The purpose of this study was to evaluate the accuracy of the second metatarsal (MT2) as a landmark for proximal tibial cutting in total knee arthroplasty (TKA). It was hypothesized that the accuracy of the MT2 is not high, especially in rheumatoid arthritis (RA) patients whose foot joints are apt to be involved.
Computer simulation studies on 48 RA knees and 45 osteoarthritis (OA) knees were performed. The deviations from the mechanical axis (MA) of the tibia when the guide rod was pointed toward the base of the MT2 or the distal part of the MT2 were measured.
The mean deviation from MA was 0.8° ± 2.1° valgus (range 8.1°-6.3° valgus) and 1.2° ± 2.9° valgus (range 11.6°-7.9° valgus) at the base of the MT2, and at the distal part of the MT2, respectively. The outlier rate when using the base of the MT2 was lower than when using the distal part of the MT2 (12.9 vs 32.3%, p = 0.0032). The outlier rate was equivalent in OA and RA patients (n.s.). However, foot involvement in RA patients demonstrated a trend toward significance (base of MT2 p = 0.078, distal part of MT2 p = 0.068).
II.
The major clinical relevance was to raise caution about using the MT2. Surgeons should aim toward the base of the MT2, but avoid using it in RA patients with foot involvement. The accuracy of the MT2 is not high and it should be used only to supplement other landmarks.