Abstract
Objectives
To evaluate the clinical significance of stress shielding in patients who have undergone an uncemented reverse shoulder arthroplasty (RSA) with a proximally coated stem for complex proximal humeral fractures (PHF) comprising 3 or 4 parts, assessed at 2 years postoperatively. Additionally, this study aims to examine the correlation between tuberosity healing and clinical outcomes.
Methods
Setting: Single Centre.
Patient Selection Criteria: 43 patients underwent surgery involving a cementless RSA with an anatomical stem (Mini Stem of Zimmer Biomet ® Comprehensive System) following an acute 3- or 4-part or 4-part with luxation PHF within 4 weeks of injury. Patients who were excluded from the study included those who passed away, did not have a minimum 2-year follow-up, or had undergone cemented RSAs or alternative humeral components, as well as those who required RSA due to fracture sequelae.
Outcome Measures and Comparisons: Stress shielding, tuberosity healing and its positioning were evaluated. Clinical-functional assessments were made using the Constant and ASES scores. Furthermore, quality of life assessments, including QuickDASH score, SF-12, satisfaction test (SANE) and Visual Analogue Scale (VAS) were performed.
Results
Of the total shoulders, 6 (13.9%) were excluded, leaving 37 PHFs that met the inclusion criteria. The mean age at the time of surgery was 72 years (range, 61–85). Stress shielding was observed in 31 shoulders (83.8%). There was no observed correlation between stress shielding and clinical-functional or quality of life assessments. The greater tuberosity in patients without stress shielding was found to be above the tray (66.7%) (P < 0.05). Tuberosity healing was observed in 94.6% (35/37) of the shoulders.
Conclusions
No significant clinical difference was observed between patients with and without stress shielding in terms of short-term functional outcomes. Successful consolidation of the tuberosities can be attained even in cases with notable stress shielding.
Level of Evidence
Level IV; Case Series; Treatment study.
Keywords: Stress shielding, proximal humeral fracture, reverse shoulder arthroplasty, clinical outcome
Introduction
The prosthesis protects the proximal humerus from its original loads and stresses primarily due to differences in elastic modulus. 1 These changes lead to various bone alterations known as stress shielding. This radiological phenomenon is characterised by proximal bone resorption, condensation lines (pedestal sign) and spot welds, 2 which may predispose to periprosthetic complications (Figure 1). While some studies have disregarded stress shielding,3–5 others have reported its prevalence in nearly all cases. 6 Few studies have examined this phenomenon in proximal humeral fractures (PHF) treated with an RSA.7,8 However, the correlation between stress shielding and clinical-functional or quality of life outcomes in patients treated with an RSA using an anatomic proximally coated stem for PHF remains unknown. This study aims to determine whether there is a correlation between stress shielding in patients treated with an RSA using a proximally coated stem for PHF and their clinical-functional and quality of life outcomes. Additionally, the study aims to correlate tuberosity healing with clinical outcomes. It was hypothesised that stress shielding does not correlate with clinical-functional PROMs and quality of life outcomes, whereas proper tuberosity healing correlates with improved PROMs.
Figure 1.
Stress shielding. (a) Proximal bone resorption. (b) Condensation lines (pedestal (sign). (c) Spot welds.
Materials and methods
Study design and patient characteristics
A descriptive, transversal ambispective study was conducted on patients who underwent RSA with a cementless anatomical stem for PHF between November 2015 and February 2019 consecutively at Hospital de Terrassa (CST). This study received approval from the Clinical Research Ethics Committee of the Hospital Consorci Sanitari de Terrassa (CEIC CST 3-February-2017). A total of 43 RSAs were performed on 42 patients at the same hospital under the surgical care of the same senior surgeon (J.M.G.). Inclusion criteria: Patients who underwent surgery with a cementless RSA utilising a Mini Stem of Zimmer Biomet ® Comprehensive System following a complex PHF within 4 weeks of injury. Exclusion criteria: A minimum 2-year follow-up, utilisation of alternative humeral components, and RSA indicated due to fracture sequelae. Six shoulders (13.9%) were excluded from the study: 3 patients due to the use of an alternate stem, and 3 patients excluded due to mortality within 2 years. Ultimately, 37 PHFs met the inclusion criteria and were included in this study.
Surgical procedure
The deltopectoral approach was utilised. A 25-mm metaglenoid was aligned to the inferior border of the glenoid with a 10° inferior tilt and a 36-mm glenosphere inferior C-offset (2.5 mm). The definitive humeral stem matched the size of the last broach used. The height of the humeral component was determined through near anatomic reconstruction of the tuberosity fragments, referencing the medial calcar. In cases where it was not feasible, the pectoralis major insertion served as a reference. The humeral version was set at 20° retroversion. Standard tray and polyethylene components were used upon request. Following prosthesis reduction and stability verification, tuberosity suturing was conducted using 3 intertuberosity N.2 non-absorbable stitches and 2 tuberodiaphyseal crossed stitches over two diaphyseal holes.
Postoperative rehabilitation protocol
Patients were initially immobilised using a sling and an anti-rotational strap. Pendulum shoulder movements began on the first day post-surgery. From the fourth week post-surgery, exercises targeting forward elevation were initiated. Active mobilisation was introduced in the sixth week.
Demographic parameters
Age (years), sex (male/female) and type of fracture (3-part, 4-part or 4-part and luxation) – Neer classification. 9
Radiological parameters
The radiographic analysis was conducted by two blinded orthopaedic surgeons who received specific training for this purpose. In case of any disagreement in their assessment, a consensus was achieved through repeated evaluations and discussions.
Stress shielding was defined by cortical resorption, condensation lines and spot welds. Cortical resorption was located at two specific regions of interest: zone 2 (lateral) and zone 7 (medial) as per Gruen's modified classification 2 and the degree of cortical resorption was graded according Inoue et al. 10 criteria: 0: No resorption; 1: Signs of osteopenia, decreased cortical density; 2: <50% cortical thinning; 3: >50% cortical thinning; 4: Disappearance of cortical.
Tuberosity healing and location of the greater tuberosity in relation to the prosthetic tray (under, at level or above) 11 (Figure 2).
Figure 2.
Localisation of the tuberosity healing in reference to the tray. (a) Below the tray, (b) at level of the tray or (c) above the tray.
Clinical-functional and quality of life parameters
Functional outcomes were measured using the Constant–Murley Score and the American Shoulder and Elbow Surgeons (ASES) Score, alongside objective measurements of shoulder range of motion, including forward elevation, abduction, and both external and internal rotation. Quality of life was evaluated with the QuickDASH, SF-12, Single Assessment Numeric Evaluation (SANE), and the Visual Analogue Scale (VAS) for pain.
Correlations were established between stress shielding and demographic data, radiological outcomes, clinical-functional tests, as well as quality of life outcomes.
Statistics
The statistical analysis was conducted using IBM SPSv219 software by the Research and Innovation Area of CST – Hospital Universitari de Terrassa (Mamen Carmona Orozco, PhD and Cristina Broceño Corrales, PhD). Descriptive statistics utilised absolute and relative frequencies for qualitative variables and means and standard deviations for quantitative variables. For inferential statistics, in the case of quantitative variables, either the Pearson's Chi 2 test or Fisher's exact test was employed as appropriate. Parametric test (T-test) or non-parametric tests (Mann–Whitney U test for independent samples) were conducted for quantitative variables, depending on whether they met the assumptions of normality (Levene's test) and sample homogeneity. Statistical significance was considered at a P-value of <0.05.
Results
Epidemiological results
A total of 44 RSAs were performed, with 37 cases (84.1%) meeting the study criteria. The cohort consisted of 31 women (83.8%) and 6 men (16.2%). The mean patient age was 72 years. Among the shoulders, 8 had a 3-part PHF (21.6%), 23 had a 4-part PHF (62.2%), and 6 had a 4-part dislocation pattern (16.2%).
Radiological results
Stress shielding was observed in 83.8% (31/37) of the shoulders:
Cortical resorption was observed predominantly in Gruen's zone 2 in 87.1% of cases with stress shielding (27/31) while cortical resorption in Gruen's zone 7 was found in 19.3% (6/31). The degree of cortical resorption in patients with stress shielding, 74.2% showed grade 4 (23/31), 9.6% showed grade 3 (3/31), and 6.5% showed grade 2 (2/31). No patient showed grade 1, and 9.6% (3/31) were classified as grade 0.
Condensation lines (pedestal formation) were found in 87.1% of the shoulders (27/31) and Spot Welds were noted in 25.8% of cases (8/31).
Tuberosity healing was observed in 94.6% of the patients (35/37), with one case of migration and one case of resorption of the tuberosities. The relationship between the position relative to the RSA humeral tray and stress shielding is elucidated in Table 1. Among patients with stress shielding, the postoperative position of the greater tuberosity was predominantly below the prosthetic tray (61.3%), whereas the position of the greater tuberosity in patients without stress shielding was above the tray (66.7%) (P < 0.05).
Table 1.
Correlation between stress shielding and greater tuberosity position.
| No stress shielding | Stress shielding | Total | ||||
|---|---|---|---|---|---|---|
| N | % | N | % | N | % | |
| At level of the tray | 1 | 16.7% | 8 | 25.8% | 9 | 24.3% |
| Below the tray | 1 | 16.7% | 19 | 61.3% | 20 | 54.1% |
| Above the tray | 4 | 66.7% | 4 | 12.9% | 8 | 21.6% |
Data are presented as N: Number of patients of the sample and %: Percentage of the sample.
Clinical-functional results at the 2-year follow-up
At the two-year follow-up, the mean Constant–Murley Score for patients with stress shielding was 59.45 (SD 9.48), compared to 55.83 (SD 4.4) for those without stress shielding (P = 0.092). The mean ASES Score was 70.89 (SD 14.42) in the stress-shielded group and 66.43 (SD 14.3) in the non-stress-shielded group (p = 0.45). No significant differences in shoulder range of motion were found between the two groups. Mean forward elevation in patients with stress shielding was 128.39° (SD 19.89) versus 115.0° (SD 18.71) in those without (P = 0.85), while mean abduction was 119.52° (SD 23.92) compared to 112.5° (SD 18.9) (P = 0.60). External and internal rotation measurements showed no significant differences related to stress shielding.
No correlation was found between stress shielding and clinical-functional tests (Figures 3 and 4).
Figure 3.
Correlations between stress shielding and clinical-functional tests.
Figure 4.
Correlations between stress shielding and forward elevation and abduction.
Quality of life results
No correlation was found between stress shielding and quality of life tests (Tables 2 and 3; Figure 5).
Table 2.
Comparison between quality of life tests and stress shielding.
| Stress shielding | No stress shielding | P | |
|---|---|---|---|
| VAS | 1.26 + −1.63 | 1 + −1.25 | 0.8 |
| QuickDASH | 28.07 + −14.47 | 36.01 + −17.89 | 0.3 |
| SF-12 Physical | 41.26 + −10.14 | 37.28 + −11.91 | 0.514 |
| SF-12 Mental | 50.52 + −8.69 | 47.98 + −13.66 | 0.054 |
| SANE | 81.94 + −16 | 75 + −13.78 | 0.81 |
Data are presented as ‘mean + −SD’.
Figure 5.
Correlations between stress shielding and quality of life tests.
Table 3.
Comparison between tuberosity healing and quality of life tests.
| Tuberosity healing | No tuberosity healing | |
|---|---|---|
| VAS | 1.29 + −1.58 | 0 + −0 |
| QuickDASH | 29.87 + −15.33 | 20.45 + −6.43 |
| SF-12 Physical | 40.21 + −10.47 | 47.74 + −6.16 |
| SF-12 Mental | 49.51 + −9.39 | 60.65 + −0.78 |
| SANE | 80.86 + −16.15 | 80 + −0 |
Data are presented as ‘mean + −SD’.
Tuberosity healing
Epidemiological results
Among patients lacking tuberosity healing, there was one male and one female (P = 0.3).
Radiological results
94.6% (35/37) of tuberosities healed, with one case of resorption and one case of migration. No correlation between stress shielding and anatomical tuberosity healing was found (P = 0.065). Given the small size of the non-healed group, no further comparative analysis was performed.
Discussion
Stress shielding is a well-recognised phenomenon in arthroplasty,2,6,12,13 yet its clinical significance in reverse shoulder arthroplasty (RSA) for PHF remains insufficiently explored.7,8,14
This study highlights a high incidence of proximal stress shielding (83.8%) in a cohort of patients treated with a cementless, proximally coated stem. Radiologically, this finding aligns with load redistribution associated with the implant's design and its interaction with surgical choices. Specifically, in fracture cases, larger stem sizes were often selected to ensure adequate fixation, resulting in higher filling ratios (FR).4,15 Evidence from multiple studies confirms that a high FR (>0.7) is a strong predictor of stress shielding and associated radiological changes, such as cortical resorption, condensation lines and distal spot welds.8,16,17
This phenomenon is particularly evident in uncemented stems designed for diaphyseal fixation, which reduce load transfer to the proximal bone, thereby promoting distal fixation and subsequent stress shielding.8,18
According to Wolff's law, the combination of high FR and the specific geometry of the implant promotes a pattern where forces are predominantly transmitted through the prosthetic component rather than the proximal metaphyseal bone. This bone is often weakened by the fracture or, in some cases, entirely absent. The redistribution of forces reduces the mechanical load on the proximal bone, inducing bone resorption in this region, and simultaneously redirects forces distally, leading to radiological changes such as pedestal signs and distal spot welds.8,17 Importantly, in this study, all prostheses demonstrated adequate fixation, with no cases of loosening, even in the challenging context of fracture cases.
It is crucial to identify these radiological alterations, avoiding misinterpretation as tuberosity healing 19 or radiolucent lines. 18
In this study, the high incidence of stress shielding showed no correlation with functional outcomes (e.g. Constant–Murley and ASES Scores) or quality of life metrics. These results align with previous studies in anatomical total shoulder arthroplasty and RSA for non-traumatic cases.1,5,15,20,21
Interestingly, this study demonstrated a remarkable tuberosity healing rate of 94.6%, which was not adversely affected by stress shielding (P = 0.065). This finding highlights the biomechanical stability achieved by the chosen implant design and surgical technique, indicating that proximal stress shielding does not impede successful tuberosity consolidation. Furthermore, the position of the greater tuberosity relative to the prosthetic tray may influence the observed stress distribution; tuberosities positioned below the tray were more frequently associated with stress shielding, indicating a potential biomechanical influence on bone healing dynamics.
Nevertheless, the long-term implications of proximal bone loss due to stress shielding remain a critical area for future research, particularly its potential to complicate revision surgery by reducing bone stock and compromising implant stability.
Surgical technique
In response to these findings, we have refined our surgical approach to mitigate the effects of stress shielding while ensuring initial prosthetic stability remains a priority. In our routine practice for atraumatic cases, we consistently use a definitive stem size that is two sizes smaller than the trial broach, resulting in lower proximal FR and minimising the risk of stress shielding. This technique has demonstrated reliable integration and has not resulted in cases of loosening, which is why we have adopted a similar approach in the context of fractures. However, given the unique challenges presented by fractures, once the trial stem is partially introduced, we prioritise filling the humeral canal with autologous graft derived from the discarded humeral head before completing the final implantation.3,8,17,22
This step ensures enhanced support and optimised integration. Importantly, if insufficient stability is detected during surgery, we adapt the approach by increasing the size of the definitive stem to achieve optimal fixation and alignment.
By achieving a balanced distribution of loads, this technique seeks to preserve proximal bone stock and optimise long-term outcomes. Combined with the use of autologous grafting and meticulous tuberosity suturing, this protocol provides a comprehensive solution that ensures both initial stability and a practical strategy for addressing stress shielding.
Limitations
This study has several limitations. Firstly, the retrospective analysis based on a relatively small sample size is consistent with available literature. Secondly, the use of only one implant type limits the investigation of stem FR and their relation to stress shielding. Thirdly, the 2-year follow-up is insufficient for definitive conclusions on stress shielding.
However, the study's strengths include a homogeneous patient population in terms of age and indications, all operated on by the same senior surgeon, and prospective data collection.
Conclusions
This study highlights the prevalence of stress shielding in RSA for PHF, with no short-term impact on functional outcomes or quality of life. However, its potential implications for bone stock preservation and revision surgery warrant consideration. Importantly, this cohort achieved a high tuberosity healing rate of 94.6%, indicating that successful consolidation of the tuberosities can be attained even in cases with notable stress shielding. Further long-term studies are necessary to better understand the implications of stress shielding in RSA.
Acknowledgements
The authors would like to extend their gratitude to Dr. Mamen Contreras for her contribution to the statistical analysis.
Footnotes
Contributorship: All authors contributed significantly to the study’s conception and design. Dr. Francesc Goñalons-Giol and Dr. Jose María Mora Guix were responsible for material preparation, data collection and analysis. Francesc Goñalons-Giol wrote the first draft of the manuscript, with all authors providing feedback on previous versions. All authors have read and approved the final manuscript.
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Ethical approval: CEIC Consorci Sanitari de Terrassa.
Funding: The authors received no financial support for the research, authorship, and/or publication of this article.
Guarantor: FGG.
Informed consent: Yes.
ORCID iD: Francesc Goñalons-Giol https://orcid.org/0000-0002-1776-5769
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