Sagittal laxity in vivo after total knee arthroplasty

Introduction A stress arthrometry study of 77 knees undergoing total knee arthroplasty was performed to determine the difference in anteroposterior (AP) laxity between posterior cruciate ligament (PCL)-retaining (PCLR) and PCL-substituting (PCLS) prostheses using the Genesis I TKA. Materials and methods Fifty-three knees had PCLR and 24 had PCLS prostheses. The selected patients had successful arthroplasties after a minimum follow-up of 5 years. AP laxity was measured with a KT-2000 arthrometer (Medmetric, San Diego, CA, USA) using standard protocols. Results At 30° of flexion, there was no statistical difference in anterior (PCLR: 4.7 mm, PCLS: 4.5 mm), posterior ( PCLR: 1.1 mm, PCLS: 0.7 mm), or total ( PCLR: 5.8 mm, PCLS: 5.3 mm) displacement. At 75°, significant differences were seen in both anterior ( PCLR: 3.3 mm, PCLS: 2.3 mm) and total ( PCLR: 4.8 mm, PCLS: 3.4 mm) displacement ( p= 0.001 and p= 0.009, respectively), although there was no statistical difference in posterior displacement ( PCLR: 1.5 mm, PCLS: 1.1 mm). Conclusion The above values are considered the suitable degree of AP laxity in total knee arthroplasty for a satisfactory clinical outcome 5 - 9 years after surgery. The PCL in a PCLR prosthesis and the central tibial spine and femoral cam in a PCLS prosthesis might play comparable roles in determining the laxity in the posterior direction in these prostheses.