Fracture risk and initial fixation of a cementless glenoid implant: the effect of numbers and types of screws.

The initial fixation of an anatomical cementless glenoid component, provided by different numbers and types of screws, and the risk of bone fracture were evaluated by estimating the bone-implant interface micromotions and the principal strains around the prosthesis. Four different fixation configurations using locking or compression screws were tested. Estimation of the micromotions at the bone-implant interface was performed both experimentally, using an in vitro model, and computationally, using a numerical model. Principal bone strains were estimated using the numerical model. Subject variability was included by modelling two different bone qualities (healthy and rheumatoid bone). For the fixation configurations that used two screws, experimental and modelling results found that the micromotions at the bone-implant interface did not change with screw type. However, screw type had a significant effect on fixation when only one screw was used; in this case, a locking screw resulted in less micromotion at the bone-implant interface compared with the compression screw. Bone strains were predicted by the numerical model, and strains were found to be independent of the screw type; however, the predicted strain levels calculated in rheumatoid bone were larger than the strain levels that may cause bone damage for most considered arm positions. Predicted bone strain in healthy bone did not reach this level. While proper initial component fixation that allows biological fixation can be achieved by using additional screws, the risk of bone failure around the screws must be considered, especially in cases of weak bone.

Interface micromotions increase with less-conforming cementless glenoid components.

BACKGROUND: The optimal degree of conformity between the glenoid and humeral components in total shoulder arthroplasty for best performance and durability is still a matter of debate. The main aim of this study is to evaluate the influence of joint conformity on the bone-implant interface micromotions in a cementless glenoid implant. MATERIALS AND METHODS: Polyethylene inlays with different degrees of conformity (radial mismatch of 0, 2, 4, and 6 mm) were mounted on a cementless metal back and then implanted in a bone substitute. These glenoid components were loaded by a prosthetic humeral head during a force-controlled experiment. Normal-to-interface micromotions and bone substitute deformations were measured at different points of the interface. Rim displacement and humeral head translation were also measured. A finite element (FE) model of the experiments was implemented to estimate the normal- and tangent-to-interface micromotions in the entire bone-implant interface. RESULTS: All measured variables increased with less-conforming PE inlays. Normal-to-interface micromotions were significantly larger (P < .05) when the radial mismatch was 6 mm compared with the fully conforming inlay. The FE model was in agreement and complemented the experimental results. FE model-predicted interface micromotions were already significantly larger when the radial mismatch was equal to 4 mm. DISCUSSION: In a force-controlled experiment with a cementless glenoid component, a non-conforming PE inlay allows larger interface micromotions than a conforming inlay, reaching a magnitude that may hamper local bone ingrowth in this type of component. This is mainly because of the larger humeral head translation that boosts the effects of the so-called rocking-horse phenomenon.

Prognostic factors in arthroplasty in the rheumatoid shoulder.

Total shoulder arthroplasty is commonly considered a good option for treatment of the rheumatoid shoulder. However, when the rotator cuff and glenoid bone stock are not preserved, the clinical outcome of arthroplasty in the rheumatoid patients remains unclear. Aim of the study is to explore the prognostic value of multiple preoperative and peroperative variables in total shoulder arthroplasty and shoulder hemiarthroplasty in rheumatoid patients. Clinical Hospital for Special Surgery Shoulder score was determined at different time points over a mean period of 6.5 years in 66 rheumatoid patients with total shoulder arthroplasty and 75 rheumatoid patients with shoulder hemiarthroplasty. Moreover, radiographic analysis was performed to assess the progression of humeral head migration and glenoid loosening. Advanced age and erosions or cysts at the AC joint at time of surgery were associated with a lower postoperative Clinical Hospital for Special Surgery Shoulder score. In total shoulder arthroplasty, status of the rotator cuff and its repair at surgery were predictive of postoperative improvement. Progression of proximal migration during the period after surgery was associated with a lower clinical score over time. However, in hemiarthroplasty, no relation was observed between the progression of proximal or medial migration during follow-up and the clinical score over time. Status of the AC joint and age at the time of surgery should be taken into account when considering shoulder arthroplasty in rheumatoid patients. Total shoulder arthroplasty in combination with good cuff repair yields comparable clinical results as total shoulder arthroplasty when the cuff is intact.

Teres major muscle activation relates to clinical outcome in tendon transfer surgery.

BACKGROUND: In massive rotator cuff tears a teres major (TMj) tendon transfer to the insertion of the supraspinatus (SSp) reverses its adduction moment arm into abduction which is supposed to be an adequate salvage procedure. Analysis of muscle function to find biomechanical ground of such success is scarce. METHODS: We compared pre- and postoperative clinical outcome of TMj transfer, i.e. Range of Motion, pain, Constant Shoulder scores and arm force. TMj activation was evaluated in 14 patients suffering massive cuff tears using activation ratios to describe the desired 'in-phase' and undesired 'out-of-phase' contribution to the external arm moment. Additionally, we analyzed activation of the latissimus dorsi (LD) and the medial part of the deltoids (DE). The activation ratios were compared to controls and TMj activation ratios were related to clinical outcome. FINDINGS: TMj tendon transfer improved arm function. Pre-operatively, we observed 'out-of-phase'abduction activation of TMj and LD. After transfer patients activated TMj according to its new anatomical position. 'Out-of-phase' LD abduction activation persisted. The clinical improvements coincided with changes in activation ratio of TMj. INTERPRETATION: 'Out-of-phase' TMj adductor activation is associated with compromised arm function in patients with irreparable cuff tears. After transfer, TMj is activated in correspondence with its new anatomical function, which was supportive for the improved arm function.

Effect of rotator cuff dysfunction on the initial mechanical stability of cementless glenoid components.

The functional outcome of shoulder replacement is related to the condition of the rotator cuff. Rotator cuff disease is a common problem in candidates for total shoulder arthroplasty; this study relates the functional status of the rotator cuff to the initial stability of a cementless glenoid implant. A 3D finite element model of a complete scapula was used to quantify the effect of a dysfunctional rotator cuff in terms of bone-implant interface micromotions when the implant is physiologically loaded shortly after surgery. Four rotator cuff conditions (from fully intact to progressively ruptured rotator cuff tendons) as well as two bone qualities were simulated in a model. Micromotions were significantly larger in the worst modeled cuff dysfunction (i.e. the supraspinatus and infraspinatus tendons were fully dysfunctional). Micromotions were also significantly different between conditions with healthy and poor bone quality. The implant's initial stability was hardly influenced by a dysfunctional supraspinatus alone. However, when the infraspinatus was also affected, the glenohumeral joint force was displaced to the component's rim resulting in larger micromotions and instability of the implant.

Clinical implications of rotator cuff degeneration in the rheumatic shoulder.

OBJECTIVE: In rheumatoid arthritis (RA) of the shoulder, loss of cartilage and soft tissue degeneration coexists with pain and reduced range of motion. We evaluated the presence of bony and rotator cuff degeneration in RA of the shoulder joint and assessed their relationship with pain and loss of functioning. We hypothesized that rotator cuff degeneration plays an important role in the presence of pain and loss of functioning of the rheumatic shoulder. METHODS: We used a cross-sectional study to assess both bony and rotator cuff involvement using plain anteroposterior radiographs, ultrasound, and computed tomography images. Additionally, we used an electromagnetic tracking device and a force transducer to evaluate range of motion and maximum force of the shoulder muscles. Between January 2003 and July 2004 we included 26 consecutive patients (51 shoulders). Twenty-one shoulders showed no or slight joint destruction, 15 showed intermediate destruction, and 15 showed severe destruction. RESULTS: Only 19 shoulders showed an intact rotator cuff. Proximal migration of the humeral head and fatty degeneration of the infraspinatus muscle especially showed a significantly strong correlation with increased pain and function loss (R2 = 0.36, P < 0.001). In a multivariate regression analysis, proximal migration and fatty degeneration of the infraspinatus muscle were related most significantly with pain and reduced functioning in the shoulder joint. CONCLUSION: Rotator cuff degeneration plays an important role in the daily functioning of the rheumatic shoulder. Prevention of rotator cuff degeneration may therefore play an important part in the treatment of the rheumatic shoulder.

Reliability of proximal migration measurements in shoulder arthroplasty.

Proximal migration is a complication in shoulder arthroplasty. Quantification of this phenomenon would help in identifying its cause. This study assessed the reliability of 4 methods of measuring proximal migration after shoulder arthroplasty on standard anteroposterior radiographs: the glenohumeral center-to-center measure, the glenohumeral arc measure, the acromiohumeral distance, and the scapular spine-humeral head center (spinohumeral) distance. Radiographs were taken of 12 embalmed shoulders, with matching metal humeral head replacements, in the neutral position and rotated both ways for 20 degrees along the vertical and horizontal axes. The measurements were tested for the reliability of the deviating projections against the neutral position, and an estimate of the interobserver and intraobserver reliability was made. Statistics included a paired t test and the interclass correlation coefficient. The acromiohumeral distance and spinohumeral center method proved most reliable. The spinohumeral center method was the least sensitive for projection errors, although only applicable as a relative measure. The coracoid process base can be used as a tell-tale sign for scapular projection.

A posterosuperior approach to the shoulder.

An approach for a better surgical exposure of the posterior part the glenoid and the rotator cuff is presented as an alternative for the long deltopectoral incision that might be considered in specially selected cases. In shoulder arthroplasty, it can be used for bone grafting to reorient the glenoid, revision of a glenoid component, or repair of a large rotator cuff tear. This posterosuperior approach consists of an osteotomy of the lateral rim and posterior corner of the acromion, with reflection of part of the medial and posterior insertion of the deltoid muscle. A special plate is used for stable fixation of the osteotomy. The method has been used since 1994 for shoulder prosthesis insertion in 79 patients. In 72 patients with a total shoulder or hemiarthroplasty, the cuff was attenuated or ruptured in 24 and could be repaired in 22. Twenty-two patients underwent a bony procedure to reorient the glenoid surface. The average correction of the superior tilt was 7 degrees . The external rotators are released from the bone and reattached, which might result in weakening of external rotation. This possible complication could not be confirmed at follow-up. The osteotomy healed in all but 1 patient. The fixation material had to be removed in 13.

Shoulder and elbow arthroplasty: one-stage or two-stage.

Multiple joint replacement in 1 operative session is frequently performed in the lower extremity but less often in the upper extremity. The purpose of the present study is to determine whether a 1-stage arthroplasty of the ipsilateral shoulder and elbow affects the clinical outcome. Replacement of the ipsilateral shoulder and elbow was performed in 34 rheumatoid patients (42 upper extremities). In 11 patients (13 upper limbs), the shoulder and elbow arthroplasty was a 1-stage procedure. The average follow-up of the whole group was 4.5 years (range, 2-12 years). The patients were in a prospective study and evaluated clinically and radiographically. With a 1-stage procedure, the hospitalization time was shorter. The overall Hospital for Special Surgery shoulder score and its items (pain, function, and strength) were similar at follow-up in the 2 groups, and only the motion score showed more improvement in the 1-stage group. The clinical outcome of elbow arthroplasty was similar in both groups, regardless of the sequence of surgery. From this study, it may be concluded that a 1-stage procedure for shoulder and elbow arthroplasty will reduce the hospitalization time and does not adversely affect the clinical outcome.

Model-based RSA of a femoral hip stem using surface and geometrical shape models.

Roentgen stereophotogrammetry (RSA) is a highly accurate three-dimensional measuring technique for assessing micromotion of orthopaedic implants. A drawback is that markers have to be attached to the implant. Model-based techniques have been developed to prevent using special marked implants. We compared two model-based RSA methods with standard marker-based RSA techniques. The first model-based RSA method used surface models, and the second method used elementary geometrical shape (EGS) models. We used a commercially available stem to perform experiments with a phantom as well as reanalysis of patient RSA radiographs. The data from the phantom experiment indicated the accuracy and precision of the elementary geometrical shape model-based RSA method is equal to marker-based RSA. For model-based RSA using surface models, the accuracy is equal to the accuracy of marker-based RSA, but its precision is worse. We found no difference in accuracy and precision between the two model-based RSA techniques in clinical data. For this particular hip stem, EGS model-based RSA is a good alternative for marker-based RSA.

Comparison of high-grade and low-grade mobilization techniques in the management of adhesive capsulitis of the shoulder: randomized controlled trial.

BACKGROUND AND PURPOSE: In many physical therapy programs for subjects with adhesive capsulitis of the shoulder, mobilization techniques are an important part of the intervention. The purpose of this study was to compare the effectiveness of high-grade mobilization techniques (HGMT) with that of low-grade mobilization techniques (LGMT) in subjects with adhesive capsulitis of the shoulder. SUBJECTS: One hundred subjects with unilateral adhesive capsulitis lasting 3 months or more and a > or =50% decrease in passive joint mobility relative to the nonaffected side were enrolled in this study. METHODS: Subjects randomly assigned to the HGMT group were treated with intensive passive mobilization techniques in end-range positions of the glenohumeral joint, and subjects in the LGMT group were treated with passive mobilization techniques within the pain-free zone. The duration of treatment was a maximum of 12 weeks (24 sessions) in both groups. Subjects were assessed at baseline and at 3, 6, and 12 months by a masked assessor. Primary outcome measures included active and passive range of motion and shoulder disability (Shoulder Rating Questionnaire [SRQ] and Shoulder Disability Questionnaire [SDQ]). An analysis of covariance with adjustments for baseline values and a general linear mixed-effect model for repeated measurements were used to compare the change scores for the 2 treatment groups at the various time points and over the total period of 1 year, respectively. RESULTS: Overall, subjects in both groups improved over 12 months. Statistically significant greater change scores were found in the HGMT group for passive abduction (at the time points 3 and 12 months), and for active and passive external rotation (at 12 months). A statistically significant difference in trend between both groups over the total follow-up period of 12 months was found for passive external rotation, SRQ, and SDQ with greater change scores in the HGMT group. DISCUSSION AND CONCLUSION: In subjects with adhesive capsulitis of the shoulder, HGMTs appear to be more effective in improving glenohumeral joint mobility and reducing disability than LGMTs, with the overall differences between the 2 interventions being small.

Limited influence of prosthetic position on aseptic loosening of elbow replacements: 125 elbows followed for an average period of 5.6 years.

BACKGROUND: Aseptic loosening of elbow replacements, seen in long-term follow-up, remains a problem. In this study, we attempted to determine the influence of cementing technique, prosthetic position, different component sizes, use of a bone plug, and intraoperative fractures on the development and progression of radiolucent lines and aseptic loosening. METHODS: We studied standard radiographs of 125 primary Souter-Strathclyde total elbow prostheses using the Wrightington method. Additionally, 104 preoperative radiographs were available for analysis. We used a Markow statistical model to detect relationships between all factors described above. RESULTS: After a mean follow-up time of 5.5 (2-19) years, 21 (17%) prostheses had loosened radiographically (10-year survival: 65%). When the humeral component was tilted more medially or more anteriorly, we found development of radiolucent lines at the medial condyle and at the posterior side of the humeral component. However, the progression of these lines was not influenced by these positions. No other prognostic factors for radiolucent lines or aseptic loosening were found. INTERPRETATION: Despite the small number of elbows studied, the weak influence of prosthetic position on aseptic loosening gives more ground for a multifactorial cause for aseptic loosening of the Souter-Strathclyde total elbow prosthesis.

Impact of adhesive capsulitis and economic evaluation of high-grade and low-grade mobilisation techniques.

The purpose of this study was to estimate the impact of adhesive capsulitis on costs and health and to compare the cost-utility of high-grade and low-grade mobilisation techniques. In a randomised controlled trial, 92 patients with adhesive capsulitis received either high-grade mobilisation techniques or low-grade mobilisation techniques and were followed for one year. Outcome measures were quality adjusted life years (QALYs) according to the Short Form 6D (SF-6D) and societal costs estimated from cost questionnaires. Estimated costs and QALYs in both randomisation groups were similar, except for the number of treatment sessions (18.6 for high-grade mobilisation techniques versus 21.5 for low-grade mobilisation techniques), with an estimated cost difference of 105 euros in favour of high-grade mobilisation techniques (p = 0.001, 95% CI 43 euros to 158 euros). In the entire sample, the average valuation of health improved from 0.597 at baseline to 0.745 after a year. The burden due to adhesive capsulitis was estimated at 0.048 QALY and 4,521 euros per patient. About half these costs were due to absenteeism which, during the first quarter, amounted to 38% of the total working hours. In conclusion, the cost-utility analysis does not allow for an evidence-based recommendation on the preferred treatment. Based on the clinical outcome measures, high-grade mobilisation techniques are still preferred to low-grade mobilisation techniques. The estimated substantial burden, both to the patient and to society, suggests that effective early treatment of adhesive capsulitis is warranted to attempt to accelerate recovery.

Identifying error pathways during elbow and knee replacements.

The aim of our study was to identify errors and error pathways during joint replacements and to propose improvements. A time-action and error analysis method was adapted for use during surgery. The error analysis consisted of identifying all possible errors, determining error paths presented in an error chart, quantifying errors, and determining the impact of errors. This method was used to evaluate joint replacements. We evaluated five knee and 11 elbow replacements done by two experienced surgeons. The main error for elbow replacements was caused by inadequate instruments. The main error for knee replacements was caused by inexperienced nurses being unfamiliar with guiding instruments. The time-action analysis showed a large variation in procedure duration. The main surgical limitations for both procedures were waiting caused by the cementing process and waiting caused by inexperienced scrub nurses. Our study identified errors and surgical limitations during joint replacements by using time-action and error analysis. Placement of both prostheses will benefit from new fixation techniques, a more experienced nursing staff, and more organized instrument tables.

Quantitative assessment of fatty degeneration in rotator cuff muscles determined with computed tomography.

OBJECTIVES: Reliable assessment of fatty degeneration of rotator cuff muscles is desirable to predict the outcome of shoulder surgery. Currently used qualitative assessments are limited by relatively high inter- and intraobserver variability. It was hypothesized that a quantitative measurement of muscle density using computed tomography (CT) was more reliable and reproducible. MATERIALS AND METHODS: Thirty shoulders from patients with rheumatoid arthritis were analyzed using parasagittal multiplanar reconstructions acquired from a 16-slice CT scanner. Three observers visually rated the severity of fatty degeneration and independently outlined the rotator cuff muscles, after which the mean density was calculated. Inter- and intraobserver agreement on both measurements was expressed by the interclass correlation coefficient (ICC) and the standard deviation of the differences (SDD) between the measurements. RESULTS: A strong correlation was found between the quantitative measure and the visual rating (R2 = 0.94; P < 0.0001). The SDD in muscle density did not exceed 2.3 Hounsfield units, and the mean rotator cuff ICC (0.98) was substantially greater than that of the visual rating (0.63). CONCLUSIONS: This study describes a reproducible method to quantify fatty degeneration of the rotator cuff muscles in CT images, with a higher interobserver agreement than the visual score, and may prove a reliable tool to evaluate the quality of the rotator cuff muscles.

Translation, adaptation and validation of the Shoulder Rating Questionnaire (SRQ) into the Dutch language.

OBJECTIVE: To translate and adapt the original English version of the Shoulder Rating Questionnaire (SRQ) into the Dutch language (SRQ-DLV) and evaluate its internal consistency, reliability, validity and responsiveness to clinical changes. DESIGN: Prospective study. SETTING: Outpatient departments of orthopaedics, radiology and physical therapy of an academic and a non-academic hospital. SUBJECTS: One hundred and seven patients treated for unilateral shoulder disorder (adhesive capsulitis 68, calcifying tendinitis 22, impingement syndrome or rotator cuff tear 17). METHODS: The original SRQ was translated and adapted following international guidelines. The SRQ-DLV was used among other measures of body function and structure, activities and societal participation in order to determine reliability, internal consistency, validity and responsiveness. Assessments were done at baseline and three months after treatment, with the SRQ-DLV being re-administered within one week before the baseline measurement and the start of the treatment for testing reliability. RESULTS: Cronbach's alpha for internal consistency was 0.89 for the total questionnaire and 0.81, 0.80, 0.72 and 0.84 for the domains pain, daily activities, sports/recreational activities and work, respectively. Test-retest reliability of the SRQ-DLV and its subscales ranged from 0.63 to 0.86. The summary score of the SRQ-DLV correlated with measures of shoulder function, daily activities and quality of life. Except for the work subscale of the SRQ-DLV, large effect sizes, reflecting its responsiveness to clinical changes after treatment, were found for both the summary and the subscales scores. CONCLUSIONS: Empirical data support that the SRQ-DLV is a reliable, valid and responsive measure to be used in clinical trials including Dutch patients with various shoulder disorders.

Factors influencing the surgical process during shoulder joint replacement: time-action analysis of five different prostheses and three different approaches.

BACKGROUND: To evaluate the per-operative process of shoulderjoint replacement, time-action analysis can be used. MATERIAL/METHODS: Forty procedures performed by 7 surgeons with different experience using 5 different prostheses and 3 different surgical approaches were analyzed. RESULTS: The surgical procedures showed a large variation in, for example, duration, tasks of team members, and protocol used. The surgical procedure was influenced by several factors, such as the prosthesis used, the surgical approach, the patient's condition, and the experience of the surgeon. Exposure of the glenoid was difficult and several retractors were needed, which were held by an extra assistant or clamped to the table or the surgeon. Two main limitations were seen in all procedures: repeated actions and waiting. Also, five errors could be identified. None of the alignment instruments was completely reliable and they allowed the surgeon to make major errors. CONCLUSIONS: Better alignment instruments, pre-operative planning techniques, and operation protocols are needed for shoulder prostheses. The training of resident surgeons should be focused on the exposure phase, the alignment of the humeral head, the exposure of the glenoid, and the alignment of the glenoid. Evaluating the surgical process using time-action analysis can be used to determine the limitations during surgical procedures. Furthermore, it shows the large variation in factors affecting surgical performance, indicating that a system approach is needed to improve surgical outcome.

Reliability of force direction dependent EMG parameters of shoulder muscles for clinical measurements.

OBJECTIVE: To assess the reliability of force direction dependent EMG parameters in shoulder muscles for future clinical research. DESIGN: EMG measurements of shoulder muscles including the rotator cuff were performed during isometrical external loading in various directions covering 360 degrees in a plane perpendicular to the humeral axis. BACKGROUND: Relating EMG to force direction bypasses problems associated with the unknown position and velocity dependence of the EMG signal. For clinical application, information on the reliability of force direction dependent parameters is required. METHODS: The EMG of shoulder muscles of healthy subjects was related to force direction. The activation patterns obtained where parameterised after least squares function fitting, returning three force direction dependent parameters, i.e. two on- and offset directions of the activity peak and the direction of highest EMG activity also called principal action. Within-trial, inter-trial, inter-day and inter-subject variabilities were estimated. RESULTS: With a group size of n = 10, the 95% confidence interval for inter-day measurements was found to be about +/-5 degrees on a scale of 360 degrees for the principal action and just below +/-10 degrees for the intercepts. CONCLUSION: The method allows for intra-individual measurements on different days with sufficient accuracy so that shoulder muscle co-ordination of patients before and after interventions like surgery or physical therapy can be evaluated. RELEVANCE: Quantitative data on shoulder muscle function in vivo are required to assess the effectiveness of interventions on the shoulder.

Evaluation of humeral head replacements using time-action analysis.

New surgical technologies are often introduced without objective evaluation of their efficiency. Commonly, their results are only related to surgical outcome and complication rate. In this study, time-action analysis was applied to evaluate the per-operative process and to measure surgical performance during 8 humeral head replacements. An overview of the operative theater and a detailed view were recorded on video simultaneously. The duration and number of actions grouped to functions, limitations, and repeated actions were determined. The duration and number of performed actions varied between procedures. The efficiency of the procedure, defined as the percentage of goal-oriented functions, was about 55%. Repeated actions were frequently observed during the alignment and insertion phase. We conclude that time-action analysis can be used to evaluate surgical performance objectively. Limitations of the surgical process that can be improved could be identified. These findings enable the evaluation of new operative techniques, protocols, and instruments.

Stress shielding and bone resorption in shoulder arthroplasty.

The radiographs of 64 patients with 70 humeral head replacements were reviewed for signs of stress shielding. Of these, 49 were implanted for rheumatoid arthritis and 21 for osteoarthritis. The radiographic follow-up averaged 5.3 years. Measurements of cortex thickness were performed in 4 regions along the stem of the implant, and the differences between the postoperative radiograph and the radiograph at follow-up were calculated. The size of the stem in relation to the diameter of the humerus was calculated with the use of validated measures, resulting in the relative stem size. In 6 patients (9%) a significant reduction in cortical thickness was observed in the proximal-lateral region of the humeral stem, 5 in rheumatoid patients and 1 in an osteoarthritic patient. In the stress shielding group, the relative stem size was found to be significantly higher than that in the non-stress shielding group (0.58 vs 0.48). Osteoporosis, especially present in rheumatoid arthritis, could well be a risk factor. It was concluded that stress shielding is a long-term complication of shoulder arthroplasty and that the relative stem size is an important factor in its genesis.