chapter 9:Patellar Subluxation

ETIOLOGY

Recurrent subluxation is relatively common. Several authors have reported this condition predominantly in women (3, 6, 7), but Hughston (8) has emphasized the notion that this condition is a frequent affliction of athletic men. That author found in his series of 60 patients that 25 (42%) were men, all of whom were athletes. Of the 35 women in his series, 10 were athletes. We believe that recurrent patellar subluxation or dislocation may occur after traumatic patellar displacement in athletes of either sex, but that women have a greater inherent predisposition to spontaneous malalignment‑related patellar instability. Bennett et al (9) have noted that differences in vastus medialis obliquus orientation in women may contribute to their increased tendency to have patellar subluxation. Like recurrent dislocation, it is a disorder of the second decade, although there are individuals who have minor recurrent subluxation for many years before any significant pain or dysfunction occurs.

Congenital Subluxation

Most patients with patellar subluxation are born with extensor mechanism imbalance. Subluxation may result from congenital deficiency of the femoral trochlea, malalignment of the lower extremity related to excessive valgus, excessive hip anteversion, external tibial torsion, pronation of the foot and ankle, or another extremity alignment problem. Soft‑tissue imbalance, particularly deficiency of the vastus medialis obliquus or excessive vastus lateralis obliquus pull on the patella, may cause or accentuate extensor mechanism subluxation. Connective‑tissue laxity will aggravate any pre‑existing extensor mechanism malalignment. Many patients with mild patellar subluxation undoubtedly go unrecognized and may, in fact, have little risk of developing arthrosis if subluxation is minimal and there is no significant tilting of the patella to pathologically increase lateral facet load.

Secondary Subluxation

Patellar subluxation may occur after extensor mechanism injury, knee surgery (Fig. 9.2), or below‑knee amputation (10). Dysplasia of the trochlea is less common in secondary subluxation, but treatment is still aimed at restoration of normal alignment.

Trauma is certainly not necessary to cause patellar subluxation, but when it can be extracted from the history, it is nearly always a rotational stress or direct blow (from the medial side). The athlete may plant a foot and change direction, with internal rotation of the femur on the fixed tibia in slight flexion and a powerful contraction of the quadriceps. This movement accentuates the lateral vector that normally exists in the extensor apparatus and can cause patellar subluxation or dislocation.

 

        

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