Aim
Current concepts in knee arthroplasty are moving towards “replacing lost bone” with prosthesis or using a “robot” to plan and/or cut the bone. The bone balancing technique for performing TKA aims to implant a well-balanced prosthesis in line with natural knee anatomy based on equal medio-lateral soft tissue tension. Our experience suggested that cutting equal bone amounts off the femur may result in an imbalanced knee or require an oblique tibial cut to restore ligament balance. Our aim was to assess the relationship between a tibial cut first technique and the size of the bone cuts required to obtain a soft tissue balanced total knee arthroplasty.
Method
We performed a prospective descriptive study on 894 individuals undergoing TKA utilizing the bone balancing technique. All alignments were measured on long leg standing X-rays. The thickness of the medial and lateral distal femoral cuts and medial and lateral posterior cuts was measured with a caliper. The flexion and extension gap size and ligament balance were measured with a tensiometer. Pre and post operative overall, femoral and tibial coronal alignments were recorded.
Results
Pre-operatively knee alignment was straight (0 degrees) in 3% of patients, valgus in 33% and varus in 64% of patients. The varus knees had mean post-operative tibial mechanical alignment of 0.8degrees varus (MPTA 89.2, 95% CI 89.0-90.7). The valgus knees had a mean tibial mechanical alignment of 0.1degree valgus (MPTA 90.1, 95% CI 89.9-90.3).
Overall soft tissue balance showed a mean medial:lateral difference of 0.5, indicating that the knees were well balanced in flexion and extension. However, the varus subgroup did tend to have slightly looser lateral compartments in flexion.
The significant findings divided into pre-operative alignment groups are as follows:-
In the varus group the posterior lateral cut measured 3.8mm less than the posterior medial cut (p<0.001). The distal lateral cut was also 3.2mm (p<0.001) larger than the posterior lateral cut.
In the valgus group, the distal medial bone cut measured on average 2mm more than the distal lateral bone cut (p<0.001), and 1.2mm less than the Posterior medial bone cut (P<0.001). However, the posterior lateral cut was 3.7mm less than the posterior medial cut (p<0.001). The distal lateral cut was 2.9mm (p<0.001) larger than the posterior lateral cuts.
In the straight group the posterior lateral cut was 4mm less than the posterior medial cut (p<0.001). The distal lateral cut was also 3.4mm (p<0.001) larger than the posterior lateral cut.
Conclusion
Using a tibial first, bone balanced technique, in which soft tissues are not released, knees that have equal flexion and extension gap sizes and soft tissue balances can show significant variations in the measured bone cuts. The chosen tibial cut angle (MPTA) will determine some of these variations.
