Before we go ahead with discussing the rationale and so forth regarding Trochlear Prosthesis I thought it might be helpful to discuss its application within a specific case.
Patient Details
Two year old Boston Terrier Female. History was of progressive hindlimb lameness towards the end of exercise sessions with intermittent skipping noted when moving at the walk and trot. The problem had progressed in the previous one month to complete non weight bearing lameness on return from even light exercise. The patient had shown depression, believed to be associated with discomfort on evenings after exercise. Current medical management was in the form of Meloxicam given once daily. The impact of the medication on the clinical problem had been minimal in terms of lameness or demeanour.
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Gait evaluation and Physical Findings
The patient showed a tendency to skip when moving at any speed above a slow walk. The foot was held in a non weight bearing position for three to four steps before being returned to the weigh bearing position. During load phase, the limb was under loaded.
On physical exam there was normal hip range of motion. There was apprehension on examination of the stifle and the patella was noted to spontaneously adopt a medial position and required manual pressure to relocate to the trochlea. Some discomfort was noted during this process. No cranial drawer nor postural thrust was noted on exam. The upper limb muscle mass was reduced compared to the contra lateral limb. No orthopaedic disease was noted in the structures distal to the stifle. Medial patellar luxation was also noted in the contralateral stifle although the patella would spontaneously return to the Trochlea in this stifle.
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Diagnosis
Grade 2 Medial Patellar Luxation with suspected retro patellar cartilage loss.
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Plan
Admit for radiographic assessment, palpation under anaesthesia and surgical management by Trochlear Prosthesis.
Radiography
Mediolateral projection showing patellar malalignment and mild effusion. Looking at the fact that the Tibial Tuberosity is clearly shown whilst the Talocrural joint is not in the mediolateral plane, proximal Tibial Torsion was present.
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Caudocranial Projection, showing the patella in the medial luxated position.
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Caudocranial Projection with lines drawn to indicate the Quadriceps Angle (Q-Angle) of 33 degrees.
Surgical Findings
A standard lateral para patellar incision extending proximal to the patella was performed. Both Cruciate ligaments were inspected and noted to be intact as were the menisci. The patellar surface was examined by applying Allis Tissue forceps proximal and distal to the patella and rolling the patella. This revealed central cartilage thinning with no clear full thickness lesion:
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The medial trochlear ridge was of reasonable height but there was clear femoral torsional deformity resulting in the Trochlea facing the lateral aspect of the femur. The medial trochlear ridge was also deficient proximally as can be seen in the second image below.
You can also see that there has been cartilage loss on the medial side of the medial trochlear ridge. The feeling then was that the patella was escaping proximally and following the straightest path for the quadriceps-patella-tibial alignment. This information would be used in considering where to position the prosthesis in the mediolateral and proximodostal plane. On this basis the measured size 2 prosthesis was considered too small to prevent proximomedial escape and thus the size 3 was chosen. The prosthesis would also need to be placed on the cut surface in a position that would mimic the preferred position of the luxated patella. The osteotomy plane would be used to correct the femoral torsion as can be seen below.
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This angle of cut gives the following appearance to the osteotomised Trochlea. The medial side is on the left. Removing the Trochlea at this angle resolves the torsional deformity of the distal femur and ensures that the prosthesis is correctly orientated in the Craniocaudal plane.
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The base plate of the Prosthesis is then placed as medial as possible based on the previous position of the luxated patella. The prosthesis will be press fit onto this base plate.
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The drill that you can see above is being used to clear bone from beneath the press fit points of the base plate. The prosthesis has three pegs that interdigitate with these holes, gentle hammering secures the prosthesis. We have found that clearing a little cancellous bone with a 1.5 mm drill allows the prosthesis to sit more flush with the base plate.
The patella is assessed for stability prior to closure of the capsule. It must be possible to maximally internally rotate the tibia in extension and full flexion and throughout range of motion without any tendency to luxate. I will never rely on capsular closure to provide stability.
Post Operative Radiographs
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You can see from the Craniocaudal projection how medial the prosthesis has been placed. On the radiograph below you can see that the Q-Angle is now around 6 degrees. This has been achieved without femoral osteotomy and without tibial tuberosity transposition. This tends to result in a far less complicated post operative process.
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I hope that this case report helps to highlight how a single procedure can resolve all of the aspects associated with even complex patellar luxation patients with various deformities present. With the groove it is possible to avoid these more complex procedures and achieve the mechanical and clinical goals of surgery.
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