Abstract
Introduction
Accurate soft tissue balancing has been recognized as important as alignment of bony cut in total knee arthroplasty (TKA). In addition, using a tensor for TKA that is designed to facilitate soft tissue balance measurements throughout the range of motion with a reduced patello-femoral (PF) joint and femoral component in place, PF joint condition (everted or reduced) has been proved to have a significant effect for intra-operative soft tissue balance. On the other hand, effect of patellar height on intra-operative soft tissue balance has not been well addressed. Therefore, in the present study, we investigated the effect of patellar height by comparing intra-operative soft tissue balance of patella higher subjects (Insall-Salvati index>1) and patella lower subjects (Insall-Salvati indexâ‰/1).
Materials and methods
The subjects were 30 consecutive patients (2 men, 28 women), who underwent primary PS TKA (NexGen LPS-flex PS: Zimmer, Warsaw, IN, USA) between May 2003 and December 2006. All cases were osteoarthritis with varus deformity. Preoperative Insall-Salvati index (ISI) was measured and patients were divided into two groups; the patella higher group (ISIï1/4ž1: 18 knees average ISI was 1.12) and the patella lower group (ISIâ‰/1; 12 knees average ISI was 0.94). Component gap and ligament balance (varus angle) were measured using offset-type tensor with 40lb distraction force after osteotomy with the PF joint reduced and femoral trial in place at 0, 10, 45, 90, 135 degrees of knee flexion. Data of two groups were compared using unpaired t test.
Results
Component gap was increased from 0 to 90 degrees of knee flexion and decreased at 135 degrees of knee flexion in both groups. Component gaps of the patella higher group in average were 10.9, 14.3, 16.6, 18.2, 16.8 mm at 0, 10, 45, 90, 135 degrees of knee flexion, respectively. Component gaps of the patella lower group in average were 9.6, 13.6, 14.6, 15.5, 14.0 mm at 0, 10, 45, 90, 135 degrees of knee flexion, respectively. When comparing two groups, component gaps of the patella higher group showed larger trend than those of the patella lower group. Especially at 90 and 135 degrees of knee flexion, the patella higher group showed significant larger values than the patella lower group (p<0.05). Varus angles of the patella higher group in average were 2.2ï1/4Œ3.4ï1/4Œ5.0ï1/4Œ5.9ï1/4Œ6.1 degrees at 0, 10, 45, 90, 135 degrees of knee flexion, respectively. Varus angles of the patella lower group in average were 1.7ï1/4Œ2.8ï1/4Œ4.4ï1/4Œ4.9ï1/4Œ4.6 degrees at 0, 10, 45, 90, 135 degrees of knee flexion, respectively. Varus angles of the patella higher group showed slight larger trend than those of the patella lower group, however there was no significant differences between two groups.
Discussion
In the present study, the patella higher group showed significant larger component gaps than the patella lower group at high flexion angles (90, 135 degree). This result suggests that smaller pressure on extensor mechanism of the patella higher group has led to larger component gaps at higher flexions. In conclusion, pre-operative measurement of ISI can help surgeons predict intra-operative soft tissue balance.
