The biomechanics of reverse anatomy shoulder replacement – A modelling study
Introduction
The normal shoulder is required to have basic mechanical characteristics of range of motion (mobility), stability and strength. However, each of these is commonly compromised in an arthritic or rotator cuff tear shoulder. Arthoplasty is a common solution for immediate pain relief and to restore shoulder functionality (Neer et al., 1982).
Many prostheses are used to address these pathologies and the designs vary in shape and dimensions and in some cases try to address stability issues by fully constraining the joint (Brostrom et al., 1992, Post et al., 1980). Reverse anatomy prostheses are introduced as an alternative solution in challenging pathologies like the arthritic shoulder with massive rotator cuff (RC) tears (Kessel and Bayley, 1979).
The DELTA® prosthesis (produced by DePuy) was first developed by (Grammont et al., 1987), but further modifications were made to the original prosthesis until the Delta® III version (Grammont et al., 1987, Grammont and Baulot, 1993) which has been extensively used in rotator cuff tear arthropathies. There are many clinical reviews reporting stability and good performance of this design (Sirveaux et al., 2004, Woodruff et al., 2003), but also indicating problems of impingement and glenoid loosening after long term use (Nyffeler et al., 2004). There are now a number of competing prostheses in the United Sates (e.g. from Encore, and Zimmer) and in Europe (e.g. Lima, Aston, Tornier, Implants Industries) following the same basic design but with modifications intended to address the above problems.
Despite the longterm use and the popularity of the Grammont type prosthesis, there are only few two dimensional (De Wilde et al., 2004) or three dimensional biomechanical studies (Terrier et al., 2008, Van der Helm, 1998) focusing either on muscle and joint contact analysis or impingement (Nyffeler et al., 2005) without examining how the different aspect of the reverse designs can affect the general joint performance. In this study we try to analyse in more depth the biomechanical properties of the DELTA® (and in general the reverse) design using an interactive biomechanical model of the upper limb (Charlton and Johnson, 2006). The comparison of the results will also lead to a discussion of the relative importance of the implantation and the design features of reverse prostheses and how they can affect the loading, stability and impingement of the prosthetic joint.
The above aim will be achieved by comparing modelling data between the reverse prosthetic and normal anatomy shoulder model on the following aspects:
- (a)
Lengthening and moment arms of deltoid and RC muscles.
- (b)
Predicted muscle and joint contact forces during standardised activities.
- (c)
Range of arm motion in reverse shoulder and prediction of impingement.
Section snippets
The reverse prosthetic biomechanical model
This modelling study was performed using a modified version of the three dimensional Newcastle upper limb model (Charlton and Johnson, 2006) which in its original state represent a normal anatomy shoulder and elbow. The model consists of six rigid bone segments; the thorax, clavicle, scapula, humerus, radius and ulna. The various bony structures and landmarks were digitised from the Visible Human male dataset (Spitzer and Whitlock, 1998). The segments are connected by three spherical joints
Muscle lengthening and moment arms after reverse joint replacement
After the DELTA® III replacement, the humerus (and as a result the whole arm) is positioned more medially and inferiorly compared to the normal shoulder, something that can be even observed visually in real subjects (Boileau et al., 2005). As a result the deltoid muscle is stretched giving a passive tension to the GH joint. The predicted lengthening of the middle deltoid was 21.6% compared to normal anatomy (Table 1). It is reported that deltoid extension more than 20%, could damage the
Moment arms, stability and loading
One of the most interesting results of the reverse geometry is how it affects the moment arms (and furthermore the function) of the muscles crossing the GH joint and especially the deltoid. The predicted results for the increased deltoid moment arms of this study agree well with the study of De Wilde et al., 2004, Van der Helm, 1998 where they used the same DELTA® geometry (Table 1). Terrier et al. (2007) using a similar type of prosthesis (Aequalis® Reversed Shoulder Prosthesis, Tornier)
Conclusions – stability over mobility
The results of this study showed the advantages of a reverse prosthesis, where the increased deltoid moment arm helps the muscle to elevate the arm compensating for the dysfunctional RC muscles. The prosthetic design also reverses the envelope of the forces providing a large glenoidal surface and stability to the increased shear forces.
The biomechanical model also confirms the impingement as the main problem on the reverse prosthesis and predicts scapular notches. The results show how the
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