Optimizing Mini-stem Humeral Component Design in Shoulder Arthroplasty

Thomas Obermeyer , MD

Read complete study: Proximal ingrowth coating decreases risk of loosening following uncemented shoulder arthroplasty using mini-stem humeral components and lesser tuberosity osteotomy

Innovation in shoulder arthroplasty prosthesis design has led to a paradigm shift in recent years to “platform” (convertible) humeral stems that obviate the need for stem extraction when converting between non-constrained anatomic total shoulder arthroplasty and reverse arthroplasty.1  Paralleling this has been a transition to mini stems that shift the location of humeral fixation from the diaphysis to the metaphysis (Figure 1).  Shorter, metaphyseal-fixing stems preserve humeral bone stock and eliminate the need for diaphyseal fixation, which eases revision and may improve long-term fixation.  Mini humeral stems have performed well in short-term follow-up studies and achieve the very favorable pain relief and functional improvement expected in total shoulder arthroplasty.2

Fig 1. Platform mini humeral stems can be converted from nonconstrained to reverse arthroplasty without resecting a well fixed stem. This stem has proximal porous coating and aggressive tapering to maximize proximal fixation.

The potential benefits of mini humeral stems are numerous.  More proximal metaphyseal fixation may eliminate the need for extended humeral osteotomy when extraction is necessary and retain native distal metaphyseal and diaphyseal bone for subsequent fixation in the revision setting.  Shorter stems may also diminish stress shielding, where the bone surrounding the proximal portion of the prosthesis remains relatively unloaded, leading to proximal bone resorption which may jeopardize long term fixation.3  Periprosthetic fracture management is often easier and shorter stems have greater applicability to preexistent humeral conditions such as post-fracture deformities and insertion above a total elbow arthroplasty.

Until recently there has not been evidence to guide specific mini stem prosthesis design.  Original mini stem designs were uncoated and had high rates of radiolucent lines and clinical loosening.4,5 The recent article by Morwood et al suggests that adding proximal porous coating to mini humeral stems may improve on early uncoated designs, with improved rates of loosening and proximal radiolucent lines.  Despite short term follow-up at two years, their retrospective analysis saw a significantly lower rate of proximal radiolucent lines and no clinical loosening in proximally coated mini humeral stems.  These are promising results.

Time will tell if recent design innovations to humeral mini stems will generate improvements in long-term shoulder replacement outcomes, but I remain optimistic.

Perhaps a concerning finding in Morwood’s study was that despite the improved fixation observed at two years with proximal porous coating, there was a 21% rate of radiolucencies.  This is slightly worrisome as the study sample size is small and two years is relatively short term follow-up in an arthroplasty study.  Will the radiolucencies progress to clinical loosening if the follow-up is prolonged to five or ten years?  This begs the question: can humeral component design be improved yet again so that, despite proximal coating, an even lower rate of radiolucent lines may be observed?  My hypothesis is yes, based on 1) generating more substantial carve-outs in the stem so that the prepared metaphyseal bone will permit improved rotational stability immediately at the time of press-fitting (Figures 2 and 3), 2) producing a more aggressive proximal press-fit so that proximal cancellous bone is more substantially loaded and the reliance on fixation at more distal metaphyseal bone is minimal, 3) avoiding distal contact of the stem with the humeral cortex, which will accelerate stress shielding (Figure 1).   Time will tell if recent design innovations to humeral mini stems will generate improvements in long-term shoulder replacement outcomes, but I remain optimistic.

Fig. 2. A potential design with more substantial carve-outs in the mini stem and aggressive tapering that maximizes proximal fixation may improve on the rate of radiolucencies observed in the Morwood study.

Fig. 3. Immediate rotational stability may be achieved by carve-outs in the broach and stem that leave this pattern of prepared cancellous bone.

 

References:
1. Crosby et al. Conversion to Reverse Total Shoulder Arthroplasty with and without Humeral StemRetention: The Role of a Convertible-Platform Stem. J Bone Joint Surg Am. 2017 May 3;99(9):736-742.
2. Harmer et al. Total shoulder arthroplasty: are the humeral components getting shorter? Curr Rev Musculoskelet Med 2016;9:17-22.
3. Raiss et al.  Radiographic changes around humeral components in shoulder arthroplasty. J Bone Joint Surg Am. 2014 Apr 2;96(7):e54.
4. Casagrande et al. Radiographic evaluation of short-stem press-fit total shoulder arthroplasty: short-term follow-up. J Shoulder Elbow Surg. 2016;25:1163-9.
5. Schnetzke et al. Radiologic bone adaptations on a cementless short-stem shoulder prosthesis. J Shoulder Elbow Surg 2016;25:650-7.

Thomas Obermeyer, MD, is a board-certified and fellowship-trained orthopaedic surgeon in Illinois, specializing in shoulder and elbow reconstruction and sports injuries. Dr. Obermeyer received his medical degree from Albany Medical College and completed his residency at Loyola University Medical Center in Chicago. He went on to complete a fellowship in shoulder and elbow at Mount Sinai Medical Center in New York City. Dr. Obermeyer is also an award-winning researcher and published author.​

Advertisements

How Should We Approach Superior Glenoid Wear?

Kevin Famer, MD

Read complete study: Reverse total shoulder glenoid baseplate stability with superior glenoid bone loss

In the article “Reverse total shoulder glenoid baseplate stability with superior glenoid bone loss” by Martin et al, the authors looked to quantify glenoid baseplate stability with worsening superior glenoid bone loss.  The authors utilized a polyurethane bone model, and created superior glenoid bone loss defects such that the attached baseplates had 100% support, 90% support, 75% support, and 50% support.  The authors found the 50% support group had significantly greater micromotion than the other defects and the native state.  The majority of micromotion occurred at the beginning of testing, indicating that some settling may occur with time.  Interestingly, micromotion in the 50% support group exceeded 150 µm, which has been shown in animal models to be the maximum amount of micromotion that allows bony ingrowth.

The authors spend a lot of time describing their testing methods, compared to other published manuscripts.  The authors applied cyclic loading at a fixed 60º glenohumeral angle, which mimics the superiorly directed force during the initiation of abduction, as opposed to other studies that apply force to the baseplate that mimics the force during the range of abduction.  The benefits of the study design of this current study is that it allows real time assessment of micromotion during the course of the analysis, as well as pre and post testing, potentially allowing a better assessment of micromotion early in the loading process.

The information presented in this study is valuable in the sense that using real time of assessment of micromotion, the authors find that increasing superior glenoid wear leads to increasing superior micromotion early in the loading process.  There also appears to be some settling over time, but the issue failure of early bony integration, and its association to loosening over time, is a concern.  Surgeons should be aware of results of this study, and the potential risk of glenoid loosening with uncorrected superior glenoid wear.

When approaching superior glenoid wear, surgeons have three main options.

Continue reading