Hemiarthroplasty of the shoulder is used to treat pain resulting from conditions that damage the quality and congruity of the proximal humeral articular surface. The most common indications for hemiarthroplasty are arthritis, osteonecrosis, and large articular defects resulting from trauma or metabolic insults. Since Neer described his initial success with hemiarthroplasty for the treatment of arthritis of the glenohumeral joint in the 1970s, multiple authors have demonstrated satisfactory improvement in pain and clinical assessment measures following shoulder hemiarthroplasty1-5. The procedure has proven successful, with results dependent on multiple factors including wear of the glenoid articular surface, the condition of the adjacent soft tissues, and the postoperative rehabilitation regimen6-8.
More recently, resurfacing of the humeral head has gained interest as an alternative to traditional hemiarthroplasty because it preserves bone stock and offers the potential benefit of an anatomic restoration of the native geometry of the glenohumeral articulation. Short and medium-term clinical follow-up of patients undergoing resurfacing has demonstrated improved Constant scores9,10. The better preservation of bone stock during resurfacing compared with traditional hemiarthroplasty may make the procedure suitable for a younger, more active population with humeral arthrosis. Short-term follow-up of such patients has revealed clinical improvement following resurfacing11.
The purpose of the current study was to characterize the biomechanical consequences of hemiarthroplasty of the humeral head with use of a stemmed implant and surface replacement with use of an anatomic resurfacing implant in a cadaveric model. We hypothesized that humeral head resurfacing would better replicate the kinematics and contact characteristics of the intact glenohumeral joint than stemmed humeral hemiarthroplasty would, by preserving the anatomy of the articular surface of the humeral head.
Specimens
Seven fresh-frozen cadaveric shoulders were used. The mean donor age was sixty-eight years (range, forty-three to ninety-seven years). Two of the donors were male and five were female. All specimens were inspected macroscopically and screened to ensure the absence of rotator cuff tears, trauma, or degenerative joint disease. Each specimen was dissected free of skin and overlying soft tissue. A 30-mm cuff consisting of the tendinous insertions of the rotator cuff, pectoralis major, latissimus dorsi, and deltoid muscles was preserved for subsequent muscle loading. The long head of the biceps brachii was tenotomized in the intertubercular groove, and the coracohumeral ligament was released. The rotator interval was opened to allow later placement of an intra-articular pressure sensor. The glenoid labrum and coracoacromial ligament were preserved.
The humerus was cut 15 cm below the inferior margin of its articular surface and transfixed with a fully threaded screw that was attached to a fiberglass rod placed in the medullary canal. The scapula was mounted onto an aluminum frame such that the medial border of the scapula was flush with the base of the frame. The scapula was fixed to the frame at the subscapularis fossa with use of three transosseous screws and plaster of Paris. Three screws were inserted in the scapula and in the humerus to function as local references for kinematic measurements, and the positions of the screws were digitized with use of a MicroScribe 3DLX (Immersion Corporation, San Jose, California). The scapular markers were consistently placed on the coracoid process and on the anterolateral and the posterior aspect of the acromion. The humeral markers were positioned on the proximal and the distal aspect of the bicipital groove and on the anterior humeral metaphysis. Each specimen was tested first in the intact condition, then following resurfacing, and finally following hemiarthroplasty; thus, each specimen served as its own control.
Resurfacing
After testing in the intact condition had been completed, humeral resurfacing was performed. This procedure was begun by reflecting the subscapularis laterally and performing an anterior capsulotomy. A guidewire was positioned centrally on the humeral articular surface. After determining and recording the appropriate implant height, the titanium anchor screw was placed, and reaming of the articular surface was performed over the guidewire. Trials confirmed that the final placement and height of the humeral resurfacing implant was appropriate. The final implant, a 35 or 40-mm-diameter HemiCAP (Arthrosurface, Franklin, Massachusetts) was then impacted into place.
Hemiarthroplasty
After testing of the humeral resurfacing had been completed, a hemiarthroplasty was performed with use of a Global shoulder humeral fracture implant (DePuy Orthopaedics, Warsaw, Indiana). Access to the glenohumeral articulation was obtained by again reflecting the subscapularis and proceeding through the prior anterior capsulotomy. The resurfacing implant was removed by reinserting the guidewire in a retrograde fashion and disengaging the screw from the head. This technique preserved the articular margin in order to facilitate implantation of the stemmed hemiarthroplasty implant. A 135° valgus humeral osteotomy was performed at the humeral anatomic neck with use of the cutting guide. The retroversion of the intact specimen was maintained by cutting along the articular margin from anterior to posterior. Appropriate version was confirmed by measuring the distance from the lateral fin of the prosthesis to the posterior border of the proximal aspect of the bicipital groove and comparing this distance with published data12. The diameter of the osteotomized humeral head was measured, and a prosthetic humeral head of the closest equivalent diameter (either 44 mm, 48 mm, or 52 mm) was implanted. The appropriate height of the humeral head was determined on the basis of a visual measurement of the intact specimen that had been made prior to the resurfacing arthroplasty. An offset head was used to improve metaphyseal coverage if necessary. Plaster of Paris was used to fix the humeral stem within the medullary canal of the humerus.
Shoulder Testing
A custom shoulder testing system that permitted loading of the rotator cuff muscles and the humerothoracic muscles was used (Fig. 1). Each specimen was positioned vertically, with the scapula in 20° of forward tilt to simulate its anatomic alignment on the thorax. The humeral rod was inserted through a fiberglass arc that maintained the desired plane of elevation and abduction angle of the humerus. Abduction was measured with use of a digital level aligned with the shaft of the humerus. The distal end of the humeral rod was attached to a rotary variable-inductance transducer (RVIT 15-60; Lucas-Schaevitz, Pennsauken, New Jersey) that measured the humeral rotation with an accuracy of 0.3°. External glenohumeral rotation of 90° was defined as the point at which the bicipital groove was aligned with the anterolateral aspect of the acromion in 60° of glenohumeral abduction in the scapular plane13. The humerus remained unconstrained along its shaft (i.e., with respect to compression or distraction).
Twenty shoulder positions were tested. Neutral rotation, 30° of internal rotation, and 30° of external rotation were each tested in 20°, 40°, 60°, and 80° of abduction in the scapular plane. Neutral rotation and 30° of external rotation were also tested at these abduction angles in 45° of horizontal adduction (anterior to the scapular plane); in 80° of abduction, 30° of external rotation could not be attained, so the specimens were tested in 15° of external rotation. The tested glenohumeral abduction angles of 20°, 40°, 60°, and 80° would correspond to abduction angles of 30°, 60°, 90°, and 120° of shoulder abduction, assuming a 2:1 ratio of humeroscapular motion.
The subscapularis, supraspinatus, combined insertion of the infraspinatus and teres minor, pectoralis major, and latissimus dorsi muscles were loaded with 20 N with use of a pulley system attached to suspended weights. The deltoid was loaded with 40 N. Equal forces were used for the subscapularis, infraspinatus-teres minor, latissimus dorsi, and pectoralis major muscles in order to balance the anterior-posterior force couple14-19. Muscle forces were applied via cables sutured to the tendons. The pulleys for the supraspinatus, the subscapularis, and the combined infraspinatus and teres minor tendons were positioned by aligning each along the orientation of the tendon fibers and the central portion of its respective osseous origin. The direction of pull for the latissimus dorsi was determined with reference to the inferior angle of the scapula. The direction of pull for the pectoralis major was estimated on the basis of the orientation of the tendon and muscle fibers, since no chest wall was available as an additional reference. The direction of pull for the deltoid muscle was determined with reference to the anterolateral aspect of the acromion.
Biomechanical Measurements
The glenohumeral contact pressure and area were recorded with use of a digital pressure sensor (Model 4000; Tekscan, South Boston, Massachusetts) placed on the glenoid surface. We used the default sensor sensitivity in order to minimize artifacts due to bending and to ensure that the maximum measurable pressure of 10.3 MPa would not be exceeded. The glenohumeral mean contact pressure and peak pressure at each position were compared among the three groups. The peak pressure was defined as the highest pressure obtained over a two-pixel-by-two-pixel region on the pressure sensor. The position of the humeral head was recorded with use of the MicroScribe three-dimensional digitizing system after the three screws inserted in the scapula and in the humerus had been digitized to define the local coordinate system of each bone.
On completion of each phase of testing (intact, following humeral resurfacing, and following hemiarthroplasty), a three-dimensional point cloud representing the surface of the glenoid and another point cloud representing the surface of the humeral head were obtained with use of the digitizing system. These data points were then used to calculate the apex of the humeral head and its position relative to the geometric center of the glenoid (Fig. 2). The apex of the humeral head was defined as the highest point on the intact or reconstructed articular surface of the humeral head, measured relative to a plane defined by digitizing the articular or implant margin20. The radius and geometric center of the humeral resurfacing or the hemiarthroplasty reconstruction relative to the center of the intact humeral head were calculated with use of a sphere-fitting algorithm. The performance of the algorithm and the required number of digitized points were validated by digitizing and analyzing 42, 92, and 142 points on one hemisphere of a perfect sphere with a 10-mm radius and comparing the calculated geometric center and radius with the actual values. The absolute error between the calculated and the actual radius was <0.02 mm when 42 digitized points were used.
Data Analysis
Two trials of each dependent variable were recorded for each testing position and condition. Given the 0.3-mm accuracy of the MicroScribe and the variability due to experimental positioning, a ≥1-mm difference between the two trials was chosen as the criterion to perform a third trial. The two trials that were within the 1-mm threshold were then averaged. Data are reported as the mean and the standard error. Statistical analysis was performed with use of repeated-measures analysis of variance, and a p value of <0.05 was considered significant. A Tukey post-hoc test was performed to analyze differences among groups.
Source of Funding
Arthrosurface provided partial funding for this study, and additional funding was provided by the Department of Veterans Affairs (Research & Development and Merit Review). Neither funding source played a role in the investigation.
The design of proximal humeral implants has focused on restoring the anatomy of the intact humeral head. Humeral offset, retroversion, neck-shaft angle, head height, and radius of curvature are patient-specific. Restoring these dimensions with an implant can pose a challenge to the surgeon21,22. The use of modular proximal humeral implants in stemmed hemiarthroplasty has addressed this, in part, by improving metaphyseal coverage by the prosthetic joint surface and by offering different height options. Resurfacing eliminates the constraint involved in mating the prosthetic articular surface to a humeral stem, and positioning and sizing the implant can be performed by referencing the remaining articular surface. These two differences between resurfacing and hemiarthroplasty have the potential to allow resurfacing to better replicate the geometric anatomy and biomechanics of the intact glenohumeral articulation.
The two-dimensional radiographic appearance of the proximal aspect of the humerus in the coronal plane has been used as a benchmark for prosthetic reconstruction. For example, Wirth et al. used measurements of the relative heights of the greater tuberosity and the humeral head and of the surface arc of the humeral head, made on such radiographs, to reconstruct the humeral heads of cadaveric specimens with use of a third-generation hemiarthroplasty implant23. Radiographs made after the reconstruction confirmed accurate restoration of the surface arc and of the relationship between the greater tuberosity and humeral head heights, and the authors concluded that a surgeon with experience in the use of modular implants could replicate the humeral anatomy on the basis of these radiographic parameters. Similarly, Thomas et al. examined positioning during the Copeland cementless surface-replacement arthroplasty in a clinical setting by comparing preoperative and postoperative coronal-plane radiographs of the shoulder24. In contrast to such studies involving two-dimensional radiographic parameters, the current study quantified the position of an implant relative to the intact humeral head in three dimensions by digitizing each specimen during testing and then developing a spherical model of the proximal aspect of the humerus. Both of the reconstruction modes that we studied successfully replicated the radius of the articular surface of the intact humerus. The humeral resurfacing implant was able to replicate the geometric center of the humerus more closely than the third-generation hemiarthroplasty did.
Restoration of the geometry of the intact humerus forms a basis on which the biomechanical soundness of a humeral hemiarthroplasty can be evaluated. Pearl et al. used a computer algorithm to optimally position a model of the implant within the proximal aspect of the humerus in three dimensions and showed that humeral hemiarthroplasty with use of a third-generation implant could closely mimic the geometric center, surface arc, and articulation point of the intact proximal aspect of the humerus25. In their study, the geometric center could be optimally restored to within slightly more than 2 mm of that of the intact condition. Clinically, however, anatomic landmarks must be used to determine the position of an implant, as was the case with both of the implants used in the current study. The positioning of the resurfacing implant was registered from the margin of the humeral articular surface. In contrast, the third-generation hemiarthroplasty implant was fitted to a cut surface, and its orientation and height were based on surrounding landmarks such as the greater tuberosity and the bicipital groove. This difference in registration between resurfacing and hemiarthroplasty may decrease the potential for malpositioning of the resurfacing implant. Under the laboratory conditions used in the current study, the geometric center of the hemiarthroplasty implant was 4.7 ± 0.3 mm from the geometric center in the intact condition. The geometric center of the resurfacing implant was slightly more than 2 mm closer to the center in the intact condition than the hemiarthroplasty implant was (see Appendix).
Despite this relatively small improvement in the distance from the calculated geometric center of the intact condition, we observed better kinematic behavior following resurfacing than following hemiarthroplasty; this emphasizes the importance of reconstructing the humeral anatomy in three dimensions. de Leest et al. also emphasized the importance of accurate three-dimensional reconstruction; based on an analysis of a three-dimensional inverse dynamic model of the shoulder, they concluded that the relationship of the center of instantaneous rotation to the humeral shaft and the greater tuberosity was the most important factor for minimizing stress on the shoulder muscles26. Cadaveric studies have also been used to investigate the acceptable tolerances for the positioning of arthroplasty implants. Subacromial impingement and changes in humeral kinematics (translation with respect to the glenoid during shoulder movement) were minimized when the offset between the humeral head and shaft following total shoulder arthroplasty was <4 mm27. Increasing the height of a humeral prosthesis relative to the tuberosities by 5 mm and by 10 mm also caused a decrease in the abduction arc and a reduction in the effective moment arm of the rotator cuff28.
The glenohumeral contact area that was measured in our study following either resurfacing or hemiarthroplasty generally decreased in the midrange of vertical abduction, and the peak pressure increased at abduction angles of ≥40°, compared with that of the intact condition. The peak pressure was significantly higher than that in the intact condition in five of the twenty glenohumeral positions following resurfacing, but in only two positions following hemiarthroplasty.
A limitation of our study is that the same set of static muscle loads was used for all shoulder positions. There are an infinite number of muscle force combinations that can result in the same shoulder position, so it is not possible to uniquely determine or estimate these forces. The muscle force simulations that we used were not designed to simulate arm motion, but only to provide a nominally physiologic loading that could maintain the arm in the positions tested.
In vivo, glenohumeral disease may make reconstruction more difficult by distorting the geometry of the humeral surface. However, each specimen in our study served as its own control, as both resurfacing and hemiarthroplasty were performed after testing of the same specimen in the intact condition. The resurfacing procedure preserved the rim of the articular cartilage, which allowed assessment of the diameter and version needed for the hemiarthroplasty. Also, since we anticipated that the humeral head height would be directly affected by the resurfacing, the articular surface of each intact specimen was visually sized to determine the best offset height for the hemiarthroplasty implant prior to implanting the resurfacing implant. The hemiarthroplasty was performed with use of a readily available third-generation fracture implant with a relatively small-diameter stem. The small stem allowed improved fit of the hemiarthroplasty implant during plaster cementation and provided more room to position the implant for optimal coverage of the cut surface. Centered heads or offset heads were used as needed to improve the fit. The two resurfacing implant diameters differed by 5 mm, and instrumentation to directly measure the implant offset that would best match the surface contour of the intact humeral head was included. The three hemiarthroplasty implant sizes that were used differed in diameter by 4 mm, and the size was selected to provide the best overall coverage of the cut humeral surface as well as restoration of the height of the articular surface.
The measured translation of the humeral head was larger than that reported in prior cadaveric studies29. This greater translation is most likely related to our inclusion of the pectoralis major and latissimus dorsi muscles, whose distances from the glenohumeral articulation provide them with a substantial moment arm14. The omission of any force on the biceps tendon may have also contributed to the increased translation by eliminating that tendon's stabilizing effect on the glenohumeral joint. However, since the same testing arrangement was used for all three conditions for each specimen, failure to include the biceps would not have influenced the direct comparisons among the three conditions.
To replicate the biomechanics of the glenohumeral joint during humeral arthroplasty, it is necessary to replicate the anatomy of the joint. The position of the apex of the reconstructed humeral head relative to the glenoid should replicate that of the intact condition throughout a range of shoulder positions. The humeral resurfacing implant was better able to replicate the calculated geometric center of the intact condition than the hemiarthroplasty implant was. For this reason, resurfacing may better limit eccentric glenoid wear because the glenohumeral joint biomechanics and the moment arms of the rotator cuff and the deltoid muscle are restored more closely to those of the intact condition.
Disclosure: One or more of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of an aspect of this work. In addition, one or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.