Tibial Plateau Symmetry and the Effect of Osteophytosis on Tibial Plateau Angle Measurements
A novel technique was developed to estimate the caudal medial tibial plateau landmark in the face of osteophytosis to improve accuracy in tibial plateau angle measurements. Using this technique, tibial plateau angles were evaluated in 31 normal dogs before and 8 months after right cranial cruciate ligament transection. There was no significant difference in mean tibial plateau angle before or after induction of osteophytosis. Additionally, it was determined that 90% of dogs had a difference of =2° between right and left tibial plateau angles, which was considered symmetrical.
Introduction
The tibial plateau leveling osteotomy (TPLO) was developed to dynamically stabilize the cranial cruciate ligament (CCL)-deficient stifle during the weight-bearing phase of ambulation. This stabilization is accomplished by decreasing the tibial plateau slope, which directs the tibial thrust from a cranial to caudal direction.1–4 The clinical recommendation of a 5° postoperative tibial plateau angle is consistent with the experimentally established, optimal postoperative tibial plateau angle of 6.5°, which achieves cranial stability and minimizes stress on the caudal cruciate ligament.4,5
Determination of the tibial plateau angle is critical prior to performing a TPLO.2,4 Although under-rotation of the osteotomized tibial plateau may result in continued cranial instability and cranial tibial translation, over-rotation results in increased stress on the caudal cruciate ligament and caudal tibial translation.1–3,6 An accurate preoperative measurement of the tibial plateau angle is imperative in determining the appropriate magnitude of the tibial plateau rotation.
To achieve consistency in tibial plateau angle measurement, anatomical landmarks must be identified. The tibial plateau landmarks are the cranial medial aspect of the CCL insertion site (the most cranial point of the medial tibial plateau) and the most caudal aspect of the medial tibial plateau adjacent to the insertion of the caudal cruciate ligament.3,4,7 The landmarks of the functional tibial axis on a lateral radiograph centered on the stifle (including the tibiotarsal joint) are the intersection point of the medial and lateral tibial intercondylar eminences and the middle of the talus.1,3,4 Accurate and consistent identification of these landmarks is paramount to reliably determining preoperative and postoperative tibial plateau angle.
Accurate tibial plateau angle measurements can be difficult to obtain in clinical cases because of osteoarthritis, which commonly occurs following CCL rupture.8 In particular, the presence of osteophytosis in the stifle at the level of the proximal tibia can obscure anatomical landmarks, making it a challenge to preoperatively determine the tibial plateau angle.4,8 A clinical study evaluating tibial plateau angle variability in dogs with CCL rupture showed that osteoarthritis at the level of the caudal tibial plateau was the only factor that significantly affected tibial plateau angle measurements between observers.8 Similarly, greater variability in tibial plateau angle was present when using a line tangential to the medial tibial plateau instead of using the anatomical landmarks described above.4
Inaccurate tibial plateau angle measurements resulting from tibial plateau osteophytosis could lead to inappropriate tibial plateau rotation and subsequent clinical failure.1,4,9 Dogs with CCL rupture and severe tibial plateau osteophytosis may have no or minimal osteophytosis in the contralateral tibial plateau. Assuming anatomical consistency between the right and left tibial plateaus, the measurement of the contralateral tibial plateau angle could be used instead of the measurement of the affected stifle.
Tibial plateau symmetry in dogs has not been defined. Although several studies have evaluated the mean tibial plateau angle between right and left stifles and found no statistical differences, standard deviations were not reported, thus limiting the usefulness of this information within individual animals.9,10 No study to date has evaluated the tibial plateau angle symmetry or the ability to reliably predict the contralateral tibial plateau angle in dogs. A human study evaluating radiographic tibial plateau slope measurements using both knees in normal individuals concluded that within an individual, tibial plateau slope measurement of one knee was unreliable in predicting tibial plateau slope of the contralateral knee; however, clinical experience has shown a high degree of similarity between tibial plateau angles of the right and left stifles in dogs.11
The purpose of this study was to evaluate the effect of osteophytosis on the measured tibial plateau angle in mongrel dogs, as well as to determine tibial plateau angle consistency between the right and left stifles. The null hypotheses were that no differences exist: (1) between the right and left tibial plateau angles in normal stifles; (2) in tibial plateau angles before and after induction of tibial plateau osteophytosis; (3) between tibial plateau angles after induction of osteophytosis and the normal contralateral tibial plateau angle; and that (4) the changes in tibial plateau angle values before and 8 months after CCL transection are not related to the severity of osteophytosis.
Materials and Methods
This study was part of a comprehensive drug evaluation and was conducted under a protocol approved by the Institutional Animal Use and Care Committee. Animals were obtained through the University Laboratory Animal Resources facility at Michigan State University. Thirty-one mongrel dogs that were free of pelvic limb orthopedic disease according to radiographic and physical examinations were included in the study.
Radiographic Procedure
Standard mediolateral and craniocaudal radiographs of the right and left stifles at time 0 and of the right stifle 8 months after CCL transection were taken under acepromazine maleatea (0.04 mg/kg intravenously [IV]) and butorphanol tartrateb (0.2 to 0.4 mg/kg IV). The radiographic beam was centered on the joint space between the femoral and tibial condyles and included the tibiotarsal joint. The stifle and tibiotarsal joint were positioned at approximately 90° of flexion for the radiographs. Lateral radiographs were accepted once superimposition of the tibial condyles was achieved.4
Surgical Procedure
The right stifle was arbitrarily chosen to undergo an arthroscopic modification of the Pond Nuki model for CCL transection to induce osteoarthritis.12,13 Thorough exploration of the intra-articular structures was conducted in a routine manner to confirm absence of joint pathology. The CCL was sharply transected at its midpoint using a specialized forward-cutting bladec [Figures 1A, 1B, 1C]. Complete transection of the CCL was ascertained by intra-operative visualization and confirmed by direct and indirect cranial drawer examinations during surgery. Each animal was then housed in a 4 × 12-ft run for the duration of the study and allowed unrestricted activity. No bandage was applied postoperatively.
Radiographic Evaluation
Tibial plateau angles were measured to the nearest 0.5° using landmarks previously described [Figure 2].3 For cases with severe tibial plateau osteophytosis, a technique was developed that estimated the caudal medial tibial plateau landmark. This technique, referred to as the extension technique, was used to determine the caudal medial tibial plateau landmark by drawing a distal to proximal line along the caudal cortex of the medial tibial ridge, just beyond the medial articular surface. A second line was then drawn following the articular surface of the medial tibial plateau, extending caudally until it intersected with the first line [Figure 3]. The intersection of those lines represented the landmark for the medial caudal tibial plateau. The cranial tibial plateau landmark was determined by identifying the most cranial aspect of the articular surface of the medial plateau. This landmark was identifiable even in the presence of osteophytosis. Tibial plateau angles were measured in one sitting and were done in random order with respect to time and sides. An extra fine-point permanent markerd was used to identify landmarks on acetate sheets overlying the radiograph. A board-certified radiologist (masked to the tibial plateau angle measurements) evaluated the lateral radiographs in order to grade tibial plateau osteophytosis. Osteophytosis scores were assigned to the cranial and caudal medial tibial plateau regions, based on osteophyte severity. Osteophyte scores were 0=none, 1=mild, 2=moderate, and 3=severe [Figures 4A, 4B, 4C, 4D, respectively].14–16 The scores for the cranial and caudal tibial plateaus were combined for a maximum score of 6.
Statistical Analysis
Results are reported as means ± standard deviations. Comparisons between two sets of angles were done with a paired t-test. Linear correlation was used to test for an association between osteophytosis scores and changes in right tibial plateau angles over time. Statistical significance was defined as a nominal type I error rate of <5% (P<0.05). Statistical power calculations were made with a commercial software package.e
Results
The 31 mongrel dogs used in the study ranged in weight from 26.6 to 36.0 kg (mean 30.4±2.6 kg). All dogs were radiographically skeletally mature and <5 years of age. Twenty-three dogs were male, and eight were female.
At time 0, no significant differences were found between the right tibial plateau angle (24.6°±1.9°, range 20° to 30°) and the left tibial plateau angle (24.3°±2.3°, range 18° to 31°) (P=0.25). The mean difference between the right and left tibial plateau angles at time 0 was 0.3°±1.8° (range 0° to 4.5°), and 90% of the dogs had an absolute difference of =2° between the right and left sides [Figure 5].
The mean right tibial plateau angle at 8 months postoperatively (25.1°±2.4°, range 17.5° to 29.5°) was statistically similar to the right (P=0.36) and left (P=0.17) preoperative tibial plateau angles. The mean right tibial plateau angle change over the 8 months after surgery was 0.5°±2.8° (range 0° to 6.5°).
Severity of osteophytosis did not have a significant effect on the differences between tibial plateau angles. Tibial plateau angle differences between time 0 and 8 months were not related linearly to osteophytosis severity based on either a composite score (P=0.86) or separate cranial and caudal location scores (P=0.34, P=0.24, respectively). The mean osteophyte composite score was 0.0 prior to CCL transection and was 3.97±1.12 at 8 months after CCL transection. Seven dogs had severe osteophytosis (composite score 5 to 6); 20 dogs had moderate osteophytosis (composite score 3 to 4), and four dogs had mild osteophytosis (composite score 1 to 2). The mean caudal tibial plateau osteophytosis score (2.2±0.79) was significantly higher than the mean osteophytosis score of the cranial tibial plateau (1.7±0.75, P=0.01) at 8 months after CCL transection.
In this study, no statistically significant differences were found between the right and left tibial plateau angles preoperatively and the right tibial plateau angle before and after osteophyte induction. Statistical power analysis revealed that 31 animals was an adequate number to identify a significant time-related difference of =1.5°, with a type II error rate of <0.2 (power =0.8).
Discussion
Accurate determination of the tibial plateau angle is critical prior to and after performance of a TPLO. Osteophytosis present on the proximal tibia can make accurate landmark identification difficult, because normal anatomical structures can be obscured. This may lead to an inaccurate tibial plateau angle measurement, with subsequent inappropriate tibial plateau rotation.4,8 Induction of tibial plateau osteophytosis 8 months after CCL transection did not significantly alter the tibial plateau angle measurement compared to preoperative values in this group of mongrel dogs. This finding was unexpected in animals with severe osteophytosis because of the difficulty in identifying proper radiographic landmarks to determine tibial plateau slope. Fettig et al. described significant variability associated with osteophytosis on the caudal tibial plateau landmark.8,9
Results of the study reported here may be related to use of the extension technique. This technique was developed to provide a consistent estimate of the medial caudal tibial plateau landmark, which can be completely obscured in cases of severe osteophytosis. The extension technique proved to be very useful in these dogs. The osteophytosis present in the region of the cranial tibial landmark did not significantly alter identification of this point. These osteophytes likely originated at the insertion of the CCL, which is typically distal and cranial to the radiographic landmark.17 The cranial tibial plateau region was also significantly less affected with osteophytosis compared to the caudal tibial plateau region, which facilitated landmark identification. Previous studies have demonstrated consistent osteophytosis production in the region of joint capsule attachment, which could explain the increase in caudal versus cranial tibial plateau osteophytosis.18,19 The difference in severity of osteophytosis may also have resulted from the body’s response to an amplified stress experienced at the caudal tibial plateau during weight bearing while the tibia was subluxated cranially.
Although osteophytosis did not significantly alter tibial plateau angle measurements in this study, the standard deviation of angle differences increased after induction of osteophytosis. That increase was likely a reflection of variability in landmark identification resulting from osteophytosis deposition that obscured normal anatomy.
The ability to use the radiographic tibial plateau angle of a normal contralateral stifle may be beneficial in situations where severe osteophytosis decreases the ability to achieve an accurate angle measurement. While previous studies have compared tibial plateau angles in both right and left stifles and found no statistical difference in the mean of those angles (i.e., no reported standard deviation), symmetry was not defined on an individual basis.9,10,20 Symmetry could be defined as a difference in tibial plateau angle of =2°. The 2° threshold for symmetry was based upon the precision of the measurement technique (i.e., marker tip size, straight edge and protractor placement). Tibial plateau angle measurement variability based on two points can be geometrically calculated using the mean diameter of the identification mark and the distance between these points. As an example, by connecting the top of one mark to the bottom of the other mark on the medial tibial plateau, a difference of nearly 2° is generated in the measurement of the tibial plateau angle. In contrast, considering the distance between the tibial axis landmarks, the potential error in the tibial plateau angle measurement would only be approximately 0.2°. Accordingly, a difference of 2° could be introduced to the tibial plateau angle simply as a result of the measurement technique, even with exact landmark identification.
In a radiographic study evaluating symmetry of the human tibial plateau slope in 33 normal individuals, it was found that regardless of radiographic technique, the majority (55%) of patients had a tibial plateau slope difference of >3° between knees.11 Considering that the human plateau slope is approximately 10°, this small discrepancy corresponds to a 30% difference in tibial plateau slope between knees.11,21–24 That study concluded that the tibial plateau slope in a healthy knee in any given patient should not be used to predict the contralateral tibial plateau slope.11,21–24 Conversely, in a canine kinematics study, symmetry was defined as up to an 8% difference in peak vertical force.25 The average tibial plateau angle of dogs in this study was 24.4°, which was similar to previously reported results.4,7,9,26,27 Interestingly, 2° also represents an 8% difference between tibial plateau angles, which further supports the concept that symmetry in tibial plateau angles could be considered =2°.
When comparing right and left tibial plateau angles prior to induction of osteophytosis, subtle differences were noted between angles in individual dogs. Of these differences, 90% were =2°; 10% of the dogs had a difference of 2° to 4.5° [Figure 5]. Differences >2° could be the result of true asymmetry between stifles, but they were more likely caused by a combination of measurement technique imprecision, landmark placement error, and performing the landmark identification in a masked fashion (unable to compare landmark location). Interestingly, based on Slocum’s TPLO rotation chartf for a 30-mm saw blade, a 1-mm error in placement of the osteotomy alignment marks would result in a 2° discrepancy between theoretical and actual tibial plateau correction. This error is similar to the difference between the right and left tibial plateau angles that was seen in the current study.
Considering the numerous sources of error when measuring tibial plateau angle (e.g., radiographic and measurement technique, osteophytosis, tibial axis shift after TPLO correction, and geometric arc calculation), it seems that consistent rather than accurate landmark identification may be more important in the evaluation of a postoperative TPLO.4,11,28 Based on the study reported here, using the extension technique may help to reduce variability in tibial plateau angle measurement in cases of severe osteophytosis, and symmetry between the two stifles may allow use of the contralateral tibial plateau angle (if there is minimal or no osteophytosis) for preoperative planning.
Conclusion
Osteophytosis did not significantly alter the measured angle of the tibial plateau when using the extension technique in 31 normal mongrel dogs. Tibial plateau angle symmetry (i.e., a difference of =2°) was present in 90% of dogs. Based on symmetry, the unaffected limb can be used as a guide to predict the angle of the affected limb in cases where landmark identification is difficult because of severe osteophytosis.
Acepromazine maleate; Boehringer Ingelheim, St. Joseph, MO 645506
Butorphanol tartrate; Fort Dodge Animal Health, Fort Dodge, IA 50501
3.0-mm Straight, V-Shape Meniscotome; Smith & Nephew, Inc., Andover, MA 01810
Sharpie Extra Fine Point Permanent Marker; Newell, Bellwood, IL 60104
PASS software; NCSS, Kaysville, UT 84037
Slocum TPLO Rotation Chart; Slocum Enterprises, Inc., Eugene, OR 97404
Citation: Journal of the American Animal Hospital Association 43, 2; 10.5326/0430093
Citation: Journal of the American Animal Hospital Association 43, 2; 10.5326/0430093
Citation: Journal of the American Animal Hospital Association 43, 2; 10.5326/0430093
Citation: Journal of the American Animal Hospital Association 43, 2; 10.5326/0430093
Citation: Journal of the American Animal Hospital Association 43, 2; 10.5326/0430093
Arthroscopic views of the intact cranial cruciate ligament of the right stifle in a 2-year-old, 30-kg, male dog prior to transection (A), during transection (B), and after complete transection (C). Note the craniomedial (1) and caudolateral (2) bands of the cranial cruciate ligament, as well as the caudal cruciate ligament (3), the caudal meniscotibial ligament of the medial meniscus (4), and the medial femoral condyle (5).
Right lateral tibial radiograph of a 2-year-old, 28-kg, male dog illustrating the position of the cranial and caudal tibial plateau landmarks used to determine the tibial plateau slope. The functional tibial axis is represented by the line joining a point midway between the tibial inter-condylar eminences and the center of the talus. The tibial plateau angle is the angle between the tibial plateau slope and a line drawn perpendicular to the functional tibial axis.
Illustration of the extension technique used to estimate the caudal landmark on the medial tibial plateau. This point is located at the intersection (white circle and dot) of a line drawn from the caudomedial tibial cortex, extending proximally beyond the articular surface (vertical curvilinear black line), and a line drawn from the articular surface of the medial tibial plateau and extending beyond the caudomedial tibial cortex (horizontal curvilinear black line).
Lateral radiographs of the right stifle of four different adult dogs, taken at 2 weeks (A) and 8 months (B, C, D) after cranial cruciate ligament transection, illustrating the osteophytosis grading scheme. The white circles in Figure 4A represent the osteophytosis grading locations on the cranial and caudal tibial plateaus. Osteophytosis of the two regions were scored as (A) none (0/0), (B) mild (1/1), (C) moderate (2/2), and (D) severe (3/3). A cumulative score ranging from 0 to 6 was used for statistical comparisons. For simplicity, the radiographs presented here show identical cranial and caudal scores. However, the majority of dogs had different scores for these two locations, with the caudal scores significantly higher (P=0.01).
Graph illustrating the probability of finding any given difference in tibial plateau angle between the right and left stifles prior to cranial cruciate ligament transection in a group of 31 normal mongrel dogs. These data indicate a high probability (88%) of finding a difference of =2° (intersection of dashed lines). TPA=tibial plateau angle.
