Abstract
Background
Soft tissue tension and intra-articular pressure distribution plays a crucial role in postoperative function and survivorship of TKA prosthesis. Although posterior stabilized (PS) and cruciate retaining (CR) knees have both been successful in relieving pain and restore function, it is reported that the joint gaps were significantly distinct between the two designs during flexion. The aim of this study is to find out what is the difference in intra-articular pressure distribution between PS and CR knees.
Methods
We prospectively included 45 consecutive patients (50 knees) scheduled for total knee arthroplasty between August, 2013 and April, 2014 in our hospital. 23 patients (25 osteoarthritic knees) received a Genesis II CR TKA (Smith & Nephew, Memphis, USA), and the other 22 patients (25 osteoarthritic knees) received Genesis II PS TKA (Smith & Nephew, Memphis, USA). During operation, after the bone osteotomy and soft tissue balance were completed, we measured and compared the intra-articular pressure distribution at 0°, 30°, 45°, 60°, 90°, and 120° flexion with a previously validated “Wireless Force Measurement System (WFMS)”. Joint gaps were measured at extension and 90° flexion. The soft tissue was not considered balanced until the medial and lateral joint gap difference ≤ 2mm at extension and 90° flexion. There are no significant differences in age, gender, BMI, varus angle and flexion deformity, and preoperative range of motion between the two groups. The medial-lateral pressure distribution and total pressure were compared at different angles between CR and PS knees.
Results
During flexion, the total pressure drops sharply at the first 30 degrees, and then goes down slowly for the rest 90 degrees, without significant difference between CR knees and PS knees at any flexion angles [fig.1]. For PS knees, pressure in the medial compartment, accounting for 65.0%∼80.4% of the total pressure, is significantly higher than that in the lateral side at all angles(p<0.05) [fig.2]. For CR knees, pressure in the medial compartment is significantly higher than the lateral side at extension (61.0%, p<0.05), but significantly lower than the lateral side at 45°, 60°, 90°, and 120° flexion (29.2%∼36.3%, p<0.05) [fig.3]. There is no difference between the medial and lateral side in CR knees at 30°[fig.3].
Discussion
Our research demonstrates that the soft tissue tension differs between CR and PS knees. The intra-articular pressure is concentrated in the medial compartment during whole ROM in PS knees. While in CR knees, pressure in the medial compartment is transferred to the lateral side during flexion. The mid flexion stability in the lateral side is significantly better in CR knees than in PS knees, and the soft tissue is better balanced at initial 0°∼30°in CR knees than in PS knees. Our results revealed that the cam-post mechanism cannot replace the PCL's role in modulating pressure distribution and soft tissue tensioning. These findings may guide the future geometrical design and soft tissue balancing techniques of PS and CR knees.
