Abstract
Introduction
Selection of an optimum thickness of polyethylene insert in total knee arthroplasty (TKA) is important for the good stability and range of motion (ROM). The purpose of this study is to investigate the amount of change of ROM as the thickness of trial insert increase.
Material and Method
The study included 86 patients with 115 knees undergoing TKA from October 2012 to February 2014. There were 17 men and 69 women with an average age of 75±8 (58–92) years. The implants posterior stabilized knee (Scorpio NRG, Stryker) was used and all prostheses were fixed with cement. The ROM was measured by the goniometer under the general anesthesia at the time of operation in increments of 1°. Preoperative flexion angle was measured by passively flexing the patient's hip 90 degrees and allowing the weight of the leg to flex the knee joint (Lee et al 1998). Extension angle was measured by holding the heel and raising the leg by another examiner. During TKA, flexion and extension angle was measured in a similar manner when each insert trial (8, 10, 12, and 15mm) was inserted. After the wound closure and removing the draping, ROM was measured again. Statistical analysis of range of motion was performed using a paired t-test to determine significance
Results
Preoperative extension angle was-11.8±7.5°and flexion angle was 125.4±14.9 °. postoperative extension angle after removing drapes was −5.0±3.4°and flexion angle was126.4±8.8°. Although extension angle was improved statistically (p<0.001), flexion angle was not improved. Intraoperative extension and flexion angle that were measured with the same thick insert trial as the polyethylene insert finally selected was −3.7±3.0°and 120.8±9.8°respectively. The thickness of polyethylene insert finally set was 8mm (28knees), 10mm (58knee), 12mm (24 knee), and 15mm (5knee). The amount of deficit in extension ROM by changing the trial inserts those were measured intraoperatively were 2.5±2.2° (n=112, 8 to 10mm, p<0.01), 3.2±2.8° (n=80, 10 to 12mm, p< 0.01), and 4.7±2.5° (n=15, 12 to 15mm, p<0.01). Flexion angle was 0.6±4.3° (8 to 10mm, n.s), 1.5±4.0° (10 to 12mm, p=0.002), 2.6±4.0° (12 to 15mm, p=0.025).
Discussion
Although it is important to select a sufficient thick polyethylene insert to prevent postoperative instability, excessive thick polyethylene can decrease ROM especially extension. In many type of prosthesis, thickness of polyethylene insert differs every 2 mm is prepared. In the current study, if the thickness of polyethylene is increased 2mm (8 to10mm and 10 to 12mm) or 3mm (12 to15mm), extension and flexion angle was decreased 2.5–4.7°and 0.6–2.6°respectively.
