Superior dislocations are the least frequent form of acute PTFJ dislocations and are associated with high-energy ankle injuries.2,10,12 Superior dislocation results in injury to the interosseous membrane between the tibia and fibula and is frequently associated with tibial shaft fracture.10,11 Atraumatic, acquired superior dislocation of the PTFJ has also been associated with congenital dislocation of the knee.10,11
SUBLUXATION/CHRONIC INSTABILITY
Subluxation of the PTFJ classically involves excessive and symptomatic anterior-posterior motion without actual dislocation of the joint.11 Subluxations of the PTFJ typically occur without any known trauma or injury and are most frequently associated with benign hyperlaxity syndrome, Ehlers-Danlos syndrome, or muscular dystrophy.4,10
Semonian and colleagues13 suggest that subluxation of the PTFJ is not given enough recognition in the literature and that instability should not be considered a rare condition. They hypothesize that many patients have joints that do not tolerate increases in fibula rotation secondary to subclinical trauma, repetitive overuse, or biomechanical variation of the joint. Semonian and associates13 state that the condition begins with the anterior capsule and anterior tibiofibular ligament attenuation as a result of excessive fibular rotation. Once stretched, the functional pull of the biceps femoris and soleus maintain the fibula in a relatively posterior and externally rotated position. Furthermore, Ogden10 found that 70% of the dislocations and subluxations he studied were of the oblique variant compared with that of the horizontal variant. Many authors suggest that the oblique variant is more at risk for injury because of its decreased joint surface area causing decreased rotational mobility.1,2,10
Early recognition is extremely important in dislocations and subluxations of the PTFJ as undiagnosed acute trauma can turn into chronic subluxation, and chronic subluxation may lead to dislocation.13 Additionally, chronic subluxation or dislocation are thought result in osteoarthritis of the PTFJ.14
OSTEOARTHRITIS
The literature on osteoarthritis of the PTFJ is limited. Eichenblat and Nathan7 studied the PTFJ in cadavers and dry bones and found that 28% had evidence of osteoarthritis. Clinically, however, osteoarthritis of the PTFJ is a rare primary diagnosis, suggesting that involvement of the PTFJ is either asymptomatic or that symptoms are associated with osteoarthritis of the knee joint. Boya and colleagues15 and Eichenblat and Nathan7 both found a high correlation between the presence of osteoarthritis of the PTFJ and osteoarthritis of the tibiofemoral joint in cadavers. The authors suggest this correlation may be related to the presence of anatomical communication between the 2 joints. Theoretically, inflammatory mediators flow freely between the joint spaces and contribute to arthritis in both joints. The possibility of degenerative arthritis of the PTFJ accompanying degenerative arthritis of the knee warrants evaluation, especially in patients considering total knee arthroplasty. Unrecognized arthritis of the PTFJ might influence outcome scores and be an unsolved source of lateral knee pain post-knee replacement.16
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