Objectives

The medially spherical GMK Sphere (Medacta International AG, Castel San Pietro, Switzerland) total knee arthroplasty (TKA) was previously shown to accommodate lateral rollback while pivoting around a stable medial compartment, aiming to replicate native knee kinematics in which some coronal laxity, especially laterally, is also present. We assess coronal plane kinematics of the GMK Sphere and explore the occurrence and pattern of articular separation during static and dynamic activities.

Introduction

Despite consequent advancement in Total Knee Arthroplasty (TKA) up to 20% of patients are not satisfied after having been operated. Beside correct implantation, the design of the TKA-system is supposed to be a key factor of a successful TKA. Consequently it has been tried to restore natural kinematics by the design of the prosthesis. A medially stabilized design therefore is supposed to allow a lateral translation with a medial pivot.

Objectives

Throughout the 20th Century, it has been postulated that the knee moves on the basis of a four-bar link mechanism composed of the cruciate ligaments, the femur and the tibia. As a consequence, the femur has been thought to roll back with flexion, and total knee arthroplasty (TKA) prostheses have been designed on this basis. Recent work, however, has proposed that at a position of between 0° and 120° the medial femoral condyle does not move anteroposteriorly whereas the lateral femoral condyle tends, but is not obliged, to roll back – a combination of movements which equates to tibial internal/ femoral external rotation with flexion. The aim of this paper was to assess if the articular geometry of the GMK Sphere TKA could recreate the natural knee movements in situ/in vivo.

Introduction

Femorotibial malalignment exceeding ±3° is a recognised contributor of early mechanical failure after total knee replacement (TKR). The angle between the mechanical and anatomical axes of the femur remains the best guide to restore alignment. We investigated where the femoral head lies relative to the pelvis and how its position varies with respect to recognised demographic and anatomic parameters. We have tested the hypothesis of the senior author that the position of the centre of the femoral head varies very little, and if its location can be identified, it could serve to outline the mechanical axis of the femur without the need for sophisticated imaging.

Introduction: The medial tibial plateau is composed of two relatively flat facets. An anterior upward sloping “extension facet” (EF) articulates with the medial femoral condyle from 0 to 20–the stance phase of gait (in Man but not in other mammals). Anatomical variation in this area might be responsible for antero-medial osteoarthritis (AMOA).

This paper reports the angle between the EF and the horizontal (the extension facet angle- EFA) in normal knees and in knees with early AMOA.

Method: MRI reports were searched to identify patients with acute rupture of the ACL on the assumption that they had anatomically normal tibiae (46 males and 18 females) and patients with MRI evidence of early AMOA without bone loss (11 males and 9 females).

A sagittal image at the midpoint of the femoral condyle was used to determine the EFA.

Results: The EFA in normal tibiae is 14 +/− 5 (range 3–25). The angle is unrelated to age. The EFA in individuals with early AMOA is 19 +/− 4 (range 13–26). The difference is significant (p< 0.001).

Discussion: There is a wide variation in the EFA in normal knees which is unrelated to age.

There is an association between an increased EFA (ie a steeper EF) and MRI evidence of AMOA. Although a causal link is not proven, we speculate that a steeper angle increases the duration of loading on the EF in stance and tibio-femoral interface shear. This may initiate cartilage breakdown.

Introduction: The medial tibial plateau is composed of two relatively flat facets. An anterior upward sloping “extension facet” (EF) articulates with the medial femoral condyle from 0 to 20° – the stance phase of gait (in Man but not in other mammals). A horizontal “flexion facet” contacts the femur from 20° to full flexion. Anatomical variation in this area might be responsible for the initiation of antero-medial osteoarthritis (AMOA).

This paper reports the angle between the EF and the horizontal (the extension facet angle - EFA) in normal knees and in knees with early AMOA.

Method: MRI reports were searched to identify patients with acute rupture of the ACL on the assumption that they had anatomically normal tibiae (46 males and 18 females) and patients with MRI evidence of early AMOA without bone loss (11 males and 9 females).

A sagittal image at the midpoint of the femoral condyle was used to determine the EFA. Repeat measurements were taken by two observers.

Results: The EFA in normal tibiae is 14 +/− 5° (range 3 – 25°). The angle is unrelated to age. The EFA in individuals with early AMOA is 19 +/− 4° (range 13 – 26°). The difference is highly significant (p< 0.001).

Discussion: There is a wide variation in the EFA in normal knees that is unrelated to age.

There is an association between an increased EFA (ie a steeper EF) and MRI evidence of AMOA. Although a causal link is not proven, we speculate that a steeper angle increases the duration of loading on the EF in stance and tibio-femoral interface shear. This may initiate cartilage breakdown.