[Causes and management of patellar instability after total knee replacement : Lateralization, subluxation and luxation].

BACKGROUND: Patellofemoral complications after total knee arthroplasty are responsible for a variety of surgical revisions.

OBJECTIVE: The causes of the various types of instability of the patella are listed in a differentiated way and the importance of clinical and imaging diagnostics as well as preventive strategies are elaborated.

MATERIAL AND METHODS: This article is based on a selective literature search in the PubMed database and on the long-standing experience of the author.

RESULTS: Besides postoperative genu valgum with malalignment of the extensor mechanisms, other risk factors for patellar maltracking are insufficiency of the medial retinaculum, weakening of the vastus medialis muscle, contracture of the quadriceps femoris or tractus iliotibialis muscle, residual valgus deformity after total knee replacement, femoral or tibial malrotation as well as malpositioning of the patella, inappropriate design of the prosthesis and asymmetrical resection of the patella. The causes with respect to incorrect component positioning, faulty preparation of the patella, leg malalignment, inappropriate design of the prosthesis and soft tissue imbalance have to be recognized in order to address the problem in a targeted way. The preferred method of choice in the case of patellofemoral instability after total knee replacement is normally surgery; however, the cause for the instability has to be identified and consequently corrected before surgery. Without a clearly identified cause surgical measures are unrewarding and almost regularly lead to an unsatisfactory outcome.

CONCLUSION: Patella maltracking after total knee arthroplasty is multifactorial and requires an accurate clarification. A surgical revision is only recommended in cases of clearly defined causes of pain or a clearly defined reason for patella malpositioning.