limbs in Labrador Retrievers. These equations were tested by use of stepwise forward and backward regressionh to determine the most adequate model for each BSP; values of α = 0.15 were used to enter or be removed from the regression. Independent variables (predictors) consisted of the morphometric measurements and body mass. The model with the highest R2 was selected for the mass, location of the COM, and mass moment of inertia for the foot segment. For the crus and thigh segments, different models were selected in accordance with the highest R2 for each limb status (ie, normal, CCL-deficient, and contralateral limbs) for the mass, location of the COM, and mass moment of inertia.27,31,38 Each R2 was higher than 0.85. The SEE was evaluated to help assess models, as described elsewhere.9,23,34,38,39 To ensure that equations were appropriate, they were placed in context to all-subsets regression by use of a statistical program.i Then, accuracy of the model was tested by use of a criterion (ie, percentage of error was < 10.5%, with percentage of error = absolute value of [{estimated value – measured value} × 100]/measured value). The R2 and SEE reflect the precision of predictive equations, and percentage of error reflects the accuracy.38 Finally, residuals and estimates were compared against measurements. When these analyses did not confirm that the estimation requirements of the linear models were met, models from the all-subsets regression were examined to identify a potentially better model and to help identify ways in which we could improve the estimation, such as incorporation of nonlinear terms.
Results
Animals—The 14 clinically normal Labrador Retrievers were significantly older (mean ± SD, 100.6 ± 23.0 months) than the 10 dogs with CCL disease (56.4 ± 20.1 months). Mean mass did not differ significantly between the 14 clinically normal dogs (36.9 ± 9.1 kg) and the 10 dogs with CCL disease (37.5 ± 7.4 kg); mean body mass for all 24 dogs was 36.2 ± 8.1 kg. The sex distribution did not differ significantly between the clinically normal dogs (5 spayed females, 3 sexually intact females, 3 castrated males, and 3 sexually intact males) and the dogs with CCL disease (6 spayed females and 4 castrated males). Mean duration of lameness for the dogs with CCL disease was 3 months (range, 1 week to 1 year).
BSPs—The BSPs of the thigh, crus, and foot segments were determined for the normal, CCL-deficient, and contralateral limbs (Table 1). Because of technical difficulties, the distal portion of the foot segment was scanned in its entirety in only 18 normal, 1 CCL-deficient, and 1 contralateral limb. Hence, mass, location of the COM, and mass moment of inertia values of the foot were only determined for 20 limbs. Morphometric measurements obtained during physical examination and DEXA of the 48 feet included in the study were compared to validate extrapolation of BSPs of the foot in normal limbs to those in CCL-deficient and contralateral limbs. Morphometry and composition of the foot were similar for normal, CCL-deficient, and contralateral limbs for this sample of Labrador Retrievers because there was not a significant difference among normal, CCL-deficient, and contralateral limbs for foot length; mediolateral tarsal width; mediolateral metatarsophalangeal width; foot length measured on DEXA images; or total area, bone mineral content, bone mineral density, lean mass, and total masses of the foot measured on DEXA images. These findings supported the extrapolation of BSPs for the foot segment between healthy, CCL-deficient, and contralateral limbs in our population of Labrador Retrievers.
Mean ± SD values for BSPs of the foot, crus, and thigh of normal, CCL-deficient, and contralateral hind limbs of Labrador Retrievers with or without unilateral CCL disease.
| Segment | Group | Mass | Mass moment of inertia | COM | |||
|---|---|---|---|---|---|---|---|
| g | %BM | g × cm2 | I/BM | cm | %L | ||
| Thigh | Normal (n = 28) | 2,231 ± 581 | 6.05 ± 0.5? | 92,875 ± 44,294 | 2,423± 561a,b | 9.0 ± 1.0 | 42 ± 5 |
| CCL-deficient (n = 10) | 1,939 ± 398 | 5.48 ± 0.34b | 79,074 ± 24,596 | 2,199 ± 276a | 9.1 ± 0.4 | 45 ± 4 | |
| Contralateral (n = 10) | 2,192 ± 374 | 6.22 ± 0.30a | 87,981 ± 23,795 | 2,459 ± 210b | 9.1 ± 0.4 | 44± 3 | |
| Crus | Normal (n = 28) | 516 ± 113 | 1.41 ± 0.16a,b | 16,242 ± 6,778 | 433 ± 35 | 6.3 ± 0.7 | 31 ± 2a |
| CCL-deficient (n = 10) | 484 ± 112 | 1.36 ± 0.09a | 15,174 ± 5,699 | 421 ± 80 | 6.3 ± 0.5 | 28 ± 1b | |
| Contralateral (n = 10) | 512 ± 116 | 1.44± 0.11b | 15,941 ± 6,210 | 441 ± 35 | 6.4 ± 0.4 | 28 ± 1b | |
| Foot | All groups (n = 20)* | 249 ± 25 | 0.70 ± 0.09 | 7,228 ± 1,223 | 203 ± 24 | 8.7 ± 0.4 | 47 ± 3 |
Represents 18 normal, 1 CCL-deficient, and 1 contralateral limb.
I/BM = Mass moment of inertia standardized on the basis of body mass.
For each segment, values in a column with different superscript letters differ significantly (P < 0.05).
None of the BSPs differed between left and right hind limbs in clinically normal dogs. The adjusted masses of the thigh and crus of CCL-deficient limbs (expressed as %BM) were less than in the contralateral limbs (Table 1). The thigh weighed less in CCL-deficient limbs than in normal limbs. The standardized mass moment of inertia of the thigh was less in CCL-deficient limbs than in the contralateral limbs. Position of the crural COM (expressed as %L) was more proximal in CCL-deficient and contralateral limbs, compared to the position in normal limbs.
Regression equations—Morphometric dimensions were determined for the limbs (Table 2). Range, mean, and SD were determined for each dimension measured in each group.
Descriptive statistics for all independent variables included in the regression analysis for 28 normal, 10 CCL-deficient, and 10 contralateral hind limbs of Labrador Retrievers with or without unilateral CCL disease.
| Variable | Group | Range | Mean | SD |
|---|---|---|---|---|
| Body mass (kg) | Normal | 25.9–55.7 | 36.9 | 8.9 |
| CCL-deficient | 26.8–50.0 | 35.4 | 6.8 | |
| Contralateral | 26.8–50.0 | 35.4 | 6.8 | |
| Thigh length (cm) | Normal | 17.9–24.9 | 21.3 | 2.1 |
| CCL-deficient | 18.2–23.7 | 20.4 | 1.7 | |
| Contralateral | 19.3–23.3 | 20.9 | 1.4 | |
| Crus length (cm) | Normal | 17.1–24.2 | 20.2 | 1.9 |
| CCL-deficient | 17.2–21.0 | 19.6 | 1.2 | |
| Contralateral | 16.9–21.2 | 19.6 | 1.3 | |
| Foot length (cm) | Normal | 8.4–10.8 | 9.5 | 0.6 |
| CCL-deficient | 8.0–11.0 | 9.3 | 0.9 | |
| Contralateral | 8.0–10.5 | 9.0 | 0.8 | |
| Thigh girth (cm) | Normal | 37.5–50.0 | 42.5 | 3.4 |
| CCL-deficient | 35.5–47.5 | 40.2 | 3.7 | |
| Contralateral | 37.5–53.0 | 42.4 | 4.8 | |
| Crus girth (cm) | Normal | 22.0–31.0 | 25.1 | 2.1 |
| CCL-deficient | 23.0–25.8 | 24.4 | 0.9 | |
| Contralateral | 23.0–27.5 | 25.2 | 1.3 | |
| Stifle width (cm) | Normal | 4.6–6.7 | 5.6 | 0.6 |
| CCL-deficient | 5.0–6.5 | 5.6 | 0.5 | |
| Contralateral | 4.8–6.5 | 5.5 | 0.5 | |
| Tarsal width (cm) | Normal | 3.0–4.6 | 3.7 | 0.4 |
| CCL-deficient | 3.1–3.9 | 3.5 | 0.3 | |
| Contralateral | 3.1–4.0 | 3.5 | 0.3 | |
| Metatarsal width (cm) | Normal | 2.9–4.5 | 3.8 | 0.4 |
| CCL-deficient | 3.1–4.1 | 3.8 | 0.3 | |
| Contralateral | 3.0–4.0 | 3.5 | 0.3 |
Regression equations were generated for the thigh, crus, and foot segments, respectively, with corresponding R2 values, absolute percentage of error, and SEE (Tables 3–5). The percentage of error varied between 1.0% and 5.7% for the mass of various segments, between 1.6% and 6.5% for the location of the COM, and between 3.4% and 10.4% for the mass moment of inertia. Mean SEE of all regression equations was 4.7% for the mass of the segment, 3.8% for the location of the COM, and 7.9% for the mass moment of inertia. Regression equations accounted for > 86% of the variance in the dependent variables.
Regression equations generated from morphometric measurements and body mass to predict the mass, location of the COM, and mass moment of inertia of the thigh in 28 normal, 10 CCL-deficient, and 10 contralateral hind limbs of Labrador Retrievers with or without unilateral CCL disease.
| Group | Variable | Regression equation* | R2 | Error (%) | SEE |
|---|---|---|---|---|---|
| Normal | Mass (g) | −2,723.4 + (30.5 × BM) + (51.2 × thigh girth) + (296.4 × stifle width) | 0.95 | 5.0 | 127.6 |
| COM (cm) | (0.80 × foot length) + (0.30 × crus girth) − (0.26 × thigh girth) + (0.70 × stifle width) + (1.3 × tarsal width) − (1.1 × metatarsal width) | 0.99 | 6.5 | 0.55 | |
| Mass moment of inertia (g × cm2) | 10(1.52 × {log10 BM}] + [0.94 × {1 + log10 cruslength}] + [0.70 × {1 + log10 thigh length}] − [0.78 × {1 + log10 tarsal width)]) | 0.89 | 10.4 | 12,906 | |
| CCL-deficient | Mass (g) | −37.7 + (55.9 × BM) | 0.91 | 5.5 | 120.1 |
| COM (cm) | (−0.04 × BM) + (0.40 × crus length) + (0.09 × crus girth) | 1.00 | 5.4 | 0.21 | |
| Mass moment of inertia (g × cm2) | 9,609.1 + (3,458.7 × BM) − (15,158.0 × metatarsal width) | 0.96 | 6.1 | 5,356 | |
| Contralateral | Mass (g) | −1,773.2 + (18.7 × BM) + (31.2 × crus girth) + (48.9 × thigh girth) − (156.3 × tarsal width) + (179.3 × stifle width) | 0.99 | 1.0 | 30.5 |
| COM (cm) | (0.40 × foot length) + (0.22 × crus girth) | 1.00 | 1.6 | 0.19 | |
| Mass moment of inertia (g × cm2) | −66,727.5 + (2,404.8 × BM) + (1,641.6 × thigh girth) | 0.97 | 3.4 | 4,073 |
Body mass (BM) is expressed in kilograms, and segment lengths, widths, and girths are expressed in centimeters.
Regression equations generated from morphometric measurements and body mass to predict the mass, location of the COM, and mass moment of inertia of the crus in 28 normal, 10 CCL-deficient, and 10 contralateral hind limbs of Labrador Retrievers with or without unilateral CCL disease.
| Group | Variable | Regression equation* | R2 | Error (%) | SEE |
|---|---|---|---|---|---|
| Normal | Mass (g) | −750.0 + (36.1 × crus girth) + (95.5 × metatarsal width) | 0.90 | 5.7 | 34.6 |
| COM (cm) | (0.20 × crus length) + (0.09 × crus girth) | 0.99 | 5.5 | 0.36 | |
| Mass moment of inertia (g × cm2) | −68,619.6 + (2,773.4 × crus girth) × (705.8 × thigh girth) + (3,167.6 × foot length) + (4,104.6 × tarsal width) | 0.93 | 9.5 | 1,797 | |
| CCL-deficient | Mass (g) | −311.0 + (11.2 × BM) + (9.9 × thigh girth) | 0.93 | 4.2 | 30.5 |
| COM (cm) | 0.70 × foot length | 1.00 | 4.5 | 0.32 | |
| Mass moment of inertia (g × cm2) | −31,413.4+ (506.1 × BM) + (3,095.7 × foot length) | 0.93 | 7.5 | 1,546 | |
| Contralateral | Mass (g) | −374.6+ (15.6 × BM) + (37.3 × foot length) | 0.94 | 4.2 | 28.8 |
| COM (cm) | (0.20 × foot length) + (0.18 × crus girth) − (1.0 × metatarsal width) + (0.90 × tarsal width) | 1.00 | 5.6 | 0.13 | |
| Mass moment of inertia (g × cm2) | −48,810.8 + (654.7 × BM) +(3,393.9 × foot length) − (4,164.4 × metatarsal width) + (7,268.8 × tarsal width) | 0.96 | 6.2 | 1,322 |
See Table 3 for key.
Regression equations generated from morphometric measurements and body mass to predict the mass, location of the COM, and mass moment of inertia of the foot in 18 normal, 1 CCL-deficient, and 1 contralateral hind limb of Labrador Retrievers with or without unilateral CCL disease.
| Group | Variable | Regression equation* | R2 | Error (%) | SEE |
|---|---|---|---|---|---|
| Mass (g) | −74.5 + (2.1 × BM) + (11.9 × foot length) + (36.3 × tarsal width) | 0.86 | 3.1 | 8.8 | |
| COM (cm) | (0.70 × foot length) + (0.18 × crus girth) − (0.60 × metatarsal width) | 1.00 | 3.1 | 0.30 | |
| Mass moment of inertia (g × cm2) | −703.5+ (124.9 × BM) + (427.0 × crus length) − (217.2 × crus girth) | 0.88 | 4.7 | 408 |
See Table 3 for key.
Discussion
The study reported here had several results. The thigh and crus of CCL-deficient limbs weighed less than their matched contralateral segments. The thigh of CCL-deficient limbs also weighed less than that of normal limbs. The COM of the crus was located more proximally in CCL-deficient and contralateral limbs than in normal limbs. For the thigh, the mass moment of inertia standardized on the basis of body mass was lower in CCL-deficient limbs than in matched contralateral limbs. Morphometric measurements used in the regression equations to estimate BSPs (ie, mass, location of the COM, and mass moment of inertia) varied with parameter, body segment, and status of the hind limb.
The age of dogs with CCL disease in our study was consistent with the epidemiologic characteristic of this condition in large-breed dogs.40 The difference in age between clinically normal and CCL-deficient Labrador Retrievers in the study reported here was expected because the minimum age requirement for inclusion in the clinically normal group was based on the decreased risk for CCL disease reported41 in older Labrador Retrievers.
The thigh and crus segments weighed less in CCL-deficient limbs than in unaffected contralateral limbs. The mass of the thigh was also less in CCL-deficient limbs, compared with the mass of the thigh in normal limbs. These findings are consistent with muscle atrophy secondary to CCL disease.42 Although the duration of lameness before examination varied, most dogs were lame for at least 1 month; therefore, the results reported were likely to reflect discomfort and disuse of the CCL-deficient limb. The decreased mass of the thigh also explains the lower mass moment of inertia of the thigh in CCL-deficient limbs, compared with values for the contralateral limbs in dogs with CCL disease. The mass moment of inertia of a segment measures resistance to change in angular motion and correlates proportionally with the segment's distribution of mass with respect to the COM. Differences appeared more pronounced in the thigh than in the crus, which suggested that muscle atrophy secondary to CCL disease affects predominantly the muscles of the thigh, rather than those of the crus. Alternatively, the magnitude of changes in the crus may be limited by the amount of tissue naturally around the tibia, compared with the muscle mass around the femur. Mass of the crus and mass moment of inertia of the thigh differed significantly between CCL-deficient and contralateral limbs in dogs with CCL disease but not between CCL-deficient and normal limbs. This could have been attributable to the difference in age between our clinically normal and CCL-deficient dogs and a potential mild atrophy of the muscles in older dogs.43–45 However, dogs with CCL deficiency redistribute their weight so that the contralateral limb bears 87% of the body weight, compared with 66% in clinically normal dogs.46 Therefore, our results are more likely to reflect this compensatory activity, thereby magnifying differences between CCL-deficient and contralateral limbs in dogs with CCL disease.
The location of the COM of the thigh segment did not differ among groups. Therefore, muscle atrophy appears to be uniform and maintains the repartition of mass along the longitudinal axis of the thigh. The COM of the crus was located in a more proximal position in CCL-deficient and contralateral limbs than in normal limbs. This difference in repartition of the mass along the crus segment is unlikely to be solely attributable to CCL disease because contralateral limbs had the same distribution. These contralateral limbs can be considered as predisposed to CCL deficiency on the basis of the high incidence of bilateral and contralateral CCL disease in dogs.47–49 Further studies are warranted to evaluate whether a relatively greater proportion of bone or muscle (or both) in the proximal portion of the crus is a factor that predisposes dogs to CCL disease.
To our knowledge, the only publication of breedspecific morphometric data in dogs was based on musculoskeletal disease–free cadaveric specimens of 3 Labrador Retrievers and 4 Greyhounds.36 Unfortunately, age of those dogs was not reported. In the study reported here, BSPs of the crus and thigh were measured directly on cadavers by use of a digital scale and a balance board technique.1 These techniques were the first methods used to measure BSPs in humans and provide a reference to which new methods of measurements can be compared. Medical imaging techniques, such as CT,15,19 magnetic resonance imaging,18,20 DEXA,21,22 and G mass scanning,16 have subsequently been found to provide accurate measurements of BSPs in live human subjects. In our study, the crural mass (mean ± SD, 1.41 ± 0.16 %BM) measured by use of a noninvasive approach based on CT was similar to that measured directly in the cadaver study36 of musculoskeletal disease–free Labrador Retrievers (1.32 %BM). The mass of the thigh and mass moment of inertia of the segments have not been reported, which prevents us from comparing results. The COM of the crus (31 %L from the proximal joint) and thigh (42 %L) appear to be located slightly more proximally in the normal limbs in our study than in the limbs of Labrador Retrievers described in another study36 (38 %L for the crus and 48 %L for the thigh). The importance of this difference is difficult to evaluate because direct measurements in that other study36 were obtained only on 3 Labrador Retrievers, rather than on a representative sample of the population. The location of the COM is primarily affected by the geometry of the segment; however, the discrepancies between results of the study reported here and that other study36 may also be caused by inaccuracies associated with the measurement of mass. Two potential sources of error can lead to overestimation of the segment mass for advanced imaging techniques: overestimation of the segmental volume and use of tissue densities higher than actual values.18 Overestimation of the volume (3% with the CT method and 6.3% with magnetic resonance imaging18,50) has been attributed to the difficulty in discriminating the tissue perimeter.20 In addition, bone was assigned a constant density regardless of its relative content of cancellous and compact bone, which potentially could have caused an overestimation of the bone mass in our study. We would expect this overestimation to be magnified in areas where bone predominates over soft tissues, such as the crus. On the other hand, direct measurements in cadavers are affected by postmortem changes, which can lead to an estimated 5% to 6% loss in body fluids and thereby cause underestimation of the mass of segments.37
Although CT allows determination of BSPs in live subjects, its application to gait analysis in dogs remains limited by cost and the need for anesthesia of the subjects. Data developed in the study reported here indicated that mass, location of the COM, and mass moment of inertia for hind limb segments in Labrador Retrievers can be predicted on the basis of simple morphometric measurements of the body. Regression equations generated from direct measurements on cadavers have become the most common method used to estimate BSPs in humans.1,9,27,31,33 We applied a similar approach to generate regression equations on the basis of simple morphometric measurements in Labrador Retrievers. The mass of each segment has been expressed as a percentage of the body mass and the location of the COM as a percentage of the length of the segment.6 This approach can only provide a crude estimation of parameters and assumes that all subjects have similar body proportions and mass repartitions. This assumption is not valid for subjects outside the tested range. Instead, we found that in addition to body mass, several morphometric measurements (segment length, girth, or width) were generally required to generate equations that met validation criteria recommended in other reports.9,23,34,38,39 These equations have been validated for dogs with morphometric dimensions similar to those of our population but cannot be recommended outside the range of our study. One of the 2 main sources of uncertainty in inverse dynamics solution in humans has been attributed to inaccuracies in estimated BSPs.51 These inaccuracies are partially attributable to the fact that anthropometric tables were derived from a relatively small and biased population.52,53 Similar limitations should be expected in dogs and justify the determination of breed-specific morphometric data. In fact, BSPs varied with the status of the hind limb in the study reported here, which prevented breed-specific extrapolation of BSPs for clinically normal dogs to those of dogs with orthopedic disease. Accurate determination of morphometric parameters is especially critical to the study of swing-phase mechanics in which segmental angular accelerations are the largest, particularly for distal segments.54,j
To our knowledge, the study reported here represents the first time that mass moment of inertia in hind limb segments of live clinically normal Labrador Retrievers and BSPs in Labrador Retrievers with CCL disease have been described. Computed tomography allows noninvasive determination of BSPs on live subjects whose gait can be concurrently evaluated. The application of this approach in small animals is limited by availability of CT equipment, cost, and the need to anesthetize subjects. Estimating BSPs on the basis of simple morphometric measurements provides a basis for nonterminal studies of inverse dynamics of the hind limbs in Labrador Retrievers. Regression equations were generated to estimate the mass, location of the COM, and mass moment of inertia in each segment in normal, CCL-deficient, and contralateral limbs of Labrador Retrievers. These equations were based on parameters that are technically simple and cost-effective and can be rapidly generated. This approach will facilitate clinical studies of gait mechanics in dogs, which should offer new strategies to investigate the pathogenesis of nontraumatic joint diseases.
ABBREVIATIONS
| %BM | Percentage of body mass |
| %L | Percentage of length of the segment |
| 3-D | Three-dimensional |
| BSP | Body segment parameter |
| CCL | Cranial cruciate ligament |
| COM | Center of mass |
| CT | Computerized tomography |
| DEXA | Dual-energy x-ray absorptiometry |
| HSD | Honestly significant difference |
| SEE | Standard error of the estimate |
QDR 4500 fan beam x-ray bone densitometer, Hologic Inc, Bedford, Mass.
Human Dynamics and Controls Laboratory, College of Engineering, University of Illinois, Urbana, Ill.
VICON motion systems, Oxford Metrics Ltd, Oxford, England.
High-speed F/X helical scanner, General Electric, Milwaukee, Wis.
Amira 4, Mercurey Computer System Inc, Chelmsford, Mass.
Pro/ENGINEER, Wildfire, version 3.0, Parametric Technology Corp, Needham, Mass.
StatXact, version 9, Cytel Software, Cambridge, Mass.
Systat, version 12, Systat Inc, Richmond, Calif.
SAS, version 9.1, SAS Institute Inc, Cary, NC.
Colborne GR, Shellard LJ, Morris KD. Sensitivity of kinetic gait analysis to accuracy of morphometric variables (abstr), in Proceedings. 50th Cong Br Small Anim Vet Assoc 2007:86.
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