Elsevier

Journal of Biomechanics

Volume 58, 14 June 2017, Pages 195-202
Journal of Biomechanics

Contact forces in the tibiofemoral joint from soft tissue tensions: Implications to soft tissue balancing in total knee arthroplasty

https://doi.org/10.1016/j.jbiomech.2017.05.008Get rights and content

Abstract

Proper tension of the knee’s soft tissue envelope is important during total knee arthroplasty; incorrect tensioning potentially leads to joint stiffness or instability. The latter remains an important trigger for revision surgery. The use of sensors quantifying the intra-articular loads, allows surgeons to assess the ligament tension at the time of surgery. However, realistic target values are missing. In the framework of this paper, eight non-arthritic cadaveric specimens were tested and the intra-articular loads transferred by the medial and lateral compartment were measured using custom sensor modules. These modules were inserted below the articulating surfaces of the proximal tibia, with the specimens mounted on a test setup that mimics surgical conditions. For both compartments, the highest loads are observed in full extension. While creating knee flexion by lifting the femur and flexing the hip, mean values (standard deviation) of 114 N (71 N) and 63 N (28 N) are observed at 0° flexion for the medial and lateral compartment respectively. Upon flexion, both medial and lateral loads decrease with mean values at 90° flexion of 30 N (22 N) and 6 N (5 N) respectively. The majority of the load is transmitted through the medial compartment. These observations are linked to the deformation of the medial and lateral collaterals, in addition to the anatomy of the passive soft tissues surrounding the knee. In conclusion, these findings provide tangible clinical guidance in assessing the soft tissue loads when dealing with anatomically designed total knee implants.

Introduction

Total knee arthroplasty involves critical interaction with the soft tissues surrounding the knee to obtain a stable, well-functioning joint following surgery. The tension in these soft tissues controls the boundaries of laxity. At any angle of flexion, the soft tissues’ effect is determined by their initial tensions and their stiffnesses. After a total knee arthroplasty (TKA), achieving normal laxity and stability during function is an important consideration, made more difficult due to the resection of one or both cruciate ligaments in most total knee designs. This problem is evident because instability can lead to increased polyethylene wear (Kretzer et al., 2010) and remains an important reason for early failure after TKA surgery, responsible for 17–26% of the early revision surgeries (Dalury et al., 2013, Lee et al., 2014). These numbers are not surprising, given the complexity in compensating for the alteration and loss of stabilizing structures during insertion of the implant. Simultaneously, a correct component alignment needs to be assured (whether kinematic or mechanical is still under debate (Cherian et al., 2014)), taking into account cartilage wear and potential deformity in the bone morphology. A large number of variables are thus involved in this process: implant design, surgical technique, soft tissue tensioning and component positioning (Walker et al., 2014). This paper focuses on the soft tissues surrounding the articulating surfaces, consisting of purely passive structures (e.g. capsule and ligaments) and combined active-passive restraints (e.g. tendons and muscles).

Dominated by the ligaments, the soft tissues control the passive stability of the knee joint (Blankevoort et al., 1991, Halewood and Amis, 2015). As a result, the degree of (in)stability can be assessed at the time of surgery by evaluating the tension in and/or strain of the ligaments. Although results have been published in the literature, a direct evaluation of the ligament strain remains impractical in a surgical setting (Fleming et al., 2003). Alternatively, the ligament tension can be indirectly assessed by evaluating the contact loads in the tibiofemoral joint. More specifically, the load transferred through the medial and lateral compartment can be quantified using sensors that quantify the forces exerted by the femur at the level of the tibia. In the remainder of this paper, these loads are referred to as compartmental loads. Such approach has the advantage of additionally accounting for the tension created by other structures (e.g. capsule) and not narrowing down to tension in a (discrete) number of ligaments.

Nowadays, these compartmental loads can be assessed intra-operatively, for instance by using instrumented tibial trial components that quantify the tibiofemoral contact forces at the level of the tibia. Use of these sensors has the potential to restore physiological load balance and increase patient satisfaction following TKA surgery (Gustke et al., 2014a, Gustke et al., 2014b). Literature recommends compartmental load levels in the range of 90–180 N (20–40 lbs), with a constant magnitude through the flexion-extension range (Asano et al., 2004) and a limited load difference between the medial and lateral compartment (<67 N/15 lbs) (Gustke et al., 2014a, Gustke et al., 2014b). However, a fundamental basis for the identification of these target load levels is missing. In an attempt to identify loads that are representative of the native anatomical situation, this study presents a quantitative assessment of the compartmental load levels in a series of cadaveric non-arthritic knees.

Section snippets

Test specimens

A total of eight fresh frozen cadaveric specimens was used. The specimens consisted of full lower limbs including a hemi-pelvis. Prior to inclusion, the knees were assessed for osteoarthritis using X-rays. Only specimens with a Kellgren-Lawrence score below grade 2 were selected (Table 1). Three additional specimens were used for method development and data from a forth additional specimen was discarded because the sensor described below failed during testing.

Test setup

The full lower limb specimens, from

Varus/valgus alignment of the knee specimens

For each specimen, the coronal alignment was assessed during a thigh pull test. One specimen is in valgus (specimen 11) and two specimens are in pronounced varus (specimen 5 and 6). The other specimens are in near-neutral alignment (Table 2).

Absolute forces transmitted through medial and lateral compartment

The absolute forces transferred through the medial and lateral compartment are first evaluated. The average data and associated standard deviation for both the heel push and thigh pull experiments are presented in Fig. 3a and b. For the heel push test,

Discussion

The main objective of this paper was to identify target intra-articular loads that may serve as reference for restoration of compartmental loads in TKA surgery. The magnitude of these loads highly depend on how knee flexion was imposed. Furthermore, these loads are non-constant through the range of motion. Eventually, the clinical importance with respect to TKA surgery along with the main limitations of the study are addressed.

First, the focus is on how knee flexion was imposed. It was clearly

Conflict of interest

As mentioned in the acknowledgement section, the research department at which the research took place received an unrestricted research grant from: OrthoSensor Inc. To the authors’ opinion, the impact of this financial support to the results presented in this paper is negligible.

Acknowledgements

The research department at which the research took place received an unrestricted research grant from OrthoSensor Inc. To the authors’ opinion, the impact of this financial support to the results presented in this paper is negligible. The international mobility of the first author was supported the Fonds Wetenschappelijk Onderzoek (FWO – grant n° V444415N). The authors also wish to thank Daniel Hennessy, for the well appreciated help and suggestions in constructing the test setup.

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