Bone quality in total shoulder arthroplasty: a prospective study correlating computed tomography Hounsfield units with thumb test and fracture risk assessment tool score

Assessing proximal humerus cortical bone thickness for total shoulder arthroplasty: A cadaveric study

Background

In reverse total shoulder arthroplasty, a humeral osteotomy is typically performed at the anatomic neck. The quality and quantity of cancellous and cortical bone impacts sizing of implants. Little-to-no information exists characterizing the transition of this bone quality at this location. This cadaveric study focused on measuring and analyzing the transition zone of the proximal humerus bone quality to better understand the implications of utilizing different implant sizes during reverse total shoulder arthroplasty. This assessment was conducted through the usage of a newly designed indentation depth probe.

Methods

An Exactech, Inc. designed indentation depth probe was used to assess the transition from cancellous to cortical bone in 45 cadavers after a humeral osteotomy was performed. The thickness of the cortical rim was measured in regard to superior, inferior, anterior, and posterior quadrants. Linear regressions were utilized to evaluate the relationship between anatomic location in regard to cortical bone thickness. Independent sample two-tailed t-tests were also conducted to examine the relationship between spring force and bone thickness. Finally, independent sample two-tailed t-tests or Mann-Whitney U tests were utilized to examine the relationship between cortical thickness and variables including sex, age, osteoporosis, and a diagnosis of cancer.

Results

All linear regressions were found to be insignificant excluding the relationships between inferior to anterior (p = 0.003) and inferior to posterior (p = 0.001). All t-tests and Mann-Whitney U tests were found to be insignificant, except for the relationship between age and the posterior aspect (p = 0.04).

Conclusions

In our cohort, cortical bone thickness was not impacted by sex, diagnosis of osteoporosis, or diagnosis of cancer. Interestingly, age was found to only have an impact on the posterior aspect of the cortical bone rim. Differences in cortical bone thickness were also found in regard to both the inferior to anterior and inferior to posterior comparisons.

Clinical relevance

This suggests that differences in cortical bone thickness can occur, and that one point of assessment might not be sufficient.

Acromion and Clavicle Stress Fractures After Reverse Total Shoulder Arthroplasty Reflect Failure to Address Osteoporosis: A Case Report and Literature Review

The exponential increase in the rate of reverse total shoulder arthroplasty (RTSA) has been accompanied by a rise in complication rates of this procedure. Of these, peri-prosthetic stress fractures can be particularly problematic due to their potential to cause significant impairment of shoulder function. Despite the association between these stress fractures and osteopenia/osteoporosis, pre-operative bone density assessment is not standard practice for elective RTSA. We report the case of a 68-year-old female patient who, at eleven weeks after elective RTSA (for rotator cuff-tear arthropathy), experienced a non-traumatic stress (insufficiency) fracture of the acromion process of the ipsilateral scapula. Thirteen weeks later, new-onset pain occurred with minimal shoulder use, and a midshaft clavicle stress fracture was detected. She was then diagnosed and treated for osteoporosis, vitamin D deficiency, and hypothyroidism. An ultrasound-based bone-growth stimulator was used to treat both fractures, but only the acromion fracture healed. The clavicle fracture became a 100% displaced chronic non-union. However, the patient felt that surgical fixation of the clavicle fracture would not provide a significant benefit. At 1.5 years after the RTSA, she was moderately satisfied with her shoulder function and highly satisfied with pain reduction, and no additional surgery was required. This is the first reported case describing a patient with acromion and clavicle stress fractures occurring in association with ipsilateral RTSA. We also review the literature of cases with clavicle stress fractures in association with RTSA and highlight key findings: (i) the prevalence of osteoporosis in the population undergoing shoulder arthroplasty is high and (ii) performing shoulder arthroplasty on patients with poor bone quality presents multiple challenges that are underappreciated. This case underscores the importance of pre-operative bone density/health screening to mitigate stress fracture risk after RTSA.

Machine learning models can define clinically relevant bone density subgroups based on patient-specific calibrated computed tomography scans in patients undergoing reverse shoulder arthroplasty

BACKGROUND

Reduced bone density is recognized as a predictor for potential complications in reverse shoulder arthroplasty (RSA). While humeral and glenoid planning based on preoperative computed tomography (CT) scans assist in implant selection and position, reproducible methods for quantifying the patients' bone density are currently not available. The purpose of this study was to perform bone density analyses including patient-specific calibration in an RSA cohort based on preoperative CT imaging. It was hypothesized that preoperative CT bone density measures would provide objective quantification of the patients' humeral bone quality.

METHODS

This study consisted of 3 parts, (1) analysis of a patient-specific calibration method in cadaveric CT scans, (2) retrospective application in a clinical RSA cohort, and (3) clustering and classification with machine learning (ML) models. Forty cadaveric shoulders were scanned in a clinical CT and compared regarding calibration with density phantoms, air muscle, and fat (patient-specific) or standard Hounsfield unit. Postscan patient-specific calibration was used to improve the extraction of 3-dimensional regions of interest for retrospective bone density analysis in a clinical RSA cohort (n = 345). ML models were used to improve the clustering (Hierarchical Ward) and classification (support vector machine) of low bone densities in the respective patients.

RESULTS

The patient-specific calibration method demonstrated improved accuracy with excellent intraclass correlation coefficients for cylindrical cancellous bone densities (intraclass correlation coefficient >0.75). Clustering partitioned the training data set into a high-density subgroup consisting of 96 patients and a low-density subgroup consisting of 146 patients, showing significant differences between these groups. The support vector machine showed optimized prediction accuracy of low and high bone densities compared to conventional statistics in the training (accuracy = 91.2%; area under curve = 0.967) and testing (accuracy = 90.5%; area under curve = 0.958) data set.

CONCLUSION

Preoperative CT scans can be used to quantify the proximal humeral bone quality in patients undergoing RSA. The use of ML models and patient-specific calibration on bone mineral density demonstrated that multiple three-dimensional bone density scores improved the accuracy of objective preoperative bone quality assessment. The trained model could provide preoperative information to surgeons treating patients with potentially poor bone quality.

Reverse Shoulder Arthroplasty Baseplate Stability Is Affected by Bone Density and the Type and Amount of Augmentation

OBJECTIVE

This study evaluated the effects of bony increased offset (BIO) and metallic augments (MAs) on primary reverse shoulder arthroplasty (RSA) baseplate stability in cadaveric specimens with variable bone densities.

METHODS

Thirty cadaveric specimens were analyzed in an imaging and biomechanical investigation. Computed tomography (CT) scans allowed for preoperative RSA planning and bone density analysis. Three correction methods of the glenoid were used: (1) corrective reaming with a standard baseplate, which served as the reference group (n = 10); (2) MA-RSA (n = 10); and (3) angled BIO-RSA (n = 10). Each augment group consisted of 10° (n = 5) and 20° (n = 5) corrections. Biomechanical testing included cyclic loading in an articulating setup, with optical pre- and post-cyclic micromotion measurements in a rocking horse setup.

RESULTS

There were no differences in bone density between groups based on CT scans (p > 0.126). The BIO-RSA group had higher variability in micromotion compared to the MA-RSA and reference groups (p = 0.013), and increased total micromotion compared to the reference group (p = 0.039). Both augmentations using 20° corrections had increased variance in rotational stability compared to the reference group (p = 0.043). Micromotion correlated with the subchondral bone density in the BIO-RSA group (r = -0.63, p = 0.036), but not in the MA-RSA (p > 0.178) or reference (p > 0.117) groups.

CONCLUSIONS

Time-zero baseplate implant fixation is more variable with BIO-RSA and correlates with bone density. Corrections of 20° with either augmentation approach increase variability in rotational micromotion. The preoperative quantification of bone density may be useful before utilizing 20° of correction, especially when adding a bone graft in BIO-RSAs.

A stemless anatomic shoulder arthroplasty design provides increased cortical medial calcar bone loading in variable bone densities compared to a short stem implant

Background

Several studies have reported proximal bone resorption in stemless and press-fit short-stem humeral implants for anatomic total shoulder arthroplasty. The purpose of this biomechanical study was to evaluate implant and cortical bone micromotion of a cortical rim-supported stemless implant compared to a press-fit short stem implant during cyclic loading and static compression testing.

Methods

Thirty cadaveric humeri were assigned to 3 groups based on a previously performed density analysis, adopting the metaphyseal and epiphyseal and inferior supporting bone densities for multivariate analyses. Implant fixation was performed in stemless implant in low bone density (SL-L, n = 10) or short stem implant in low bone density (Stem-L, n = 10) and in stemless implant in high bone density (SL-H, n = 10). Cyclic loading with 220 N, 520 N, and 820 N over 1000 cycles at 1.5 Hz was performed with a constant valley load of 25 N. Optical recording allowed for spatial implant tracking and quantification of cortical bone deformations in the medial calcar bone region. Implant micromotion was measured as rotational and translational displacement. Load-to-failure testing was performed at a rate of 1.5 mm/s with ultimate load and stiffness measured.

Results

The SL-H group demonstrated significantly reduced implant micromotion compared to both low-density groups (SL-L: P = .014; Stem-L: P = .031). The Stem-L group showed significantly reduced rotational motion and variance in the test results at the 820-N load level compared to the SL-L group (equal variance: P = .012). Implant micromotion and reversible bone deformation were significantly affected by increasing load (P < .001), metaphyseal cancellous (P = .023, P = .013), and inferior supporting bone density (P = .016, P = .023). Absolute cortical bone deformation was significantly increased with stemless implants in lower densities and percentage reversible bone deformation was significantly higher for the SL-H group (21 ± 7%) compared to the Stem-L group (12 ± 6%, P = .017).

Conclusion

A cortical rim-supported stemless implant maintained proximally improved dynamic bone loading in variable bone densities compared to a press-fit short stem implant. Biomechanical time-zero implant micromotion in lower bone densities was comparable between short stem and stemless implants at rehabilitation load levels (220 N, 520 N), but with higher cyclic stability and reduced variability for stemmed implantation at daily peak loads (820 N).

Preoperative 3-dimensional computed tomography bone density measures provide objective bone quality classifications for stemless anatomic total shoulder arthroplasty

BACKGROUND

Reproducible methods for determining adequate bone densities for stemless anatomic total shoulder arthroplasty (aTSA) are currently lacking. The purpose of this study was to evaluate the utility of preoperative computed tomography (CT) imaging for assessing the bone density of the proximal humerus for supportive differentiation in the decision making for stemless humeral component implantation. It was hypothesized that preoperative 3-dimensional (3-D) CT bone density measures provide objective classifications of the bone quality for stemless aTSA.

METHODS

A 3-part study was performed that included the analysis of cadaveric humerus CT scans followed by retrospective application to a clinical cohort and classification with a machine learning model. Thirty cadaveric humeri were evaluated with clinical CT and micro-CT (μCT) imaging. Phantom-calibrated CT data were used to extract 3-D regions of interest and defined radiographic scores. The final image processing script was applied retrospectively to a clinical cohort (n = 150) that had a preoperative CT and intraoperative bone density assessment using the "thumb test," followed by placement of an anatomic stemmed or stemless humeral component. Postscan patient-specific calibration was used to improve the functionality and accuracy of the density analysis. A machine learning model (Support vector machine [SVM]) was utilized to improve the classification of bone densities for a stemless humeral component.

RESULTS

The image processing of clinical CT images demonstrated good to excellent accuracy for cylindrical cancellous bone densities (metaphysis [ICC = 0.986] and epiphysis [ICC = 0.883]). Patient-specific internal calibration significantly reduced biases and unwanted variance compared with standard HU CT scans (P < .0001). The SVM showed optimized prediction accuracy compared with conventional statistics with an accuracy of 73.9% and an AUC of 0.83 based on the intraoperative decision of the surgeon. The SVM model based on density clusters increased the accuracy of the bone quality classification to 87.3% with an AUC of 0.93.

CONCLUSIONS

Preoperative CT imaging allows accurate evaluation of the bone densities in the proximal humerus. Three-dimensional regions of interest, rescaling using patient-specific calibration, and a machine learning model resulted in good to excellent prediction for objective bone quality classification. This approach may provide an objective tool extending preoperative selection criteria for stemless humeral component implantation.

Bone Density Changes at the Origin of the Deltoid Muscle following Reverse Shoulder Arthroplasty

Background: Reverse total shoulder arthroplasty (RSA) significantly impacts deltoid length, tension, and structure. Studies have extensively investigated various modifications in deltoid characteristics, such as perfusion, elasticity, caliber, histological changes, and strength post-RSA. However, to date, there is a notable absence of research evaluating changes in bone mineral density (BMD) at the deltoid muscle origin after the RSA procedure. Methods: A retrospective analysis of a consecutive series of RSAs performed between May 2011 and May 2022 was conducted. Inclusion criteria comprised primary RSAs with both preoperative and last follow-up shoulder CT scans and a minimum follow-up of 12 months. Trabecular attenuation measured in Hounsfield units (HU) was calculated using a rapid region-of-interest (ROI) method. BMD analysis involved segmenting three ROIs in both pre- and postoperative CT scans of each patient: the acromion, clavicle, and spine of the scapula. Results: A total of 44 RSAs in 43 patients, comprising 29 women and 14 men, were included in this study. The mean follow-up duration was 49 ± 22.64 months. Significant differences were observed between preoperative and postoperative HU values in all analyzed regions. Specifically, BMD increased in the acromion and spine, while it decreased in the clavicle (p-values 0.0019, <0.0001, and 0.0088, respectively). Conclusions: The modifications in shoulder biomechanics and, consequently, deltoid tension post-implantation result in discernible variations in bone quality within the analyzed regions. This study underscores the importance of thorough preoperative patient planning. By utilizing CT images routinely obtained before reverse shoulder replacement surgery, patients at high risk for fractures of the acromion, clavicle, and scapular spine can be identified.

A survey and biomechanical analysis of the feasibility of the thumb test for determining the cancellous bone quality for stemless shoulder prosthesis

BACKGROUND

The effect of the thumb test for assessing the cancellous bone quality at the resection plane of the proximal humerus on determining the application of a stemless shoulder prosthesis remains unclear. This study was conducted to survey the current utilization of the thumb test among surgeons and to investigate biomechanical features of the thumb test.

METHOD

A survey among shoulder surgeons who had experience with stemless prostheses was conducted to investigate the current utilization of preoperative assessments and intraoperative thumb test when applying stemless prosthesis. Biomechanical experiments for the thumb test using artificial bone models were performed to assess the compression force, contact pressure and area. According to the preliminary survey, three compression techniques were assessed: compression perpendicular to the surface with thumb pad (P-pad technique) or tip of the thumb (P-tip technique), or compression in the vertical direction simulating compression along the longitudinal axis of the humeral shaft with tip-pad of the thumb (H-axis technique). The contact area was separated into three subregions (proximal, middle and distal) to assess the distribution of contact pressure.

RESULTS

Among 38 surgeons, 66% utilized the thumb test intraoperatively. The P-pad technique was more frequently applied than the P-tip or H-axis techniques (80%, 4% and 16%, respectively). Although with wide variation among the examiners, biomechanical assessments revealed the P-pad technique showed larger contact area and less compression force than the P-tip technique. The P-pad technique provided no significant localized differences in the mean contact pressure on the compressed plane, whereas the P-tip and H-axis techniques showed significant differences among subregions.

CONCLUSION

This survey demonstrated relatively frequent application of the thumb test on applying the stemless shoulder prosthesis. Biomechanical assessment revealed the thumb test can hinder objective reproducibility among examiners; therefore, further investigations to identify feasible assessments of the bone quality is required.