Autologous distal clavicle versus autologous coracoid bone grafts for restoration of anterior-inferior glenoid bone loss: a biomechanical comparison

BACKGROUND

Treating anterior glenoid bone loss in patients with recurrent shoulder instability is challenging. Coracoid transfer techniques are associated with neurologic complications and neuroanatomic alterations. The purpose of our study was to compare the contact area and pressures of a distal clavicle autograft with a coracoid bone graft for the restoration of anterior glenoid bone loss. We hypothesized that a distal clavicle autograft would be as effective as a coracoid graft.

METHODS

In 13 fresh-frozen cadaveric shoulder specimens, we harvested the distal 1.0 cm of each clavicle and the coracoid bone resection required for a Latarjet procedure. A compressive load of 440 N was applied across the glenohumeral joint at 30° and 60° of abduction, as well as 60° of abduction with 90° of external rotation. Pressure-sensitive film was used to determine normal glenohumeral contact area and pressures. In each specimen, we created a vertical, 25% anterior bone defect, reconstructed with distal clavicle (articular surface and undersurface) and coracoid bone grafts, and determined the glenohumeral contact area and pressures. We used analysis of variance for group comparisons and a Tukey post hoc test for individual comparisons (with P <.05 indicating a significant difference).

RESULTS

The articular distal clavicle bone graft provided the lowest mean pressure in all testing positions. The coracoid bone graft provided the greatest contact area in all humeral positions, but the difference was not significant.

CONCLUSION

An articular distal clavicle bone graft is comparable in glenohumeral contact area and pressures to an optimally placed coracoid bone graft for restoring glenoid bone loss.

LEVEL OF EVIDENCE

Basic Science Study; Biomechanics.

Correlation Between Clinical Outcomes and Anatomic Reconstruction with Anatomic Total Shoulder Arthroplasty

Many glenohumeral studies have demonstrated how anatomy varies across the population. Third and fourth generation shoulder prosthesis designs were developed to provide greater modularity and size ranges to better reproduce this anatomy and thus achieve better results in terms of shoulder function. This study quantifies the quality of anatomic reconstruction and compares that to long-term clinical outcomes using one fourth generation platform shoulder system.

METHODOLOGY

One hundred and forty primary total shoulder arthroplasties were performed by one experienced single surgeon between 2001 and 2009, using the same fourth generation modular prosthesis. Pre- and postoperative clinical assessments were quantified with the Constant, ASES, SPADI, SST, and UCLA scores, and active range of motion was measured. Five anatomic parameters were defined, measured, and compared pre- and postoperatively on the anterior-posterior (AP) radiographs: Humeral Head Height (HHH), Humeral Head Centering (HHC), Humeral Head Medial Offset (HHMO), Humeral Head Diameter (HHD), and Humeral Neck Angle (HNA). The differences between each of the parameters were then calculated and rated from 0 to 2 and then summed for each patient to obtain the Anatomic Reconstruction Index (ARI), which objectively quantifies and assesses the quality of the anatomic reconstruction. Patients were sorted based upon their ARI score into two groups (ARI 5 to 7 and ARI 8 to 10), and their latest follow-up outcomes were compared using the Mann-Whitney test to identify differences in preoperative and postoperative results, where p < 0.05 denoted a significant difference.

RESULTS

Of the 140 primary prostheses performed, 78 patients were lost to follow-up, and 13 were excluded for complications that were not related to the anatomic reconstruction. Forty-nine patients (75.8 yrs., 31F/18M) were included with an average follow-up of 9.1 years. The average score for HHH was 1.9 ± 0.4, 1.8 ± 0.5 for HHC, 1.7 ± 0.5 for HHMO, 1.7 ± 0.5 for HHD, and 1.5 ± 0.7 for HNA. Thus, all reconstructions were rated good to excellent with 86% of very good/excellent reconstruction (ARI 8 to 10) and 14% good reconstruction (ARI 5 to 7). A comparison of radiographic anatomic parameters was performed for these two cohorts: HHC (< 0.0001), HNA (0.000), and ARI (<0.0001) were significantly greater for the ARI 8 to 10 cohort. Four of five postoperative clinical outcome metrics for the ARI 8 to 10 cohort were significantly greater than the mean values for the ARI 5 to 7 cohort. Additionally, mean postoperative pain on a daily basis and shoulder function for the ARI 8 to 10 cohort were significantly greater than that for the ARI 5 to 7 cohort.

DISCUSSION

The relatively small number of good reconstructions (14%) compared to very good/excellent reconstructions (86%) and the absence of fair/poor reconstructions limited the ability for any strong linear correlations between anatomical reconstruction and clinical parameters. Despite this, patients with larger mean ARI scores were associated with significantly better outcomes for some measures. This study is limited by the use of 2D assessments from standard AP radiographs; this method can be further refined by the use of 3D quantitative assessment of each parameter.

CONCLUSION

This study confirmed that an improved anatomic reconstruction results in better postoperative clinica outcomes. The fourth generation prosthesis used for this study allows continuous in-situ adjustment of the humeral head orientation through use of the spherical taper on the replicator plate and also a double adjustment of offset through the use of the offset humeral head and offset taper on the replicator plate.

Optimization of Cemented Glenoid Peg Geometry. A Comparison of Resistance to Axial Distraction

INTRODUCTION

Glenoid loosening is one of the most common complications of anatomic total shoulder arthroplasty (aTSA). Numerous glenoid pegged designs exist within the market place; however, little effort has been made to optimize peg geometry, and as a result, there is no consensus regarding the superiority of one design over another. The aim of this study was to determine the impact of peg design on the fixation strength by comparing the force and displacement associated with five different geometries of cemented glenoid components when each is axially displaced from two different densities of polyurethane bone substitute substrates.

METHODS

An axial pull-out test was conducted on five different cemented peg geometries in both low- and high- density polyurethane bone-substitute blocks. All substrates were prepared utilizing a drill, which created a 7.3 mm diameter hole to a depth of 26.8 mm. Cemex ® brand bone cement was prepared and used to cement all pegs. After cementation of each peg, an electromechanical load frame applied a linear ramp displacement of 10 mm/minute axially to each peg while the polyurethane block was fully con strained. Load and displacement of each peg was sampled at 100 Hz until failure and axial distraction of each peg where the peak pull-out force and associated displacement were recorded. The average load to failure and associated displacement for each peg geometry were compared utilizing the Student's t-test where a p-value < 0.05 determined significance.

RESULTS

Cemented peg design #3 was associated with the greatest axial load to failure (675.3 N ± 18.8 N in low density and 707.3 N ± 11.7 N in high density) for both densities of bone-substitute blocks. Peg designs #5 and #2 were associated with the next highest axial loads to failure in both low and high density blocks. Finally, peg designs #1 and #4 were associated with the lowest axial loads to failure in both low and high density blocks. Only design #3 had a statistically significant difference between peak pull-out forces between the low- and high-density bone substitute blocks, as compared to all other designs.

CONCLUSIONS

The results of this study demonstrate that glenoid peg geometry can significantly influence the resistance to axial distraction where the continuous threaded geometry exemplified by peg design #3 demonstrated significantly superior cemented fixation relative to the other peg designs. It can be concluded that overall macrostructure and design of the peg itself plays a key role in pull-out force; however, performance in a clinical setting is required to confirm these biomechanical results.

Analysis of Glenoid Fixation with Anatomic Total Shoulder Arthroplasty in an Extreme Cyclic Loading Scenario

INTRODUCTION

ASTM F2028-14 was adopted to recom mend a cyclic eccentric glenoid edge loading test that simulates the rocking horse loading mechanism beleived to cause aTSA glenoid loosening. While this method accurately simulates that failure mechanism, the recommended 750 N load may not be sufficient to simulate worst-case loading magnitudes, and the recommended 100,000 cycles may not be sufficient to simulate device fatigue-related failure modes. Finally, if greater loading magnitude or a larger number of cycles is performed, the recommended substrate density may not be sufficiently strong to support the elevated loads and cycles. To this end, a new test method is proposed to supplement ASTM F2028-14.

METHODS

A series of cyclic tests were performed to evaluate the long-term fixation strength of two different hybrid glenoid designs in both low (15 pcf) and high (30 pcf) density polyurethane blocks at elevated loads relative to ASTM F2028-14. To simulate a worst case clinical condition in which the humeral head is superiorly migrated, a cyclic load was applied to the superior glenoid rim to induce a maximum torque on the fixation pegs for three different cyclic loading tests: 1. 1,250 N load for 0.75 M cycles in a 15 pcf block, 2. 1,250 N load for 1.5 M cycles in a 30 pcf block, and 3. 2,000 N load for 0.65 M cycles in a 30 pcf block.

RESULTS

All devices completed cyclic loading without failure, fracture, or loss of fixation regardless of glenoid design, polyurethane density, loading magnitude, or cycle length. No significant difference in post-cyclic displacement was noted between designs in any of the three tests. Post-cyclic radiographs demonstrated that each device maintained fixa - tion with the metal pegs within the bone-substitute blocks with no fatigue related failures.

DISCUSSION

These results demonstrate that both cemented hybrid glenoids maintained fixation when tested according to each cyclic loading scenario, with no difference in post-cyclic displacement observed between designs. The lack of fatigue-related failures in these elevated load and high cycle test scenarios are promising, as are the relatively low displacements given the extreme nature of each test. This cyclic loading method is intended to supplement the ASTM F2028-14 standard that adequately simulates the rocking horse loading mechanism but may not adequately simulate the fatigue-related failure modes.

Bone Grafting the Glenoid Versus Use of Augmented Glenoid Baseplates with Reverse Shoulder Arthroplasty

BACKGROUND

Large glenoid defects are a difficult reconstructive problem for surgeons performing reverse shoulder arthroplasty (rTSA). Options to address glenoid defects include eccentric reaming, bone grafting, and augmented glenoid baseplates. Augmented glenoid baseplates may provide a simpler, cost-effective, bone-preserving option compared to other techniques. No studies report the use of augmented baseplates to correct glenoid deformity in rTSA relative to the use of glenoid bone graft.

MATERIALS AND METHODS

We retrospectively reviewed 80 patients that received a primary rTSA and received either a structural bone graft or an augmented glenoid baseplate to address a significant glenoid defect. There were 39 patients in the augmented baseplate cohort and 41 patients in the bone graft cohort. The augmented baseplate cohort contained 24 8° posterior augment implants and 15 10° superior augment baseplates. The bone graft cohort consisted of 36 autograft humeral heads and 5 allograft femoral heads. The average follow-up for rTSA patients with an augmented baseplate was 28.3 ± 5.7 months, and the average follow-up for rTSA patients with glenoid bone graft was 34.1 ± 15.0 months. Each patient was scored preoperatively and at latest follow-up using the SST, UCLA, ASES, Constant, and SPADI metrics. Range of motion data was obtained as well.

RESULTS

All patients demonstrated significant improvements in pain, ROM, and functional scores following treatment with rTSA using either augmented baseplates or glenoid bone graft to correct glenoid defects. The database contained no complications for the augmented glenoid baseplate cohort, and six complications (14.6%) for the glenoid bone graft cohort (including two glenoid loosenings and graft failures). Additionally, the augmented baseplate cohort showed a lower scapular notching rate of 10% as compared to the bone graft cohort which had a notching rate of 18.5%.

DISCUSSION

The results of this study suggest that either augmented glenoid baseplates or glenoid bone graft can be used to address large glenoid defects during rTSA with significant improvement in outcomes. Augmented glenoid baseplates may achieve a lower complication and scapular notching rate, but additional and longer-term clinical follow-up is required to confirm these results.

Preliminary Results of a Novel Hybrid Cage Glenoid Compared to an All-Polyethylene Glenoid in Total Shoulder Arthroplasty

INTRODUCTION

The aim of this study was to evaluate the preliminary outcomes of a hybrid cage glenoid design in comparison to pegged all-polyethylene glenoid components in anatomic total shoulder arthroplasty (aTSA).

MATERIALS AND METHODS

Ninety-two patients undergoing primary anatomic total shoulder arthroplasty with minimum two-year follow-up were reviewed. Forty-six patients had an ultra-high molecular weight polyethylene (UHMWPE) cemented pegged glenoid component, and 46 had a hybrid cage glenoid component. Patient data was retrospectively reviewed from prospectively acquired data in a multi-institutional IRB approved database. These age, gender, and follow-up matched patients were evaluated and scored preoperatively and a latest follow-up using the SST, UCLA, ASES, Constant, and SPADI scoring metrics. Additional measures included active abduction, elevation, and external rotation. Radiolucent line assessment of the glenoid was performed by use of a Grashey and axillary radiograph at latest follow-up. A Student's two tailed, unpaired t-test was used to identify differences in preoperative and postoperative results, where p < 0.05 denoted a significant difference.

RESULTS

All patients demonstrated significant improvements in pain and function following treatment with the primary aTSA. The database contained three complications for the aTSA patients with a cage glenoid, and three complications for patients with a UHMWPE pegged glenoid. Radiographic data was available for 37 of 46 cage glenoid patients and 29 of 46 UHMWPE pegged glenoid patients. Five of 37 cage glenoid patients had a radiolucent line (13.5%) with an average radiographic line score of 0.22. Eight of 29 UHMWPE peg glenoid patients had a radiolucent line (27.6%) with an average radiographic line score of 0.57. Cage aTSA patients were associated with significantly less blood loss than aTSA UHMWPE pegged glenoid patients (avg. blood loss = 242 vs. 337; p = 0.022).

CONCLUSION

At minimum two-year follow-up, hybrid cage aTSA components show equal clinical outcomes to UHMWPE pegged glenoids. However, the hybrid cage components had significantly fewer radiolucent lines and less intra-operative blood loss. Additional and longer-term clinical and radiographic follow-up is necessary to confirm these promising early results.

Preliminary Results of a Posterior Augmented Glenoid Compared to an all Polyethylene Standard Glenoid in Anatomic Total Shoulder Arthroplasty

INTRODUCTION

Osteoarthritis of the shoulder often results in significant posterior glenoid wear. The options for treating this have been eccentric glenoid reaming and occasionally bone grafting. More recently reverse total shoulder arthroplasty (rTSA) with or without bone grafting and posterior augmented glenoids (PAGs) has been introduced. The PAG restores the native joint line while reaming a minimal amount of glenoid bone. The purpose of this study is to compare osteoarthritic shoulders with significant posterior glenoid wear treated with anatomic total shoulder arthroplasty (aTSA) using a PAG to shoulders without glenoid wear treated with aTSA using a standard all poly pegged glenoid.

METHODS

The patients' data in this study were retrospectively queried from prospectively acquired data in a multi-institutional IRB approved database. The study population consisted of 24 patients with osteoarthritis and posterior glenoid wear who were treated with aTSA using a PAG with a minimum of two-year follow-up. This population was age, sex, and follow-up matched to patients treated with an all poly non-augmented pegged glenoid (NAG) for osteoarthritis. Seven females and 17 males with an average age of 65.8 ± 11.5 years received a posterior augmented glenoid. The control group consisted of 7 females and 17 males with an average age of 66.4 ± 9.1 years who underwent aTSA for osteoarthritis using an all poly standard glenoid. These age, gender, and follow-up matched patients were evaluated and scored preoperatively and at latest follow-up using the SST, UCLA, ASES, Constant, and SPADI scoring metrics; active abduction, elevation, and external rotation were also measured. A Grashey and axillary lateral radiograph was evaluated at two-year follow-up. The Shoulder Arthroplasty Subluxation Index was used to determine the degree of humeral component subluxation on the glenoid component. A Student's two-tailed, unpaired t-test was used to identify differences in preoperative and postoperative results, where p < 0.05 denoted a significant difference.

RESULTS

All patients demonstrated significant improvements in pain and function with the primary aTSA. Sixty per-cent of PAG shoulders had a radiolucent line with an average radiographic line score of 1.10, and 33.3% of NAG had a radiolucent line with an average radiographic line score of 0.438. One glenoid in the PAG group is radiographically but not clinically loose. In the PAG group, the Grashey view showed that 18/20 humeral heads were centered with the two remaining joints demonstrating superior subluxation. On the axillary lateral in the PAG group, 17/20 humeral heads were centered, and three were anteriorly subluxated; none were posteriorly subluxated. There were no differences in any of the measured postoperative clinical outcomes or any difference in improvement between the two groups.

DISCUSSION

At a minimum of two-year follow-up, there were no statistical clinical differences between the PAG and NAG groups despite the PAG group being disadvantaged with posterior worn glenoids. There were no revisions in either group. No humeral heads resubluxated posteriorly. The PAG group had a higher incidence of lucent lines. Based on this short-term follow-up, a posterior augmented glenoid is a viable option for the posterior worn osteoarthritic glenoid.

Reverse Shoulder Arthroplasty Augments for Glenoid Wear. Comparison of Posterior Augments to Superior Augments

INTRODUCTION

Patients who are candidates for a reverse total shoulder arthroplasty (rTSA) may have varying amounts and patterns of glenoid wear. The usual treatment of these deformities has been eccentric reaming or bone grafting. Eccentric reaming often removes a large amount of subchondral bone. Bone grafting is technically more difficult and introduces another mode of failure if the graft does not heal. The purpose of this study is to evaluate patients undergoing a rTSA with concomitant superior or posterior glenoid wear who were treated with a superior augmented baseplate (SAB) or posterior augmented baseplate (PAB) without eccentric reaming or bone grafting.

MATERIALS AND METHODS

Prospectively obtained data were queried from a multi-institutional IRB-approved database. Preoperative and postoperative data were analyzed from 39 patients who received a primary rTSA with either an 8° PAB or a 10° SAB and a minimum of 2 years follow-up. Twenty-four (10 females and 14 males, aged 72.3 ± 8.2 years) received a primary rTSA shoulder with a PAB. Fifteen patients (4 females and 11 males, aged 71.7 ± 9.2 years) received a primary rTSA shoulder with a SAB. Each patient was scored preoperatively and at latest follow-up using the SST, UCLA, ASES, Constant, and SPADI metrics. Active abduction, forward flexion, and active and passive external rotation with the arm at the side were also measured. The average follow-up for rTSA patients with a PAB was 25.6 ± 3.1 months, and the average follow-up for rTSA patients with a SAB was 32.5 ± 6.5 months. A Student's two-tailed, unpaired t-test was used to identify differences in preoperative and postoperative results, where p < 0.05 denoted a significant difference.

RESULTS

All patients in both groups demonstrated significant improvements in pain and function following treatment with the reverse shoulder arthroplasty. The PAB rTSA cohort had a scapular notching rate of 6.3%, whereas the SAB rTSA cohort had a scapular notching rate of 14.3%. The PAB outperformed the SAB with the ASES, Constant, and active forward elevation measures.

DISCUSSION

The PAB group outperformed the SAB group with the ASES and Constant outcome scores and forward flexion. The reason for this is unknown; however, it may be due to the posterior augment baseplate itself tensioning the remaining external rotators better than the superior augment, or it may be that the posterior augment group had a better posterior cuff. Both implant groups had no revisions or dislocations and had a low notching rate. It appears that a SAB for superior glenoid wear and a PAB for posterior glenoid wear are viable simple solutions in patients undergoing a rTSA, where each preserves glenoid bone and eliminates the need for glenoid bone grafting.

Evaluation of Preoperative Implant Placement in Total Shoulder Arthroplasty

INTRODUCTION

New technology to assist with glenoid placement in shoulder arthroplasty has evolved to include preoperative planning tools and intraoperative guides. These tools provide surgeons with a more complete understanding of glenoid anatomy prior to surgery. However, there have been no studies identifying the information that most influences surgical decision making. Further, there have been few studies that quantify intraoperative identification of scapular landmarks required to execute a preoperative plan. The purpose of this study is to examine the variables that are considered when making a preoperative plan in shoulder arthroplasty.

METHODS

The first part of this study was a cadaveric lab in which three surgeons identified the neutral axis in surgical simulation. The second part of the study utilized a preliminary software tool in which surgeons were able to place glenoid implants in a set of CT reconstructions utilizing standard pegged glenoid components. In the third part of the study, surgeons utilized a novel planning software that included the ability to view the 3D reconstructed glenoid in all planes simultaneously and place either standard or augmented glenoid implants. The results of these three studies were compared.

RESULTS

The center of the glenoid identified in the cadaver lab was 1.69 mm ± 1.58 mm anterior and 1.99 mm ± 2.49 mm superior to center. The identified neutral axis was tilted 14.2° ± 9.2° superior to the Friedman axis with 11.8° ± 7.9° of retroversion relative to that axis. Using the novel preoperative planning tool, the surgeons placed implants less than 0.5 mm from the center of the glenoid (AP = -0.07 mm ± 0.42 mm, SI = 0.44 mm ± 0.82 mm) with an average retroversion of less than 1° (-0.96° ± 3.04°).

CONCLUSION

There was a discernible difference between the neutral axis identified in the cadaveric simulation (aver age of 14.2° superior and 11.8° retroverted) and the implant orientation planned using preoperative software (average of 3.26° superior and 0.96° retroverted). Based on the variability of position and orientation seen cadaverically, it is concluded that additional intraoperative guidance is needed alongside a preoperative plan in order to execute ideal placement of the glenoid component.

Distribution of Glenoid Implant Options for Correcting Deformities Using a Preoperative Planning Tool

Preoperative planning tools in shoulder arthroplasty are a recently developing technology with the advantage of being able to clearly assess patient anatomy and deformities before entering the OR. Addressing retroverted glenoids remains one of the most difficult aspects of primary shoulder arthroplasty. In this study, five surgeons were provided with a preoperative planning tool with posterior augmented glenoid implant options (0°, 8°, and 16°) to treat 10 cadaveric cases with a range of versions from 7.8° anteversion to 25.1° retroversion. Surgeons were able to remove less bone using 8° augmented implants over standard non-augmented implants (2.8° reamed vs. 6.4° reamed) and were able to correct each case on average within ± 1.8° of neutral version. Slight glenoid vault perforation was observed in 18% of the plans. Eight degrees posterior augmented implants were used in scans averaging 9.0° retroversion, and 16° posterior augmented implants were used in scans averaging 20.6° retroversion. Results were then compared to 14 preoperative CT scans provided by one of the surgeons in which both 8° and 16° posterior augmented glenoid implants were used in actual patients, showing 8° posterior augmented implants were used in cases averaging 12.3° retroversion, and 16° posterior augmented implants were used in cases averaging 20.7° retroversion. The study shows that surgeons can effectively and predictably use a preoperative planning tool to correct glenoid abnormalities using augmented implant solutions while minimizing both scapular bone removal and vault perforation and maximizing version correction.

Optimizing Deltoid Efficiency with Reverse Shoulder Arthroplasty Using a Novel Inset Center of Rotation Glenosphere Design

INTRODUCTION

Paul Grammont's hemispherical gleno sphere concept medializes the center of rotation (CoR) to the glenoid face to increase deltoid abductor moment arms and improve muscle efficiency. Reducing glenosphere thickness to less than half its spherical radius further medializes the CoR and offers the potential for even greater improvements in efficiency. To that end, this study quantifies deltoid abducttor moment arms for six different rTSA prostheses during scapular abduction from 0° to 140°.

METHODS

A 3D computer model was developed in Uni graphics to quantify deltoid moment arms during scapular abduction for the normal anatomic shoulder, the 36 mm Grammont Delta III (Depuy, Inc.), 36 mm BIO-RSA ® (Tornier, Inc.), the 32 mm RSP® (DJO, Inc.), and the Equinoxe® rTSA (Exactech, Inc.) with three different glenosphere geometries: 38 mm x 21 mm, 46 mm x 25 mm, and the novel 46 mm x 21 mm. Each muscle was simulated as three lines from origin to insertion as the arm was elevated; positional data was exported to Matlab where the abductor moment arms were calculated for the anterior, middle, and posterior deltoid from 0° to 140° humeral abduction in the scapular plane using a 1.8:1 scapular rhythm.

RESULTS

The 46 mm x 21 mm glenosphere had the larg est average abductor moment arms and also the largest efficiency for all three heads of the deltoid, having a 4.8% to 40.7% increase in the average deltoid efficiency relative to all other designs tested. The glenosphere design with the next most efficient deltoid was the 36 mm Delta III, which had the next most medialized CoR. The two least efficient designs were the BIO-RSA ® and the DJO RSP® , which had the most lateral CoR.

DISCUSSION

These results provide new biomechanical insights on the impact of glenosphere geometry on deltoid abductor moment arms and demonstrate that subtle changes in rTSA prosthesis design can result in dramatic improvements. Increasing glenosphere diameter while also decreasing thickness to be less than half its spherical radius may minimize the muscle forces required to perform activities of daily living. Clinical follow-up is necessary to demonstrate a reduction in complications related to joint over-loading and also demonstrate greater increases in range of motion for patients with weak musculature.

The Impact of Posterior Wear on Reverse Shoulder Glenoid Fixation

INTRODUCTION

Achieving glenoid fixation with posterior bone loss can be challenging. The purpose of this study was to quantify the impact of two different sizes of posterior glenoid defects (10° and 20°) on reverse shoulder arthroplasty (rTSA) glenoid baseplate fixation and determine if utilizing different sizes of posterior augmented baseplates (8° and 16°) with off-axis reaming provides comparable fixation to using a standard baseplate with different amounts of eccentric reaming.

METHODS

We quantified the impact of 10° and 20° posterior glenoid defects on rTSA baseplate fixation in composite scapulae using the ASTM F2028-14 rTSA glenoid loosening test method. Forty-two total implants (N = 7 for each size defect and for each type of baseplate) were tested at 750 N for 10,000 cycles. Baseplate displacement was measured before and after cyclic loading in the superior-inferior and anterior-posterior directions. Statistical analysis was performed with a two-tailed unpaired Student's t-test (significance defined as p < 0.05) to compare prosthesis displacements relative to each scapula (10° and 20° posterior defects for each type of baseplate versus the non-defect control) before and after cyclic loading.

RESULTS

All glenoid baseplates remained well-fixed after cyclic loading in composite scapulae without a defect and in scapulae with posterior defects. Increased pre- and post-cyclic displacement was observed with increased posterior defect size and differences in displacement were observed between standard and augmented baseplates. Augmented baseplates were observed to remove significantly less bone than standard baseplates when correcting posterior defects, regardless of size.

DISCUSSION

Both standard baseplates with eccentric reaming and two different sizes of augmented baseplates with off-axis reaming successfully maintained fixation following cyclic loading in composite scapula with corrected 10° and 20° posterior glenoid defects. Augmented glenoids may be more advantageous long-term from a fixation perspective as they preserve more subchondral glenoid bone due to the minimal reaming occurring by the off-axis method. Mid and long-term clinical follow-up comparisons of outcomes are necessary between these two techniques.

Reverse Shoulder Arthroplasty Prosthesis Design Classification System

Multiple different reverse total shoulder arthroplasty (rTSA) prosthesis designs are available in the global marketplace for surgeons to perform this growing procedure. Subtle differences in rTSA prosthesis design parameters have been shown to have significant biomechanical impact and clinical consequences. We propose an rTSA prosthesis design classification system to objectively identify and categorize different designs based upon their specific glenoid and humeral prosthetic characteristics for the purpose of standardizing nomenclature that will help the orthopaedic surgeon determine which combination of design configurations best suit a given clinical scenario. The impact of each prosthesis classification type on shoulder muscle length and deltoid wrapping are also described to illustrate how each prosthesis classification type impacts these biomechanical parameters.

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.

Effect of return to overuse activity following an isolated supraspinatus tendon tear on adjacent intact tendons and glenoid cartilage in a rat model

Rotator cuff tears are common conditions that can alter shoulder mechanics and may lead to damage of intact joint tissues. These injuries are of particular concern in populations who perform tasks requiring repetitive overhead activity (e.g., athletes and laborers) and who are likely to return to aggressive pre-injury activity levels despite limited understanding of the potentially damaging effects on the remaining tissues. Therefore, we investigated the effect of returning to overuse activity following a supraspinatus tear on shoulder function and the mechanical properties of the remaining intact tendons and glenoid cartilage. Forty rats underwent 4 weeks of overuse activity to create a tendinopathic condition followed by detachment of the supraspinatus tendon and were then randomized into two groups: continued overuse or cage activity. Ambulatory measurements were performed throughout the 8 weeks prior to euthaniasia, and properties of the adjacent tendons and cartilage were evaluated. Results demonstrated that shoulder function was not compromised in the return to overuse group. However, alterations of the glenoid cartilage and biceps tendon properties occurred. Our results help define the contributory roles of common mechanical injury mechanisms and provide a framework by which physicians could better prescribe long-term treatment strategies for patients.