Eligibility of all patients between 55 and 80 years, scheduled to undergo unilateral TKA on for primary knee osteoarthritis will be evaluated. The exclusion criteria are rheumatological joint diseases, previous knee surgery, neuromuscular diseases, bilateral TKA, or insufficiency of collateral ligaments. Patients will be randomized in a 1:1 ratio via computer-generated randomization using Microsoft Excel 2016 (Microsoft Corporation, Seattle, WA, USA) to be allocated in the UC insert or PS insert group before the operation. Unblinded senior resident will implement the randomization. Patients and physiatrists performing isokinetic measurements will be blinded to group allocation.

During the UC and PS operations, Vanguard® Complete Knee System prosthesis (Zimmer Biomet Inc., Warsaw, IN, USA) will be implanted using the same surgical technique in all patients. The Vanguard anterior-stabilized (AS) insert is a UC deep-dish design with a 10 mm prominent anterior lip and 5mm posterior lip. This bone-conserving design prevents anterior femoral subluxation because of the prominent anterior lip. The insert design allows it to be used with the Vanguard cruciate-retaining femoral component and the highly congruent articulating surface increases rotational stability. There is more contact area between the femoral component and the weight-bearing surface to decrease the shear stress between the femur and polyethylene insert.

A tourniquet will be inflated to pressure of 300 mmHg after spinal anesthesia. All operations will be performed with the same surgical technique by a single senior surgeon. Patellar surface arthroplasty will be performed in all the cases. Both femoral and tibial prostheses will be implanted with pressured bone cement. A suction drain will be placed inside the knee capsule. On the morning of the first day after surgery patients will be mobilized under the supervision of the physical therapist. All patients will undergo the same rehabilitation procedure during the outpatient period and will be examined by the same physiatrist at the regular 3, 6, and 12-month follow-up visits.

The primary outcome is isokinetic performance, measured as peak knee extensor and flexor torque values in Newton-meters, on the operated knee. Measurements wilil be done preoperatively and at 3, 6, and 12-month follow-up examinations under the supervision of the same physiatrist. At same time points, the Knee Society Score (KSS) will be evaluated as well. Isokinetic measurements will be performed using a Biodex System III Isokinetic Dynamometer, version 3.03 (Biodex Medical Inc., Shirley, NY, USA) by the same senior physiatrist. Patients will be positioned on the dynamometer with the hip in 90° in a sitting position for the knee flexion and extension measurements. Lateral movement of the knee will be prevented during full extension and flexion of the knee by a thigh strap on the operated leg. The physical therapist will help the patients to achieve proper positioning before each test. Concentric isokinetic knee flexion-extensions will be assessed at a preset velocity of 60º/sec, over a range of motion of 0º to 110º for both parameters. A fixed number of 10 flexion-extension repetitions will be completed by each patient. Instructions will be provided, and one trial repetition will then performed by all patients before the measurements were taken.

Sample size estimation will be performed using the extension peak torque as a primary effect variable. As there is no similar study with isokinetic measurements regarding this subject, a difference in mean values of 10 Nm and standard deviation of 12 Nm were assumed for each group. The group sample sizes of 31 and 31 achieved a power of 0.90 to detect a difference of 10 Nm between the two groups with estimated group standard deviations of 12 for each group and with a significance level (alpha) of 0.05 using a two-sided, two-sample test. Considering the loss to follow-up, two patients were added to each group. Thus, recruitment will end after 33 patients were assigned to either groups. All data will be calculated as mean and standard deviation. The Student's t-test will be used for statistical analysis of the patient data. Statistical calculations will be performed with SPSS 22.0 software (IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY: IBM Corp.). A value of p\< 0.05 will be considered statistically significant.