Objectives
Patient selection for distal femoral corrective osteotomy as a treatment for patellar luxation currently depends on measurement of varus/valgus deformity. An alternative consideration could be alignment of the tibial tuberosity with the trochlea using the trochlear groove-tibial tuberosity distance (TT-TG). A novel approach to determining this alignment from computed tomography scans without prior multiplanar reconstruction was tested for repeatability in two configurations. Trochlear groove frontal plane curvature was assessed.
Methods
Twelve computed tomography scans of canine stifles clinically unaffected by disease were imported into ImageJ. A caudal condylar axis and cranial trochlear ridge axis were identified, as well as a diaphyseal landmark to define a plane. The trochlear sulcus depths proximally and distally were identified in relation to each axis in turn, to define a second orthogonal plane. The distance from the tibial tuberosity to this second plane was calculated as the TT-TG. Repeatability was estimated using within-subject standard deviations. The angle between the proximal, middle, and distal trochlear landmarks was calculated.
Results
Mean TT-TG was 2.3mm laterally for the condylar axis and 1.5 mm laterally for the trochlear axis, with repeatability of 0.4 mm (95% confidence interval 0.2; 0.6) and 0.6 mm (95% confidence interval 0.3; 0.8), respectively. Trochlear curvature was 171° ±7° (condylar axis) and 175° ±5° (trochlear axis), with slight lateral deviation of the mid-trochlea. Curvature repeatability was 2.1° (95% confidence interval 1.2; 3.0) and 3.3 (95% confidence interval 1.8; 4.7), respectively.
Impact
The caudal condylar axis appears slightly more reliable for measuring TT-TG. The trochlea is naturally curved.