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EFFECTS OF NAVIGATED MINI SUB-VASTUS TOTAL KNEE ARTHROPLASTY ON NATIVE PATELLA

Bay Pines Orthopedic Research Institute, Bay Pines Health Care System, PO Box 517 Bay Pines, FL 33744, USA.

Most common complication of non-navigated classic total knee arthroplasty (TKA) relates to patella. Not resurfacing the patella makes exposure more difficult in a mini-approach which may add to its potential complications. Effect of navigated mini sub-vastus TKA on native patella is clinically and radiologically studied, observing also, whether severity of deformity or obesity adds to patellae complications in such approach.

92 of 100 subjects were eligible. Peri-operative radiological and navigation data with follow up visits to 24 months provided alignment, patella tilting or displacement data. Clinical outcome gauged by "KSS" documented pain from patella movements, or pain generated from stair climbing, or rising from a chair. Patella is considered subluxated if it displaced 5mm. No exclusion by obesity or severity of deformity. Results were evaluated with descriptive statistics.

Of the 92 patients, 3 had patella pain (3%). 72% had <5° of patella tilting (of which 3 had patella pain) while 28% had a 5°–17° tilting. As for patellae displacement, 12% displaced laterally (3 mm) but with no pain. None had patellar displacement 4mm (which we define as subluxation), and none had a dislocation. Pre-operative knee deformity ranged from 19° varus to 13° of valgus. 70% of subjects had pre-operative varus/valgus deformity of <10°. The other 30% had deformity of 10°. Post-operative mean mechanical axis alignment was 0° (± 1°) with a mean range of motion of –3.8° to 133.6°. No vascular injuries, skin necrosis, deep infection, or fractures.

The BMI ranged from 25–46 Kg/m². 16% had a BMI 40 with no patellar pain, tilting or displacement.

Incidence of native patella pain in a navigated mini sub-vastus TKA was low irrespective to body mass or pre-operative deformity. Perhaps navigation helped align the components ideally and thus reducing the complication rate of a mini-approach. However, 28% of native patella tilted > 5° but unlike tilting of a resurfaced patella, it did not correlate with patella pain. In this study, whether non-resurfacing caused the 3% of patella pain is undetermined. Nevertheless, the pain level was not severe to make the patients seek a revision of the patella. Finally, as we compare with other studies, we cannot conclude that mini sub-vastus approach is superior; however its low patella complication rate is comparable if not superior to classic approach.

Correspondence should be addressed to Mr K Deep, General Secretary CAOS UK, Dept of Orthopaedics, Golden Jubilee National Hospital, Glasgow G81 4HX, Scotland. Email: caosuk{at}gmail.com