Background

Shoulder pain and loss of function are directly associated with obesity (Kane et al. 2010). High body mass index (BMI) increases incidence of rotator cuff tendinitis (Wendelboe et al. 2004), arthropathy, rotator cuff tear size (Gumina et al. 2014), and the need for elective shoulder replacement (Bostman 1994). The annual volume of total shoulder arthroplasty (TSA) and reverse total shoulder arthroplasty (RSA) has increased 2.5 fold in the U.S. from 2000 to 2008 (Beck et al. 2013; Bostman 1994). The percentage of overweight and obese patients undergoing TSA or RSA ranges from 19 to 75% (Gupta et al. 2014) depending on the cohort (Gupta et al. 2014; Werner et al. 2015b). TSAs and RSAs are often indicated for treatment of osteoarthritis of the shoulder. RSAs are typically indicated for rotator cuff tear arthropathy, massive rotator cuff tears, proximal humerus fractures, revision arthroplasty, and glenohumeral osteoarthritis in the context of irreparable rotator cuff tears (Saltzman et al. 2014) and the indications for use are continuing to expand (Urch et al. 2016).

The relationship between BMI and shoulder replacement outcome is complex. High BMI increases the surgical complexity and risk for complications out to 6 months (Beck et al. 2013; Griffin et al. 2014; Gupta et al. 2014; Werner et al. 2015a, b), but may not impact 30-day complications (Jiang et al. 2016). Obesity does not necessarily inhibit gains in outcome scores and quality of life (QOL) over the short-term (Li et al. 2013). Few studies have assessed obesity’s collective impact on pain, range of motion (ROM), or QOL after shoulder replacement (Li et al. 2013; Linberg et al. 2009; Pappou et al. 2014). Limited evidence suggests that obese patients can attain similar 2-year improvements in physical health as non-obese patients after TSA (Li et al. 2013) and can attain good surgical outcomes after RSA (Statz et al. 2016). What remains unclear is whether shoulder-specific outcomes, physical function, and patient-reported QOL are different beyond 2 years or longer across different BMI strata. This evidence gap is significant; comparative patient-centric metrics are not yet available to facilitate the shared decision-making process between patient and surgeon. An improved understanding of the mechanics of RSA and prostheses permitted the expansion of indications that are considered appropriate for this procedure (Urch et al. 2016), thereby increasing the likelihood that its use in patients with high BMI values becomes common. Moreover, as the indications for the use of RSA expand, it is not clear whether obesity differentially impacts these actual or patient-reported outcomes over a longer time frame.

Previous work has shown that morbidly obese patients have significantly worse functional outcomes than non-obese counterparts after rehabilitation for lower body elective procedures such as hip and knee arthroplasty (Vincent and Vincent 2008; Vincent et al. 2007). It is still unclear whether mid-term TSA and RSA actual and patient-reported outcomes are different based on BMI value, which precludes establishment of expected mid-term elective shoulder replacement outcomes and risk analysis. It is possible that despite the different indication for TSA and RSA and different initial functional ability, obese patients may report similar perceived improvements in QOL as non-obese patients. Clarifying the relationship of BMI strata to outcomes after TSA and RSA will provide the first complete patient experience that can be used to establish expectations for improvement in patient subgroups that may benefit from additional interventions. Comparative evidence of sustained outcomes is essential as health care moves toward a pay-for-performance and value-based system (Li et al. 2013). Thus, our study first compared surgery-specific and general health QOL outcomes between non-obese, obese, and morbidly obese patients with a TSA or RSA. We hypothesized that significant interactions would exist between the surgery type and obesity status on functional and QOL outcomes over the long term. Although all patients would experience improved functional outcomes and QOL, we anticipated that morbidly obese patients would report lower QOL and physical function gains than non-obese patients after TSA and RSA.

Methods

Study design

A retrospective review was conducted of prospectively collected data on patients who underwent TSA or RSA between January of 1992 and May of 2012 at the University of Florida. Both TSA and RSA were included in this analysis as RSA represents approximately one-third of the nationally reported shoulder replacement surgeries in the U.S. (Westermann et al. 2015). Diagnoses included osteoarthritis, acute fracture, rotator cuff tear, and rotator cuff arthropathy, and other (osteonecrosis, avascular necrosis, and synovial chondromatosis).

Of the enrolled patients, 310 had complete follow-up data for our research questions. Patients underwent TSA or RSA and had a minimum 2-year follow up and a final mid-term mean follow up of 5.0 ± 2.5 years (range 3–17 years). Age ranged from 21 to 95 years. Study documents and procedures were approved by the University of Florida Institutional Review Board and procedures conformed to the guidelines set by the Declaration of Helsinki for the treatment of human subjects.

Patients were stratified into groups based on BMI values: non-obese (BMI < 30 kg/m2), obese (30–39.9 kg/m2), and morbidly obese (≥40 kg/m2) for statistical analysis (Kane et al. 2010). Demographics (age, height, weight, BMI, and ethnicity/race), surgery type (TSA or RSA), comorbidities, perioperative analgesic use, and tobacco use were collected preoperatively.

Patient-reported outcome measures

Surveys were administered to each participant on subjective shoulder-specific functional and pain outcomes, perceived physical activity, and general health.

American Shoulder and Elbow Surgeons Evaluation form (ASES)

The ASES permits the patient to self-evaluate pain, shoulder instability, and activities of daily living (Richards et al. 1994). Eleven items are used to generate a score (pain has one item, function ten items). The physician-assessment section documents ROM, physical signs, strength, and stability. The final score is derived from the pain and cumulative activities of daily living score (Richards et al. 1994). A minimal clinically important difference is 6.4 points (Roy et al. 2009).

Shoulder Pain and Disability Index (SPADI)

The SPADI assesses shoulder pain and disability in the outpatient setting (Breckenridge and McAuley 2011), with 13 items categorized into two subscales: pain (five items) and disability (eight items) (Roach et al. 1991). This instrument has high internal consistency with Chronbach’s α values ranging from 0.86 to 0.95 for the subscales. The SPADI responds to change over time in various patient populations, and can discriminate between patients with improving and deteriorating conditions (Beaton and Richards 1996; Roy et al. 2009; Williams et al. 1995). A minimal clinically important difference is 8 points (Roy et al. 2009).

University of California at Los Angeles Shoulder Rating scale (UCLA)

This scale has separate domain areas: pain, function, strength of forward flexion, active forward flexion, and overall patient satisfaction (Amstutz et al. 1981). The scale is weighted by pain, and function accounts for 20 points. The other domains account for 15 points, giving a total score of 35 points (<27 is poor and fair and >27 is good to excellent). This has been used in patients with shoulder replacement due to osteoarthritis, rotator cuff disease, and instability (Kirkley et al. 2003).

Constant score

This score is comprised of subjective assessments relating to pain, activities of daily living, and objective measures of shoulder motion and strength. The maximum score is 100 points, with higher scores representing less pain and better shoulder function (Constant et al. 2008). Construct validity is 6 = .673, and the Constant score correlates moderately well with the UCLA score (r = 0.673) (Oh et al. 2009).

Medical Outcomes Short Form 12 (SF-12)

The SF-12 is a standardized, validated health related QOL questionnaire comprised of 12 items of the Medical Outcomes Short Form 36 original instrument (Ware et al. 1996). The SF-12 summary score (PCS-12 and MCS-12) ranges from 0 to 100 points. The SF-12 has a high test–retest reliability (r = 0.89) and has been validated for use in independently living older adults with chronic illnesses (Resnick and Nahm 2001).

Radiological outcomes and adverse events

Development of radiolucency lines during the follow-up (specifically within the glenoid) and scapular notching was identified by the surgeon in the study (Choi et al. 2013). Adverse events included infections, bone graft failure, loosening, dislocation, rotator cuff failure, wound complications, fracture, and pain and stiffness. The number of revisions in each surgery category was documented. The criteria for revisions included the following: infections, loosening or instability, dislocation, bone graft failure, and rotator cuff repair failure. If revisions occurred, the follow-up time was determined from the initial procedure.

Objective functional measures

Passive and active shoulder ROM was assessed by an experienced athletic trainer using a goniometer. Active external rotation and active elevation measures were performed by the patient. Each participant lifted progressively higher weights in increments of 2.2 kg during a lateral straight arm raise to estimate strength. The strength value was defined as the maximal weight the patient could lift with appropriate form once.

Statistical analyses

Data were analyzed using the Statistical Package for the Social Sciences (SPSS version 24.0, IBM Corp; Chicago, IL). Patient characteristics were compared between the three BMI strata (non-obese, overweight, and obese) using one-way analysis of variance (ANOVA) and a Scheffé post hoc analysis for continuous variables and by Kruskal–Wallis tests for categorical variables. Chi square tests were used to determine whether distributions of the shoulder pain mechanisms, radiolucency lines, and scapular notching were different among the groups. To address the purposes of the study, generalized linear models were used, where the dependent variables were study outcomes (ASES, SPADI, UCLA, Constant scores, SF-12, range of motion and strength) and the predictors were obesity status (non-obese, obese, and morbidly obese) and surgical type (RSA or TSA). Covariates were diagnosis and age. Significance was established at p < .05 a priori for all tests.

Results

Table 1 shows patient characteristics. The morbidly obese group had a higher prevalence of women and Diabetes Mellitus than the other groups (p < .05). The obese group had a greater proportion of patients with hypertension and those who took analgesics for pain than the remaining groups (p < .05). A total of 56.9% of patients underwent a TSA and 43.1% underwent a RSA. There was a significant difference in the distributions of shoulder pain mechanisms among the BMI groups (χ2 = 52.417; p < .0001).

Table 1 Participant characteristics of the study groups
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Adverse events that did and did not require revision are shown in Table 2 for the surgery types and BMI groups. The numbers of radiolucent lines (humeral and glenoid) and scapular notching at follow-ups are shown in Table 3. The distributions of the radiolucency lines (glenoid and humeral) and scapular notching were not different among the BMI groups. The revision procedures that were performed included the following: removal and replacement of long-stem humeral implant, revision to hemiarthroplasty, removal of loose glenoid and revision, conversion of TSA to RSA, reduction and revision, revision to an antibiotic spacer, and bone grafting for injury after a fall.

Table 2 Adverse events (AE) in patients from each body mass index (BMI) stratum
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Table 3 Radiolucency lines and scapular notching by the final follow-up time point
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Patient-reported functional outcomes are provided in Table 4. There were no significant interactions of obesity status by time for any outcome variable. There were no significant main effects of obesity or surgery type for any outcome. Improvements were maintained at the final follow-up.

Table 4 Patient-reported functional outcomes
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Patient-reported QOL SF-12 scores are presented in Table 5. There were no significant interactions of obesity status by time for the SF-12 scores. However, there was a main effect for obesity for all SF-12 scores (p < 0.05). All BMI groups improved the SF-12 scores over time, but the morbidly obese group attained lower SF-12 scores compared to the remaining groups at each time point (p < .0001).

Table 5 Quality of life (QOL) represented by the Medical Outcomes Short Form 12 (SF-12) total score, physical component score (PCS), and mental component score (MCS)
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Objective functional scores of active and passive ROM and shoulder strength are presented in Table 6. Only one significant obesity status by time interaction existed for active external rotation (p = .021), with main effects of both obesity and surgery type reaching significance.

Table 6 Shoulder motion and strength
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Discussion

The interactions between surgery type and obesity status on several mid-term shoulder-specific general outcomes were tested. The novel findings are (a) morbidly obese patients made significant improvements in functional and QOL outcomes over the mid-term; (b) there were no significant main effects for surgery type on ASES, SPADI, UCLA, and Constant scores; and (c) shoulder replacement adverse events and radiological outcomes were not different among the BMI groups. These findings demonstrate sustained benefit of both surgery types on clinically meaningful outcomes even in patients with high BMIs. The magnitude of improvement in QOL was less in the morbidly obese patients compared with the remaining groups by follow-up.

General health QOL and ROM/strength assessments were compared among non-obese, obese, and morbidly obese patients. Surgery benefits on outcomes were maintained past 2 years, suggesting that shoulder arthroplasty effectively relieves symptoms and improves function in patients across the BMI spectrum. Because morbidly obese patients had worse function and QOL before the surgery, they did not achieve the same absolute level of improvement as the other BMI groups on the ASES, SPADI, and UCLA scores. Improvements in these outcomes ranged from 48 to 65% in the morbidly obese group, and from 60 to 115% in the other groups. Despite the lesser gain in perceived function, morbidly obese groups achieved similar relative improvements in the SF-12 physical component score and over twice the improvement in the SF-12 mental component score than the other BMI groups. Few directly comparable data are available in obese patients. However, patient-reported QOL after RSA has been shown to reach comparable levels to that of healthy age-matched norms using the SF-36. Also, SF-36 domains have been shown to be higher than those reported by a normalized age-matched cohort after TSA (Gruson et al. 2010).

Recent studies measured shoulder replacement outcomes in obese patients. First, a case control study of obese and non-obese patients (N = 84) with RSA similarly found that obese patients did not achieve the same ASES improvement as non-obese controls after surgery (86 vs. 105%) (Pappou et al. 2014). Pain subscores improved similarly in the two patients groups (78–80%), but the greatest change occurred in the ASES function subscores in the non-obese and obese patients (142 vs. 89%). Second, a prospective study of patients enrolled in a shoulder registry (N = 76) collected preoperative and 2-year ASES and SF-36 scores, and visual analogue scales of pain and fatigue (Li et al. 2013). Patients were stratified into ‘‘normal’’ weight, overweight, and obese groups based on BMI. The ASES scores improved by 108 and 123% in the normal weight and obese patients, respectively. Improvements on the SF-36 physical and mental subscores were less in the obese patients than the normal weight patients, however. The obese patients experienced 2.1–12% improvements in the physical SF-36 subscores compared to the normal weight patients, who experienced improvements ranging from 11.3 to 40.2%. Third, a study of morbidly obese patients with TSA (N = 45 shoulders) found a 53.5% reduction in pain out to an average of 4.6 years of follow up (Linberg et al. 2009). These studies and ours suggest that obese patients may report improvements in shoulder-related function, but potentially less improvement in reported QOL.

Individuals with low and high BMI values can improve active elevation, external rotation, and internal rotation abduction and forward flexion after TSA and RSA (Constant et al. 2008; Gupta et al. 2014). We observed improvements in active (external rotation, active elevation) and passive (external rotation, elevation) ROM between morbidly obese, obese, and non-obese patients. Shoulder raise strength gains were made across all groups, with continued strength gain in the morbidly obese group. Obesity reduces upper body muscle strength and endurance (Cavuoto and Nussbaum 2014), compromises shoulder ROM as much as 38.9% for actions like shoulder abduction (Park et al. 2010), and increases upward scapular rotation during movement (Gupta et al. 2013). The combination of low muscle strength and shoulder ROM negatively impacts the ability to perform daily activities. Thus, improvements in ROM and strength in obese patients after surgery might translate to sustained performance of activities of daily living over time (Maier et al. 2014).

Case–control data comparing outcomes between TSA and RSA revealed that ASES scores, pain severity, elevation, abduction, and internal rotation improved similarly after the two procedures (Kiet et al. 2015). After RSA and TSA, a significant proportion of patients continue to participate in medium or high-demand activities (84 and 89%, respectively), but there are specific activities that people with RSA are unable to do well (Ware et al. 1996). Morbidly obese patients may experience unique difficulties with shoulder movements that are not captured with current instruments. Development of task inventories that may be relevant to an obese person may provide researchers and clinicians with better insight into the impact of the surgery on shoulder function. Surgeons and patients would benefit from discussing functional and QOL goals to determine which procedure would yield the best results (Schwarzkopf et al. 2013). Here, the morbidly obese group’s pain may be more related to degenerative disease and less to rotator cuff issues, whereas the non-obese group’s pain may be more linked with rotator cuff arthropathy and acute fracture. The fact that there were no differences in adverse events and radiological outcomes in the three patient groups indicates that obesity does not compromise the success of shoulder replacement. Surgical possibilities are open to both procedures, even in morbidly obese patients.

To our knowledge, this is the first study to examine the mid-term changes in function and patient-reported QOL after two shoulder replacement surgery types. Some limitations and strengths deserve comment. First, the patient subgroup sizes are different, with the morbidly obese subgroup being the smallest size. Based on clinical tracking of patient population demographics, we believe that this distribution represents the actual proportions of patients seen in this tertiary care institution. This study collected a battery of standardized assessments, most of which were self-report surveys. These surveys were joint-specific and general, providing a more comprehensive assessment of the patient experience. The same investigators administered the surveys and performed functional testing in all patients, minimizing interrater error.

Conclusions

Shoulder replacement procedures improve function and QOL outcomes in patients across the BMI spectrum out to an average of 5 years. The interactions of obesity status and surgery type for key outcomes were not different in the non-obese, obese, and morbidly obese patients. Adverse event frequencies and radiological outcomes were not higher in obese and morbidly obese patients. Morbidly obese patients can achieve meaningful shoulder outcomes and QOL without elevated risk for adverse events or worse radiological outcomes over the mid-term.