Abstract
To stabilise the dislocating patella, one can increase the medial vectors, decrease the lateral vectors, or combine these options.
Oblique strengthening of vastus medialis increases the medial vectors. This muscle is an active secondary constraint for stabilising the patella. Strengthening this muscle is the cornerstone of treatment of patellar instability, but it is often unsuccessful if the medial patello-femoral ligament is deficient. The medial patellofemoral ligament is the primary passive constraint to lateral dislocation of the patella. Reconstruction of this ligament, which tenses in extension, stabilises the patella in most cases, without the danger of secondary late-stage patellofemoral degeneration. However, in high-riding patellae, effective ligament reconstruction may cause an extensor lag. An elevation of more than 3 mm affects the contact pressures by disturbing the unique relationship between patella and trochlea. Because it can lead to late stage patellofemoral degeneration, trochleaplasty is rarely indicated.
Tibial tubercle transfer decreases the lateral vectors and is indicated in cases of severe patella alta, a markedly increased Q-angle and lateral patellar tilt. The tubercle can be transferred distally or medially or internally rotated. The procedure changes the patellofemoral relationship, increasing the load in the medial tibio-femoral compartment and giving rise to the possibility of late-stage degeneration in both the patellofemoral and the tibiofemoral joints. It should not be undertaken lightly and the amount of shift should be conservative. The lateral retinaculum, which becomes lax in extension in right in flexion, provides about 10% of patellar stability to lateral dislocation. Because most patellae dislocate in early flexion, lateral retinacular release is seldom indicated except in the rare cases where the patella dislocates in late flexion.
In severe cases of patellar instability, it might be necessary to combine reconstruction of the medial patello-femoral ligament with tibial tubercle transfer and even with lateral retinacular release.
The abstracts were prepared by Professor M.B.E. Sweet. Correspondence should be addressed to him at PO Box 47363, Parklands, Johannesburg 2121, South Africa.
