There remains considerable debate about the most appropriate method for treatment of the canine stifle afflicted by cranial cruciate ligament (CCL) deficiency.
The models proposed by Slocum [1], based on the tibial compression test, and by Tepic [2] focus primarily on either the gastrocnemius or quadriceps muscles as drivers of stifle (in)stability. Since treatments (TPLO, TTA) based on these theories seem clinically efficacious, the truth probably lies somewhere in the middle. Fluoroscopic studies of patients operated lege artis with both TPLO and TTA have revealed ongoing dynamic instability despite good clinical results, in contrast with existing ex vivo models which suggest the joint should be stable [3,4]. Further evidence of clinical instability has been found radiographically for both TTA and TPLO [5,6].
During weight bearing, the quadriceps serves primarily to prevent uncontrolled flexion of the stifle, and the gastrocnemius does the same at the hock. Looking at stifle craniocaudal stability alone, the quadriceps and gastrocnemius appear to have antagonistic roles. Both muscle groups cross two joints, and other muscles (e.g. hamstrings) are also involved in a complex balance of co-contraction. In vivo canine quadriceps and gastrocnemius forces during stance have not been reported: limited and insufficient model-derived data exist on muscle forces in the canine limb during stance [7]. Modelling a combination of gastrocnemius and quadriceps forces in the stifle therefore presents some challenges.
Models investigating stifle stability with cranial cruciate ligament deficiency and TTA or TPLO generally are static (single fixed angle) and use adjustable linkage systems to replace the quadriceps +/- gastrocnemius muscles [3,4,8–11]. A force applied to the proximal femur causes the linkage systems to generate a load on the stifle. The magnitudes and ratios of these simulated muscle forces are unknown. Multi-angle evaluations have been performed by two groups [12,13].
We developed an ex vivo model designed to test the effect of triple tibial osteotomy (TTO) on cranial cruciate and medial meniscus deficient stifle stability dynamically throughout a range of stifle angles. We made some rough estimates of limb loading during transition from standing to sitting and found an area under the curve for the ratio of forces of 1:3.3 for gastrocnemius:quadriceps force. Empirically, gastrocnemius loads in excess of this produced consistent cranial subluxation of the tibia in cruciate deficient stifles even in full flexion, when caudal cruciate ligament loading is expected. Once the tibia subluxated, increasing the quadriceps force did not produce reduction: only complete unloading was sufficient.
Nine hind limb preparations were used from dogs with body mass of 30.6kg ±2.6kg, and with initial tibial plateau angles (TPA) and patellar tendon angles (PTA) of 25.5°±2.2° and 113.9°±2.6°, respectively. Steel beads were implanted to enable measurement of cranial cruciate ligament attachment site separation (ASS). Following assessment of the effects of CCL section and meniscal release [14], stifles underwent TTO by removing a wedge of (PTA-90°)*0.6+7.3°. Resultant TPA and PTA were 7.2°±3.0° and 96.2°±4.7°, respectively, consistent with published values from patients with good clinical outcomes [15,16].
Mean TTO-stabilized ASS values did not exceed intact ASS values until a caudal joint angle of approximately 135° (maximum stability angle or MSA). Grouping the stifles by PTA above or below median PTA produced groups with mean PTA of 99.6°±2.8° and 92.7°±4.3°, with MSAs of 125° and 145° respectively, a 20° difference. Alternatively, grouping by TPA magnitude produced groups with mean TPA of 9.6°±2.6° and 4.9°±1.4°, with MSAs of 135° and 130° respectively, only a 5° difference.
This model reflected real-life experience in terms of objective instability in about half of the stifles despite ‘appropriate’ correction, in contrast to the majority of published models, which could be due to the gastrocnemius:quadriceps force ratio used. Although the results suggest a primary role for PTA in explaining stifle stability, achievement of a PTA of 90° did not appear necessary to achieve MSA targets in ca. half of these stifles. With refinement, this model might be a step towards a more realistic ex vivo testing platform. Key steps include: acquisition of in vivo data regarding contraction forces in the quadriceps, gastrocnemius, and the hamstring, muscles; preservation of the gastrocnemius origin with use of a tendon-attached traction device; improvement in the axial loading system.
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