Knee Implants: The Importance of Fit

Standardized knee replacement implants come close to fitting the knee sizes of many people however there is some evidence that if the fit is off by as little as 3mm there may be painful side effects that persist after the surgery recovery period.1

Two factors are attributed to improper fit of an implant. Overhang occurs when the implant is bigger than the bone surface, which may impinge upon the soft tissues surrounding the knee joint causing pain. Likewise underhang occurs when the implant does not cover all the exposed cut bone which may also result in pain.

In a traditional knee replacement procedure, the surgeon chooses an implant from a standardized selection of six to ten sizes depending upon the manufacturer of the implant. Despite the wide variations in anatomy from patient-to-patient, it is surprisingly easy for an appropriately skilled surgeon to fit standard implant designs well. However occasionally, in order to fit standardized implants to non-standardized knees, surgeons may have to make compromises in the rotational alignment of the implant. In other words, while trying to get the best fit possible the surgeon may have to rotate the knee implant a bit left or right of center. Such adaptations in alignment may result in pain that persists after surgery. In a small study (of 437 performed by one surgeon) among knee replacement patients with ongoing painful knees, 44% were found to have significant rotational errors of the tibial (shin bone) component of the implant.1

Total Knee Replacement, Precise Fit (image courtesy of ConforMIS)

Precise Fit, Total Knee Replacement (image courtesy of ConforMIS)

As the number of patients undergoing knee replacement surgery continues to grow, some knee replacement manufacturers are offering a broader range of standardized implant sizes to address this issue while a few offer patient-specific, or custom implants created from CT-scan data that closely matches the natural shape and curves of the knee. Using the same scan data, computer-assisted surgical instrumentation is utilized by the surgeon during the procedure, simplifying many of the sizing and alignment steps that are necessary throughout surgery.


  1. Mahoney, et al. Overhang of the Femoral Component in Total Knee Arthroplasty: Risk Factors and Clinical Consequences. The Journal of Bone and Joint Surgery; 2010, 92: 1115-1121.