Background

While posterior cruciate retaining (PCR) implants are a more common total knee arthroplasty (TKA) design, newer bi-cruciate retaining (BCR) TKAs are now being considered as an option for many patients, especially those that are younger. While PCR TKAs remove the ACL, the BCR TKA designs keep both cruciate ligaments intact, as it is believed that the resection of the ACL greatly affects the overall kinematic patterns of TKA designs. Various fluoroscopic studies have focused on determination of kinematics but haven't defined differentiators that affect motion patterns. This research study assesses the importance of the cruciate ligaments and femoral geometry for Bi-Cruciate Retaining (BCR) and Posterior Cruciate Retaining (PCR) TKAs having the same femoral component, compared to the normal knee.

The extension facet angle (EFA) of the medial compartment of the knee has been implicated as a potential mechanical cause for anteromedial knee osteoarthritis.

We developed a novel sagittal plane flexion osteotomy of the medial tibiofemoral compartment. We then performed a cadaveric study to study the effect of the osteotomy on the intra-articular knee pressures under axial load mimicking the stance phase of gait. A Tekscan K400 pressure sensor was inserted submeniscally into the joint and 700N applied using an Instron machine. A topographical map of the pressure areas was then assessed pre- and post-osteotomy for the 10 cadaveric knees specimens.

We found that the intra-articular pressures are greatest in the anteromedial compartment in the native knee and after the osteotomy the area of highest pressure moves posterolaterally spread over a greater surface area.

We conclude that a flexion osteotomy of the medial compartment reduces intra-articular knee pressures concentrated anteromedially in full extension and may be beneficial in patients with an elevated EFA with anteromedial symptoms.

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.

Introduction and Aims: To assess the tibiofemoral kinematics of the PCL deficient knee using vertical open-access ‘dynamic’ MRI.

Method: Tibiofemoral motion was assessed using open-access MRI, weight-bearing in a squat, through the arc of flexion from zero to 90 degrees in six patients with isolated rupture of the PCL in one knee [diagnosed from conventional MRI scanning and clinical assessment] and a normal contralateral knee. Mid-medial and mid-lateral sagittal images were analysed in all chosen positions of flexion in both knees to assess the relative tibiofemoral relationships. Passive sagittal laxity was assessed by performing the posterior and anterior drawer tests, while the knees were scanned, again using the same MRI scanner. The tibiofemoral positions during this stress MRI examination was measured from mid-medial and mid-lateral sagittal images of the knees.

Results: Rupture of the PCL leads to an increase in passive sagittal laxity in the medial compartment of the knee [P< 0.006]. In the weight-bearing scans, PCL rupture alters the kinematics of the knee with persistent posterior subluxation of the medial tibia so that the femoral condyle rides up the anterior upslope of the medial tibial plateau. This ‘fixed’ subluxation was observed throughout the extension-flexion arc being statistically significant at all flexion angles (P< 0.018 at 0°, P< 0.013 at 20°, P< 0.014 at 45°, P< 0.004 at 90°). The kinematics of the lateral compartment were not altered by PCL rupture to a statistically significant degree. The posterior drawer test showed increased laxity in the medial compartment.

Conclusion: PCL rupture alters the kinematics of the medial compartment of the knee resulting in ‘fixed’ anterior subluxation of the medial femoral condyle [posterior subluxation of the medial tibial condyle]. This study helps to explain the observation of increased incidence of osteoarthritis in the medial compartment and specifically femoral condyle, in PCL deficient knees.

Purpose: To assess if ACL reconstruction restores normal knee kinematics.

Methods: Tibiofemoral motion was assessed weight-bearing through the arc of flexion from 0 to 90° in ten patients who were at least 6 months following successful hamstring graft ACL reconstruction. Lachman’s test was also performed using dynamic MRI. Mid-medial and mid-lateral images were analysed in all positions to assess the tibiofemoral relationship.

Results: The laxity of the reconstructed knees was reduced to within normal limits. However the normal tibiofemoral relationship was not restored after ACL reconstruction with persistent anterior subluxation of the lateral tibial plateau throughout the arc of flexion 0–90°(p< 0.001).

Conclusion: Successful ACL reconstruction reduces joint laxity and improves stability but it does not restore normal knee kinematics.