Contact stresses in the knee joint in deep flexion

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Abstract

The contact stresses in the knee that arise from activities involving deep flexion have not been given due consideration in view of social and cultural practice amongst many Asians that frequently cause the engagement of these activities. Excessively large stresses (>25 MPa) can cause cartilage damage and may be the precursor to the development of degenerative disease of the joint. In this study, forces in the knee derived from previous studies of human walking and squatting were applied to five cadaver knees that underwent quasi-static mechanical testing. This was conducted using a materials-testing machine and a custom-made apparatus that allowed secure and consistent loading of the knee specimen in flexion beyond 120°. A thin-film electronic pressure transducer was inserted into the cadaver tibiofemoral joint space to measure force and area. Throughout the various positions simulating specific phases of walking, it was found that stresses peaked to 14 MPa (standard deviation was 2.5 MPa). In deep flexion, the peak stresses were significantly larger by over 80%, reaching the damage limits of cartilage. The results from this biomechanical study suggest that the adequacy of articular cartilage to support loads in the knee joint during deep flexion may be questionable.

Introduction

Accurate and functionally relevant intra articular contact stresses in the natural knee joint is difficult to determine and there are no known published data on the stresses that result in the tibiofemoral knee joint in walking and squatting. The knee joint reaction forces from walking have been estimated to be as high as three to five times bodyweight [1], [2]. This duly raises concern when strength studies of cartilage explants have shown that damage occurs with as low as 5–10 MPa of cyclic stress [3]. Furthermore, given the evidence that osteoarthritis and cartilage damage can occur in the knee as a result of frequent or high contact stresses [3], [4], the relevance in measuring these stresses becomes especially significant for population groups where cultural and social habits commonly include high weight-bearing daily activities of deep flexion such as squatting and kneeling. The role of the meniscus, cartilage and soft tissue to neutralise high stresses becomes increasingly diminished in larger flexion angles where the tibiofemoral contact is largely reduced [5]. Studies of deep flexion biomechanics in the past have been largely focused on the patellofemoral joint [6], [7], [8], [9], looking at forces and stresses in this joint and its impact on cartilage. The possibility of failure in the tibiofemoral articular cartilage from high stresses in deep flexion, perhaps a cause for the development of knee osteoarthritis in these compartments, has not been investigated.

Evidence shows that the tibiofemoral compartments are commonly involved in knee osteoarthritis where clinical observation indicates that the medial side is the compartment most frequently affected [10], [11] resulting in varus-knee deformities. Recent findings show that Chinese subjects have significantly more varus alignment of the lower extremity compared with Westerners [12]. The concurrence of varus-knee deformity and knee osteoarthritis [13] and the influence of race on the prevalence of knee osteoarthritis [14], [15] pointing to a significantly higher incidence of knee osteoarthritis among the Chinese and Japanese, raises some questions. If not at least it makes it relevant to further understand the relationship between knee joint mechanics and activity, and the consequential onset of osteoarthritis, in particular for our Asian populations. Mechanical factors for knee osteoarthritis in relation to differences that exist between Asian and Western populations groups should not be ignored. Such as the need for frequent deep flexion activity among Asian populations which remains a significant difference observed when compared with Western populations [16], [17].

The higher incidence of knee osteoarthritis in Asian populations where deep flexion activity is common, together with information from epidemiological studies conducted in non-Asian populations [18], [19], [20], make it meaningful to investigate further the cause-and-effect relationship of high stresses in the knee joint and the incidence of knee osteoarthritis from frequent deep flexion activity. The previous studies, conducted on Western population groups, show that physical factors such as excess weight [18], knee bending demands in the work place [19] and squatting [20] are positive factors that correlate significantly to the incidence of osteoarthritis of the knee.

As a starting point therefore, this study investigates the contact stresses that arise in the weight-bearing knee in various degrees of flexion simulating walking and squatting. It is envisaged that by determining the contact stresses in the natural knee joint, insight would be provided on the mechanical risks on the articular cartilage when an individual performs functional activities such as walking and squatting. The adequacy of the articular cartilage as a weight bearing structure for loads during these activities is subsequently discussed.

Section snippets

Method

In vitro mechanical testing was carried out in this study on five cadaver knees. Loading conditions in various phases of walking and squatting were derived from previous studies [1], [21] and applied to quasi-static mechanical testing on the cadaver knees in which pressure transducers were inserted in the articulation to measure contact stresses in these various phases. Flexion angles of 5.5°, 15.5°, and 4.5° were selected [22], [23] to represent knee flexion angles involved at heel strike,

Results

To achieve the ‘equilibrium’ position of the knee joint undergoing the 700–1000 N load, there was some significant adjustment in the joint orientation in the different degrees of flexion. In heel strike (HS) and toe-off (TO) simulations, the knee specimens adjusted such that there was a slight varus angulation of 2°. For the single limb stance (SLS) at 15.5° flexion, besides the varus angulations, some internal rotation had to be allowed as well. This internal rotation was about 5°. At deep

Discussion

Previous studies have used other methods to estimate contact stresses in the natural knee joint, such as, casting techniques, the use of pressure sensitive film and stereophotogrammetry [25]. A comparison of the stresses calculated in the present study is tabulated together with some of the previous reports (Table 1). The values from the present study appear to be larger than those from the previous study where pressure sensitive ink film was used. One of the issues of using the ink film is

Conclusion

The results from this biomechanical study suggest that the adequacy of articular cartilage to support loads in the knee joint during deep flexion may be questionable.

Acknowledgment

This work was supported by a research grant from the National Medical Research Council, Singapore.

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