Comparison of Computed Tomographic and Standard Radiographic Determination of Tibial Torsion in the Dog

INTRODUCTION

TIBIAL TORSION (TT) describes the angular relationship of the proximal articular axis relative to the distal articular axis of the tibia about its longitudinal axis.¹ TT beyond the normal, physiologic range because of torsional deformities^2,3 or malalignment after fracture repair^4,5 has been reported to cause gait alterations,⁶ as well as stifle joint pathology in humans.^7–10

Different methods have been developed for assessment of TT in humans, such as direct goniometric measurement,¹¹ and several radiologic techniques, e.g., fluoroscopy,¹² ultrasound,¹³ computed tomography,^14–16 magnetic resonance imaging,¹⁷ or computer-assisted quantification of periaxial bone rotation.¹⁸ In veterinary medicine, a radiographic method using conventional radiographs to evaluate TT has been reported.¹⁹

Disadvantages of conventional radiographic techniques are the projection of a complex three-dimensional structure in a two-dimensional plane resulting in superimposition of the anatomic structures used in tibial axis determination. In addition, the rotational position of the proximal aspect of the tibia is evaluated using femoral landmarks. True caudocranial position of the femur, however, may not necessarily guarantee true caudocranial positioning of the tibia. Suboptimal radiographic positioning, because of the natural recurvation of the femur, or rotation about the stifle joint, which may be enhanced by cranial cruciate ligament (CCL) deficiency, can create a radiographically perceived torsional deformity, or can reduce the apparent magnitude of an actual deformity. Accurate assessment of TT is necessary before corrective osteotomy, such as tibial plateau leveling osteotomy (TPLO), because surgical correction of torsional deformities and inadvertent torsional malalignment are possible with this procedure. Thus, accurate assessment is also required to evaluate the effectiveness of such procedures postoperatively.

Our purpose was to compare the standard radiographic technique with computed tomographic (CT) assessment of TT in the dog, based on the hypothesis that CT assessment was more precise than standard radiographic technique, and would not be affected by minor rotational malpositioning of the tibia.

MATERIALS AND METHODS

Six paired (n=12) cadaveric hind limbs from mature dogs of grossly normal conformation, visually assessed to be free of orthopedic disease of the stifle, were prepared by removal of all soft tissues proximal to the stifle with careful preservation of the stifle joint capsule and collateral ligaments. The CCL was transected through a lateral arthrotomy to provide a model of a clinically affected CCL deficient stifle. The paired legs were positioned in a custom made holding device (Fig 1A), which rigidly fixed the femur, and allowed quantifiable, independent rotation of the tibia. The angle of the stifle joint was held at 132°; this angle was determined based on the measurement of a number of clinical cases (n=6; mean, 131.9°) during routine caudocranial TPLO radiography. The tibiae were aligned parallel to the base of the holding device (Fig 1B).

Imaging

Caudocranial radiographs of both limbs were obtained (Prestige II, GE Medsystems, Milwaukee, WI) and true caudocranial alignment of the femur was confirmed as previously described by Slocum, with the patella centered in the trochlear groove of the femur, and the fabellae bisected by the distal femoral cortices. Each tibia was positioned in a neutral position, such that the medial aspect of the calcaneus was aligned with the base of the sulcus of the talus (Fig 2A). Without altering the position of the limbs within the holding device, transverse, 1 mm thick, contiguous CT (PQS Spiral CT scanner, Picker International, Highland Heights, OH, 130 kV, 125 mA, matrix size 512 × 512, FOV 100, bone algorithm) slices were obtained from the proximal 2 cm of all tibiae including at least 4–6 slices of the distal femur, and from the distal 2 cm of all tibiae including at least 3–4 slices of the proximal aspect of the tarsus (Fig 3). The tibiae were internally rotated 15° about the stifle and fixed in this position within the holding device; this magnitude was chosen because this is the degree of internal rotational instability in the normal, extended canine stifle.²⁰ Caudocranial radiographic images (Fig 2B) and transverse CT images were repeated. The radiographic images were digitized on a flatbed scanner (Epson Perfection^®1660 Photo, Epson America, Inc., Long Beach, CA) and measurements were made using commercially available software (Adobe Photoshop^®, Version 7.0, Adobe Systems, Inc., San Jose, CA). According to the guidelines of TT assessment for TPLO (Seminar titled: Tibial plateau leveling osteotomy for cranial cruciate ligament repair; Slocum Enterprises, Eugene, OR), the distance (d) between the medial border of the calcaneus at the level of the talocrural joint and the base of the sulcus of the talus was identified and measured (Fig 2). CT determination of TT was based on the angular relationship of proximal and distal axes drawn between reference points. The CT slice within which the specific anatomic landmarks that define each tibial axis could most clearly be identified was chosen for analysis. The proximal transcondylar (TC) axis was defined as the line connecting the caudolateral extent of the extensor sulcus to the prominence at the medial collateral ligament insertion. The distal cranial tibial (CnT) axis was defined as a line parallel to the cranial tibial cortex immediately proximal to the talocrural joint (Fig 3). The axis angle was determined by calculating the angular orientation of these axes with the vertical plane, using commercially available software (eFilm Workstation™, Version 1.8.3, Merge eFilm, Milwaukee, WI). The torsion angle, expressed in degrees, was calculated by determining the difference between the axis angles, as described by Aper et al.²¹ Deviation from parallelism was described using positive and negative values. For example, a clockwise offset of the distal axis in a left tibia was assigned a negative value.

RESULTS

The mean distance (d) for the neutral radiographic group (−0.19±0.90 mm) was not significantly different from the hypothetical mean value of 0 mm (P=.473); however, the mean distance (d) for the 15° radiographic group (6.92±0.94 mm) was significantly different from 0 mm (P<.0001; Fig 4). The mean difference between the initial and 15° CT determination of TT (−0.583±1.93°) did not differ from the hypothetical mean value of 0° (P=.317).

DISCUSSION

In human medicine, various CT techniques for determination of TT have been developed, based on identification of reference axes on transverse images of the proximal and distal tibia, femur, or fibula. These techniques differ primarily with regard to the anatomic landmarks and therefore position of the axes. One of the reasons for this may be the rounded form of the human tibial bone and the lack of obvious anatomic ridges.¹⁵ A commonly used and reproducible proximal reference axis is a tangent drawn on the dorsal border of the tibia just below the joint space and above the fibular head. However, variations in distal reference point determination exist, including using the lateral malleolus of the fibula for the trans- or bimalleolar axis. This point may not accurately reflect the distal tibial axis, because of the influence of the position of the fibula.²² Jend et al¹⁴ developed a method where the distal reference line is formed by joining the center of a circle fitted to the pilon tibiale including the fibular notch, with the midpoint of a line across the fibular notch of the tibia. Waidelich et al²³ assess TT with a distal reference axis defined by a line joining the midpoint of an ellipse fitted to the base of the medial malleolus and the midpoint of a line across the incisura fibularis. Nevertheless, nonconformity of axis determination contributes to variability in normal torsion values and indicates that there is no consensus in the most reliable, accurate, or reproducible method or anatomic landmarks.

Canine tibial anatomy offers more pronounced reference points for definition of proximal and distal tibial axes. Aper et al²¹ demonstrated correlation between direct anatomic measurements, and CT measurement of 2 proximal and 2 distal tibial axes in canine cadaveric tibial bones. Using 2 of these reported axes, TT was measured in this study, and the CT method was compared with the standard radiographic method. Selection of the proximal TC axis and the distal CnT axis was based on the investigators ability to identify the specific anatomic landmarks that define these axes in all of the tibiae studied.

The standard radiographic technique described by Slocum is based on the determination of TT as a linear measurement, whereas the CT method assigns an angular value; therefore these 2 methods cannot be compared directly. For this reason, the results of each method, in a neutral tibial position were compared with the results after 15° internal rotation. Because the real magnitude of TT was unchanged by tibial rotation, significant changes in perceived TT would be artifactual and caused by positioning. The standard radiographic technique relies on the inherent stability of the stifle in extension to minimize tibial rotation. In practice, it is difficult to achieve complete extension of the stifle during positioning for the caudocranial radiographic projection, and for this reason, a stifle joint angle comparable with that obtained clinically was used in our study. Rotational malpositioning of the tibia at the stifle may occur during routine stifle radiography, and because rupture of the CCL results in an increase in range of motion in internal rotation,²⁰ this may increase the likelihood of malpositioning, and result in inaccurate evaluation of TT. The linear measurements obtained with the standard technique demonstrated a significant increase after internal tibial rotation about the stifle. This finding demonstrates that this method is susceptible to rotational artifacts, resulting in inaccurate determination of TT.

In contrast, CT assessment of TT was not affected by 15° internal rotation of the tibia about its long axis. This finding demonstrates that the CT method of TT measurement is unaffected by internal tibial rotation of up to 15°, unlike the standard radiographic method. Interestingly, a study using human cadaveric tibiae demonstrated that angled CT slice orientation, with respect to the long axis of the tibia did not have an effect on torsional assessment.¹⁶

A variety of clinical problems have been associated with torsional malformation of the tibia in both human and veterinary patients, such as patellar instability,^24–26 stifle osteoarthritis,¹³ and Osgood–Schlatter's disease.²⁷ The magnitude of TT varies widely between individuals, and more detailed information regarding physiologic versus pathologic magnitudes of TT in veterinary patients is required. Clinical studies focused on breed specific TT values and inter-observer differences are necessary to define developmental or acquired malformation of the tibia. This information would be particularly helpful if corrective tibial osteotomy is to be performed.

The standard radiographic technique is adequate to determine malalignment of limb axes in the frontal and sagittal plane and can be a helpful in screening for TT. However, if accurate assessment of TT is required, CT determination is indicated, as the CT method is unaffected by minor malpositioning.