Fixation of humeral prostheses and axial micromotion

Time-elapsed microstructural imaging of failure of the reverse shoulder implant

BACKGROUND

Reverse Shoulder Arthroplasties (RSA) have become a primary choice for improving shoulder function and pain. However, the biomechanical failure mechanism of the humeral component is still unclear. The present study reports a novel protocol for microstructural imaging of the entire humerus implant under load before and after fracture.

METHODS

A humerus specimen was obtained from a 75-year-old male donor. An expert surgeon implanted the specimen with a commonly used RSA implant (Aequalis reversed II, Stryker Orthopaedics, USA) and surgical procedure. The physiological glenohumeral contact force that maximized the distal implant migration was selected from a public repository ( orthoload.com ). Imaging and concomitant mechanical testing were performed using a large-volume micro-CT scanner (Nikon XT H 225 ST) and a custom-made compressive stage. Both when intact and once implanted, the specimen was tested under a pre-load and by imposing a constant deformation causing a physiological reaction load (650 N, 10 degrees adducted). The deformation of the implanted specimen was then increased up to fracture, which was identified by a sudden drop of the reaction force, and the specimen was then re-scanned.

RESULTS

The specimen's stiffness decreased from 874 N/mm to 464 N/mm after implantation, producing movements of the bone-implant interface consistent with the implant's long-term stability reported in the literature. The micro-CT images displayed fracture of the tuberosity, caused by a combined compression and circumferential tension, induced by the distal migration of the implant.

CONCLUSION

The developed protocol offers detailed information on implant mechanics under load relative to intact conditions and fracture, providing insights into the failure mechanics of RSA implants. This protocol can be used to inform future implant design and surgical technique improvements.

Reverse total shoulder arthroplasty with proximal bone loss: a biomechanical comparison of partially vs. fully cemented humeral stems

BACKGROUND

The appropriate amount of cementation at the time of reverse total shoulder arthroplasty with significant proximal bone loss or resection is unknown. Extensive cementation of a humeral prosthesis makes eventual revision arthroplasty more challenging, increasing the risk of periprosthetic fracture. We analyzed the degree of subsidence and torque tolerance of humeral components undergoing standard cementation technique vs. our reduced polymethyl methacrylate (PMMA) protocol. Reduced cementation may provide sufficient biomechanical stability to resist physiologically relevant loads, while still permitting a clinically attainable torque for debonding the prosthesis.

METHODS

A total of 12 cadaveric humeri (6 matched pairs) underwent resection of 5 cm of bone distal to the greater tuberosity. Each pair of humeri underwent standard humeral arthroplasty preparation followed by either cementation using a 1.5-cm PMMA sphere at a location 3 cm inferior to the porous coating or standard full stem cementation. A 6-degree-of-freedom robot was used to perform all testing. Each humeral sample underwent 200 cycles of abduction, adduction, and forward elevation while being subjected to a physiologic compression force. Next, the samples were fixed in place and subjected to an increasing torque until implant-cement separation or failure occurred. Paired t tests were used to compare mean implant subsidence vs. a predetermined 5-mm threshold, as well as removal torque in matched samples.

RESULTS

Fully and partially cemented implants subsided 0.49 mm (95% CI 0.23-0.76 mm) and 1.85 mm (95% CI 0.41-3.29 mm), respectively, which were significantly less than the predetermined 5-mm threshold (P < .001 and P < .01, respectively). Removal torque between fully cemented stems was 45.22 Nm (95% CI 21.86-68.57 Nm), vs. 9.26 Nm (95% CI 2.59-15.93 Nm) for partially cemented samples (P = .021). Every fully cemented humerus fractured during implant removal vs. only 1 in the reduced-cementation group. The mean donor age in our study was 76 years (range, 65-80 years). Only 1 matched pair of humeri belonged to a female donor with comorbid osteoporosis. The fractured humerus in the partially cemented group belonged to that donor.

CONCLUSION

Partially and fully cemented humeral prostheses had subsidence that was significantly less than 5 mm. Partially cemented stems required less removal torque for debonding of the component from the cement mantle. In all cases, removal of fully cemented stems resulted in humeral fracture. Reduced cementation of humeral prostheses may provide both sufficient biomechanical stability and ease of future component removal.

Primary Stability and Bone Contact Loading Evaluation of Suture and Screw based Coracoid Graft Fixation for Anterior Glenoid Bone Loss

BACKGROUND

Reconstruction techniques for anterior glenoid bone loss have seen a trend from screws to suture-based fixations. However, comparative biomechanical data, including primary fixation and glenoid-graft contact pressure mapping, are limited.

HYPOTHESIS

Suture-based bone block cerclage (BBC) and suspensory suture button (SB) techniques provide similar primary fixation and cyclic stability to double-screw fixation but with higher contact loading at the bony interface.

STUDY DESIGN

Controlled laboratory study.

METHODS

In total, 60 cadaveric scapulae were prepared to simulate anterior glenoid bone loss with coracoid autograft reconstruction. Graft fixation was performed with 3 different techniques: (1) an interconnected all-suture BBC, (2) 2 SB suspensions, and (3) 2 screws. Initial compression was analyzed during primary fixation. Cyclic peak loading with 50 N and 100 N over 250 cycles at 1 Hz was performed with a constant valley load of 25 N. Optical recording and pressure foils allowed for spatial bone block tracking and contact pressure mapping at the glenoid-graft interface. Load-to-failure testing was performed at a rate of 1.5 mm/s with ultimate load and stiffness measured.

RESULTS

Initial graft compression was higher with screw fixation (141 ± 5 N) compared with suture-based fixations (P < .001), with BBC fixation providing significantly higher compression than SB fixation (116 ± 7 N vs. 91 ± 5 N; P < .001). Spatial bone block migration and ultimate failure load were similar between the BBC and screw groups. The SB group showed significantly increased bone block translation (3.1 ± 1.0 mm; P≤ .014) and rotation (2.5°± 1.4°; P≤ .025) and significantly lower ultimate failure load (180 ± 53 N) compared with the BBC (P = .046) and screw (P = .002) groups. Both suture-based fixations provided significantly increased graft-glenoid contact loading with higher pressure amplitudes (P≤ .032) and contact pressure after cyclic loading (+13%; SB: P = .007; BBC: P = .004) compared with screw fixation.

CONCLUSION

Both SB and interconnected cerclage fixation improved dynamic contact loading compared with screw fixation in a biomechanical glenoid bone loss model. Cerclage fixation was biomechanically comparable with screw fixation but with a greater variability. SB fixation showed significantly lower primary fixation strength and greater bone block rotation and migration.

CLINICAL RELEVANCE

Suture-based bone block fixations improved graft-glenoid contact loading, but the overall clinical consequence on healing remains unclear.

Early fixation of the humeral component in stemless total shoulder arthroplasty : a radiostereometric and clinical study with 24-month follow-up

AIMS

Stemless humeral implants have been developed to overcome stem-related complications in total shoulder arthroplasty (TSA). However, stemless implant designs may hypothetically result in less stable initial fixation, potentially affecting long-term survival. The aim of this study is to investigate early fixation and migration patterns of the stemless humeral component of the Simpliciti Shoulder System and to evaluate clinical outcomes.

METHODS

In this prospective cohort study, radiostereometric analysis (RSA) radiographs were obtained in 24 patients at one day, six weeks, six months, one year, and two years postoperatively. Migration was calculated using model-based RSA. Clinical outcomes were evaluated using the visual analogue scale (VAS), the Oxford Shoulder Score (OSS), the Constant-Murley Score (CMS), and the Disabilities of the Arm, Shoulder and Hand (DASH) score.

RESULTS

At two years, median translation along the x-, y-, and z-axis was -0.12 mm (interquartile range (IQR) -0.18 to 0.02), -0.17 mm (IQR -0.27 to -0.09), and 0.09 mm (IQR 0.02 to 0.31). Median rotation around the x-, y-, and z-axis was 0.12° (IQR -0.50 to 0.57), -0.98° (IQR -1.83 to 1.23), and 0.09° (IQR -0.76 to 0.30). Overall, 20 prostheses stabilized within 12 months postoperatively. Four prostheses showed continuous migration between 12 and 24 months. At two-year follow-up, with the exception of one revised prosthesis, all clinical scores improved significantly (median VAS difference at rest: -3.0 (IQR -1.5 to -6.0); OSS 22.0 (IQR 15.0 to 25.0); CMS 29.5 (IQR 15.0 to 35.75); and DASH -30.0 (IQR -20.6 to -41.67) (all p < 0.001)) with the exception of one revised prosthesis.

CONCLUSION

In conclusion, we found that 20 out of 24 implants stabilized within 12 months postoperatively. The significance of continuous migration in four implants is unclear and future research on the predictive value of early migration for future loosening in TSA is required. Clinical results revealed a clinically relevant improvement. Cite this article: Bone Joint J 2022;104-B(1):76-82.

Cemented humeral stem versus press-fit humeral stem in total shoulder arthroplasty: a systematic review and meta-analysis

AIMS

The aim of this study was to evaluate the differences in revision and complication rates, functional outcomes, and radiological outcomes between cemented and press-fit humeral stems in primary anatomical total shoulder arthroplasty (TSA).

MATERIALS AND METHODS

A comprehensive systematic review and meta-analysis was conducted searching for studies that included patients who underwent primary anatomical TSA for primary osteoarthritis or rheumatoid arthritis.

RESULTS

There was a total of 36 studies with 927 cemented humeral stems and 1555 press-fit stems. The revision rate was 5.4% (95% confidence interval (CI) 3.9 to 7.4) at a mean of 89 months for cemented stems, and 2.4% (95% CI 1.1 to 4.7) at a mean of 40 months for press-fit stems. A priori subgroup analysis to control for follow-up periods demonstrated similar revision rates: 2.3% (95% CI 1.1 to 4.7) for cemented stems versus 1.8% (95% CI 1.4 to 2.9) for press-fit stems. Exploratory meta-regression found that longer follow-up was a moderating variable for revision (p = 0.003).

CONCLUSION

Cement fixation had similar revision rates when compared to press-fit stems at short- to midterm follow-up. Rotator cuff pathology was a prevalent complication in both groups but is likely not related to fixation type. Overall, with comparable revision rates, possible easier revision, and decreased operative time, humeral press-fit fixation may be an optimal choice for primary anatomical TSA in patients with sufficient bone stock. Cite this article: Bone Joint J 2019;101-B:1107-1114.

The Humeral Implant in Shoulder Arthroplasty

Humeral hemiarthroplasty, anatomic total shoulder arthroplasty, and reverse total shoulder arthroplasty all rely on a prosthetic articular surface fixed to the proximal humerus. Humeral implant designs have changed considerably as a result of improved understanding of proximal humeral anatomy and prosthetic biomechanics. Fixed, monoblock implants have been superseded by modular implants with variable inclination, offset, version, and stem length. Press-fit designs now commonly have surface coatings that allow bony ingrowth. Metaphyseal fixation is often favored over diaphyseal fixation. Both cemented and noncemented fixation continue to be used, and each of these techniques has advantages and disadvantages. Although aseptic loosening rarely requires revision, complications, such as osteolysis, stress shielding, radiolucent lines, and proximal humeral bone loss, can occur. Humeral periprosthetic fractures continue to be a disabling complication and are difficult to manage. Innovations such as short-stemmed implants, stemless implants, and platform stems are currently under clinical investigation.

Long-term outcomes of cemented versus cementless humeral components in arthroplasty of the shoulder

Aims

In the initial development of total shoulder arthroplasty (TSA), the humeral component was usually fixed with cement. Cementless components were subsequently introduced. The aim of this study was to compare the long-term outcome of cemented and cementless humeral components in arthroplasty of the shoulder.

Patients and Methods

All patients who underwent primary arthroplasty of the shoulder at our institution between 1970 and 2012 were included in the study. There were 4636 patients with 1167 cemented humeral components and 3469 cementless components. Patients with the two types of fixation were matched for nine different covariates using a propensity score analysis. A total of 551 well-balanced pairs of patients with cemented and cementless components were available after matching for comparison of the outcomes. The clinical outcomes which were analysed included loosening of the humeral component determined at revision surgery, periprosthetic fractures, post-operative infection and operating time.

Results

The overall five-, ten-, 15- and 20-year rates of survival were 98.9%, 97.2%, 95.5%, and 94.4%, respectively. Survival without loosening at 20 years was 98% for cemented components and 92.4% for cementless components. After propensity score matching including fixation as determined by the design of the component, humeral loosening was also found to be significantly higher in the cementless group. Survival without humeral loosening at 20 years was 98.7% for cemented components and 91.0% for cementless components. There was no significant difference in the risk of intra- or post-operative fracture. The rate of survival without deep infection and the mean operating time were significantly higher in the cemented group.

Conclusion

Both types of fixation give rates of long-term survival of > 90%. Cemented components have better rates of survival without loosening but this should be weighed against increased operating time and the risk of bony destruction of the proximal humerus at the time of revision of a cemented humeral component. Cite this article: Bone Joint J 2017;99-B:666–73.

In vitro initial stability of a stemless humeral implant

BACKGROUND

Stemless humeral prostheses have been recently introduced. We measured for the first time their in vitro primary stability and analyzed the influence of three clinically important parameters (bone quality, implant size and post-operative loading) on micromotion. We also assessed if displacement sensors are appropriate to measure implant micromotion.

METHODS

A stemless humeral implant (Sidus® Stem-Free Shoulder, Zimmer GmbH, Winterthur, Switzerland) was implanted in 18 cadaveric humeri. Three-dimensional motion of the implant was measured under dynamic loading at three load magnitudes with displacement sensors. Additionally, the relative motion at the bone-implant interface was measured with an optical system in four specimens.

RESULTS

Micromotion values derived from the displacement sensors were significantly higher than those measured by the optical system (P<0.005). Analysis of variance (ANOVA) indicated that bone density (P<0.0005) and load (P<0.0001) had a significant effect on implant micromotion, however the effect of implant size was not statistically significant (P=0.123).

INTERPRETATION

Micromotion of this stemless design was shown to be significantly dependent on cancellous bone density. Patients must therefore have adequate bone quality for this procedure. The influence of load magnitude on micromotion emphasizes the need for controlled post-operative rehabilitation. Measurements with displacement sensors overestimate true interface micromotion by up to 50% and correction by an optical system is strongly recommended.

Results of uncemented hemiarthroplasty as primary treatment of severe proximal humerus fractures in the elderly

OBJECTIVES

The purpose of this study was to analyze functional and radiologic results of a consecutive series of elderly patients who underwent uncemented hemiarthroplasty as primary treatment of complex proximal humeral fractures.

METHODS

Twenty-one patients with severe proximal humerus fracture (three or four fragments and three- or four-fragment fracture dislocations) were treated with uncemented proximal humerus hemiarthroplasty. Patients were evaluated using the Constant-Murley Score, the Quick scale Disabilities of Arm, Shoulder and Hand (Quick-DASH) Score, range of motion, residual pain, radiographic parameters, and complications including clinical and radiologic data of mobilization.

RESULTS

After a mean postoperative period of 20.57 months (range 12-42 months), the mean Constant-Murley Score was 44 points (20-57), the mean Quick-DASH score was 24 points (16-39), postoperative pain according to a mean visual analogue scale was 1 (0-8), active abduction was 50° (30-135), and active flexion 70° (20-120). There were no cases of infection, deep vein thrombosis, dislocation, blood transfusions, or reoperation because of prosthetic loosening. Bivariate analysis of demographic data, radiologic findings, and other variables showed associations between duration of surgery and a higher Quick-DASH score (0.606; p = 0.037), and the number of sessions of rehabilitation with a higher Quick-DASH score (0.708; p = 0.015).

CONCLUSIONS

The results of treatment of severe proximal humerus fractures in the elderly with an uncemented hemiarthroplasty are safe and promising; however, a comparative cohort study (cemented vs. uncemented) and long-term follow-up are still needed.

Short-stem uncemented primary reverse shoulder arthroplasty

Short uncemented humeral stems can preserve humeral bone stock and facilitate revision surgery but may be prone to malalignment or loosening, especially when used in reverse total shoulder replacement (TSR). We undertook a retrospective review of 44 patients with a mean age of 76 years (59 to 92) who underwent primary reverse TSR with a short uncemented humeral stem. There were 29 females. The indications for joint replacement included cuff tear associated arthropathy (33), avascular necrosis (six), post-traumatic arthritis (two), and inflammatory arthritis (three). At a mean follow-up of 27 months (24 to 40), pain was rated as mild or none in 43 shoulders (97.7%). The mean active elevation improved from 54° (sd 20°) to 142° (sd 25°) and the mean active external rotation from 14° (sd 13°) to 45° (sd 9°). The outcome, as assessed by the modified Neer score, was excellent in 27 (61.3%), satisfactory in 15 (34.1%), and unsatisfactory in two shoulders (4.5%). Stems were well-positioned, without evidence of significant valgus or varus malalignment in 42 TSRs (95.5%). There was no radiological evidence of loosening of the humeral stem in any patient; 13 TSRs (29.5%) had evidence of proximal humeral remodelling and scapular notching was noted in three (6.8%).

Cite this article: Bone Joint J 2014;96-B:526–9.

Cemented versus uncemented fixation of humeral components in total shoulder arthroplasty for osteoarthritis of the shoulder: a prospective, randomized, double-blind clinical trial-A JOINTs Canada Project

BACKGROUND

Although cemented humeral fixation is recognized as the standard of care in total shoulder arthroplasty (TSA), uncemented fixation has the potential to provide stable fixation, decrease operative time, and simplify potential revision procedures. This prospective, randomized, double-blind clinical trial compared cemented and uncemented humeral fixation in TSA for primary shoulder osteoarthritis.

METHODS

Patients with primary shoulder osteoarthritis requiring replacement were screened for eligibility. After providing informed consent, subjects received baseline clinical and radiologic assessments, computed tomography scans, and standardized TSA. After glenoid component insertion, patients were randomized to either a cemented or uncemented humeral component. The primary outcome was the WOOS (Western Ontario Arthritis of the Shoulder Index) score at 2 years. Other outcomes included the Short Form 12 score, American Shoulder and Elbow Surgeons score, McMaster-Toronto Arthritis Patient Preference Disability Questionnaire, operative time, complications, and revisions. Patients were assessed by a blinded evaluator at 2 and 6 weeks and 3, 6, 12, 18, and 24 months postoperatively.

RESULTS

In total, 161 patients consented to be included and were randomized: 80 in the cemented group and 81 in the uncemented group. There were no significant differences in demographics or baseline evaluations between groups, except for gender. The 12-, 18-, and 24-month WOOS scores showed a significant difference in favor of the cemented group. The cemented group also had better strength and forward flexion. As expected, the operative time was significantly less for the uncemented group.

CONCLUSIONS

These findings provide level I evidence that cemented fixation of the humeral component provides better quality of life, strength, and range of motion than uncemented fixation.

Primary uncemented hemiarthroplasty for severe fractures of the proximal humerus

OBJECTIVES

The purpose of this study is to evaluate a consecutive series of proximally porous-coated hemiarthroplasty for the treatment of severe proximal humeral fractures with special emphasis on aseptic loosening and functional outcome.

DESIGN

Observational cohort of a consecutive series of a single prosthetic design.

SETTING

Orthopaedic clinic, nonacademic institutional hospital.

PATIENTS/PARTICIPANTS

All patients (n = 30) treated with hemiarthroplasty for severe fractures of the proximal humerus with informed consent and at least 1 year follow-up were included. Follow-up averaged 37 months (range, 13-62 months).

INTERVENTION

Uncemented proximally porous-coated hemiarthroplasty.

MAIN OUTCOME MEASUREMENTS

We evaluated clinical and radiologic aseptic loosening, Constant score, patient satisfaction and visual analog scale score, range of motion, radiologic parameters such as head-to-tuberosity distance, lateral projection, and lateral offset.

RESULTS

: The overall Constant score was 68 (standard deviation [SD] 18), the mean patient satisfaction 7.1 (SD 1.8), and the average visual analog scale score 3.7 (SD 2.5). We observed no cases of radiographic tilting or subsidence. In 17 cases (57%), radiolucent lines 1.0 mm or less were observed in less than three zones. None of the cases met the criteria for radiographic loosening. No revision was performed for aseptic loosening. The average head-to-tuberosity distance was 7.6 (SD 8.7 mm). We found that traumatic neurologic deficit and head-to-tuberosity distance significantly influenced Constant score. Lateral projection significantly correlated with Constant score, active forward flexion, and abduction.

CONCLUSIONS

Proximally porous-coated hemiarthroplasty is a valuable alternative alongside cemented hemiarthroplasty for the treatment of severe fractures of the proximal humerus. The radiographic finding of increased lateral projection was associated with an improved range of motion.

Stem diameter and micromotion of press fit radial head prosthesis: a biomechanical study

BACKGROUND

Aseptic loosening of the stem, ranging from mild periprosthetic lucency to symptomatic loosening leading to implant removal, has been reported in press fit radial head prostheses.

HYPOTHESIS

The purpose of this study was to determine the effect of the stem diameter and insertion force on initial stability with a press fit radial head prosthesis designed for bone ingrowth.

MATERIALS AND METHODS

Cadaveric radii were implanted with radial head prostheses of increasing stem diameter. The insertion forces for each rasp and stem were measured. Micromotion of the stem at the isthmus of the canal and stem tip were measured under circumstances simulating eccentric loads.

RESULTS

Insertion forces for all submaximum-sized rasps were similar. However, the insertion force for the maximum-sized rasp was approximately twice as large, and the insertion force for the oversized rasp was twice as large again, potentially indicating that the insertion force may be useful as a guide for determining appropriate stem size. Micromotions of the maximum diameter stem (isthmus, 41 microm; tip, 64 microm) were near the threshold for bone ingrowth, whereas the micromotions of the submaximum stem (isthmus, 253 microm; tip, 394 microm) were above this threshold.

DISCUSSION

The maximum diameter stem achieved greater stability (minimum micromotion) compared with the submaximum diameter stem.

CONCLUSION

The best fixation strength in the press fit radial head prosthesis was achieved by maximum sizing in the neck canal.

Fixation for the millennium: the shoulder

Charles Neer designed his original prosthesis in 1951. It was made of cast cobalt chrome alloy and was only press-fit. There was not any significant loosening in his original series, but patients complained of easy fatigability. This finding was attributed to glenoid issues and a cemented glenoid replacement was introduced in 1973. High rates of lucencies were reported and raised issues regarding fixation. Cementless glenoid components have been found to have fewer lucencies but require metal backing. Higher failure rates have been reported for the cementless design versus the cemented glenoids. With humeral fixation, symptomatic loosening is not a problem with cement or coated press-fit fixation. Long-term outcome studies with cemented and uncemented stems have shown the incidence of humeral loosening to be low. Fixation of humeral components should be based on bone quality, patient age, prosthetic design, and surgeon preference.