Shoulder & Elbow

Understanding the glenoid in reverse shoulders X-ref

The reverse shoulder arthroplasty is currently viewed as a panacea, with the range of indications growing almost daily. Despite the early clinical results looking excellent, those naturally cautious surgeons are becoming increasingly concerned about the different biomechanics and how these may affect long-term longevity. A sound understanding of the biomechanics is likely to result in a clearer understanding of appropriate surgical technique and therefore improved longer-term surgical outcomes. We enjoyed this paper from Hsinchu (Taiwan) which examines the differences in stress variation seen with different designs of glenoid components in reverse total shoulder arthroplasties.⁴ The authors undertook a finite element analysis (FEA) study to determine the stress variations in the glenoid components. In what is one of the most accessible FEA papers on the topic, the investigators summarise succinctly the best methods for reducing stress at the glenoid component interface. The investigators conclude that distal placement of the glenosphere and lateral offset protects the glenoid from higher stresses at the baseplate junction. Conversely, inferiorly tilting the glenoid and use of the increased bony offset method will incur higher stresses at the glenoid screws, which in themselves have differential stress. The inferior screw suffers greater stresses than the superior, and this is concentrated around the base of the screw. In an environment with ever-increasing utilisation of reverse total shoulder replacement for a wide variety of indications, this paper is a good point for the inexperienced shoulder surgeon to begin to understand the biomechanics of the glenoid component in these replacements, and therefore how to choose the appropriate implant and alignment for a given indication.

Glenoid conformity and stress distribution

In an insightful look at the more traditional total shoulder arthroplasties, surgeons from New York (USA) investigate the potential advantages of non-conformity of the glenohumeral articulation.⁵ The interplay between constraint, conformity, wear and stress uncoupling has been investigated in knee arthroplasty, where less conforming implants offload sheer forces, although this has the potential disadvantage that smaller contact areas give rise to higher contact stresses and more wear. As the weak link in shoulder replacement continues to be the glenoid component, the study team constructed computer models of nine patients’ scapulae from CT scan images, and undertook analysis of three glenoid component designs: conforming, semi-conforming and hybrid designs. The finite element analysis modeling established that although the glenoid component was subjected to a similar level of maximum stresses at the centre, the conforming design was subject to significantly higher levels of maximum stress at the superior margin. There are clear differences in the designs of prosthesis, and the effect they have on the glenoid prosthesis interface. This should obviously be considered when designing new prostheses. However, given the range of glenoid designs currently available, the findings of this study could also be taken into account when undertaking anatomic total shoulder arthroplasty already.

Is cuff tear a genetic phenomenon? X-ref

Understanding the pathophysiology of orthopaedic disease is central to developing treatments (particularly biological) to combat a range of pathologies. Rotator cuff disease, like other enthesopathies (such as Dupuytren’s disease), has been shown to be due in part at least to matrix biology, with a range of pathways including the MMPs and WNT pathway implicated in the pathophysiology of disease. What has been inferred, but not necessarily proven up to this point, is that this may be a heritable factor. Research teams in Salt Lake City (USA) have undertaken the most complete genotyping study of patients both with and without rotator cuff tears.⁶ The study revolved around 311 patients with rotator cuff disease and 2641 genetically matched controls sourced from the Illumina Inc. (San Diego, CA) Controls database. The study involves full genotyping for specific heritable factors associated with rotator cuff disease. The authors have been able to identify two single nucleotide polymorphisms (SNPs) associated with rotator cuff tears. While clearly many of these injuries are traumatic in nature, an understanding based on this paper and other related studies is beginning to unravel the pathophysiology of degeneration in rotator cuff disease. Given time, it will be possible to understand how to discriminate the shoulder at risk of developing rotator cuff tears and perhaps even tailor treatments according to the underlying disease pattern.

MCID in reverse shoulder arthroplasty

Assessing outcomes is a complex topic. It is not enough to show just a simple statistical significance; that significance has to be clinically relevant. Clinically relevant differences are even more difficult to assess – clearly a small change on a score may not be noticed by a patient, but how much of a change in needed for the patient to subjectively say that their shoulder functions better than before? Investigators in Barcelona (Spain) have published their paper which carries a very important message: publication of patient outcome information is not enough, and it may be difficult to understand in terms of likely outcomes.⁷ The authors use a longitudinal cohort study of 60 patients, all with cuff deficient shoulders treated with a reverse shoulder arthroplasty. The ‘anchor’ questionnaire method is used in combination with the Constant score at the one-year follow-up in order to allow for calculation of the MCID for reverse arthroplasty in this group. During the course of the study the mean Constant score improved from 30 pre-operatively to 58 post-operatively. Although the composite scores increased by the MCID in around half of patients, the component scores varied, with just 20% of patients exceeding the MCID in forward flexion. The authors make a valid point concerning the need to reach the MCID. However, although just 50% of patients reached the MCID in this series, it is clear from previous methodology papers that the MCID should be taken for the overall score, not the subcomponents.

Superobesity and shoulder arthroplasty

The term ‘superobese’ is a relatively new one, and is usually taken to refer to patients with a BMI of 50+. However, as the incidence of obesity is increasing, more and more patients are presenting in the various stages of obesity, including ‘superobese’. Other than the subjective heart sink surgeons feel when dealing with patients with a large soft-tissue envelope due to the increasing technical difficulty of the surgery, there may well be also some specific risks of surgery to the superobese. Researchers in Charlottesville (USA) undertook a database study using the PearlDiver database to establish what the perceived effect of superobesity was on complications following shoulder arthroplasty.⁸ As would be expected, there were a large number of patients included in this study. The results of 144 239 patients, including 23 864 obese, 13 759 morbidly obese and 955 superobese patients, were reported. The study team was able to identify a significantly higher rate of major complications (including infection, dislocation, loosening, revision, VTE and medical complications) following shoulder arthroplasty in the superobese group. This paper outlines some early experience with this group of patients, and should inform surgeons and primary care physicians of the risks that obese patients face when undergoing shoulder arthroplasty surgery. Given that this group of patients is not going to disappear, it may be wise to examine measures to reduce complications and optimise outcomes while superobesity is still a rarity.