Sergio Gutierrez

Range of impingement-free abduction and adduction deficit after reverse shoulder arthroplasty. Hierarchy of surgical and implant-design-related factors.

BACKGROUND Evaluations of functional outcomes of reverse shoulder arthroplasty have revealed variable improvements in the range of motion and high rates of scapular notching. The purpose of this study was to systematically examine the impact of surgical factors (location of the glenosphere on the glenoid and tilt angle of the glenosphere on the glenoid) and implant-related factors (implant size, center-of-rotation offset, and humeral neck-shaft angle) on impingement-free abduction motion. METHODS A computer model was developed to virtually simulate abduction/adduction motion and its dependence on five surgical and implant-related factors. Three conditions were tested for each factor, resulting in a total of 243 simulated combinations. The overall motion was determined from 0 degrees of abduction until maximum abduction, which would be limited by impingement of the humerosocket on the scapula. In those combinations in which 0 degrees of abduction could not be achieved, the adduction deficit was recorded. RESULTS The largest average increase in the range of impingement-free abduction motion resulted from a more lateral center-of-rotation offset: the average increase was 31.9 degrees with a change in the center-of-rotation offset from 0 to 10 mm, and this change resulted in an increase in abduction motion in eighty of the eighty-one combinations. The position of the glenosphere on the glenoid was associated with the second largest average increase in abduction motion (28.1 degrees when the glenosphere position was changed from superior to inferior, with the change resulting in an increase in seventy-one of the eighty-one combinations). These factors were followed by glenosphere tilt, humeral neck-shaft angle, and prosthetic size in terms of their effects on abduction motion. The largest effect in terms of avoiding an adduction deficit was provided by a humeral neck-shaft angle of 130 degrees (the deficit was avoided in forty-nine of the eighty-one combinations in which this angle was used), followed by an inferior glenosphere position on the glenoid (deficit avoided in forty-one combinations), a 10-mm lateral offset of the center of rotation, inferior tilt of the glenosphere, and a 42-mm-diameter prosthetic size. CONCLUSIONS An understanding of a hierarchy of prosthetic design and implantation factors may be important to maximize impingement-free abduction motion as well as to avoid inferior impingement.

Evaluation of abduction range of motion and avoidance of inferior scapular impingement in a reverse shoulder model.

The purpose of this study was to determine the effects of prosthetic design and surgical technique of reverse shoulder implants on total abduction range of motion and impingement on the inferior scapular neck. Custom implants in three glenosphere diameters (30, 36, and 42 mm), with 3 different centers of rotation offsets (0, +5, and +10 mm), were placed into a Sawbones scapula (Pacific Research Laboratories, Vashon, WA) in 3 different positions: superior, center, and inferior glenoid. Humeral sockets were manufactured with a 130 degrees , 150 degrees , and 170 degrees neck-shaft angle. Four independent factors (glenosphere diameter, center of rotation offset, glenosphere position on the glenoid, and humeral neck-shaft angle) were compared with the 2 dependent factors of range of motion and inferior scapular impingement. Center of rotation offset had the largest effect on range of motion, followed by glenosphere position. Neck-shaft angle had the largest effect on inferior scapular impingement, followed by glenosphere position. This information may be useful to the surgeon when deciding on the appropriate reverse implant.

Effects of tilt and glenosphere eccentricity on baseplate/ bone interface forces in a computational model, validated by a mechanical model, of reverse shoulder arthroplasty

HYPOTHESIS/BACKGROUND Reverse shoulder arthroplasty is being used with greater frequency for patients with severe rotator cuff deficiency. There are several commercially available reverse shoulder devices, each with different glenosphere options. The purpose of this study was to determine: (1) forces at the baseplate-bone interface in glenospheres with centers of rotation located concentrically and eccentrically to the center of the baseplate; and (2) if baseplate-bone forces can be optimized by altering tilt of the baseplate. METHODS A validated computer model was used to compare concentric glenospheres with neutral offset to eccentrically offset glenospheres (6 mm inferior or 6 mm lateral) in 3 baseplate tilts: 15° inferior, neutral, or 15° superior. A baseplate, simulated bone, screws, and humeral component were modeled, and forces underneath the baseplate were calculated as the arm was abducted through 90° of glenohumeral motion. RESULTS For lateral and concentric glenospheres, inferior tilt provides the most even distribution of forces (mean difference in force between superior and inferior portions of baseplate: 11.3 N and 24.7 N, respectively) and superior tilt provides the most uneven distribution of forces (109.3 N and 78.7 N, respectively). For inferior eccentric glenospheres, inferior tilt provides the most uneven distribution of forces (58.7 N) and neutral tilt provides the most even distribution of forces (27.7 N). CONCLUSION This is the first study to investigate force distribution under the baseplate in inferior eccentric glenospheres. Although inferior tilting of the baseplate is recommended for concentric and laterally offset glenospheres, this same recommendation may be detrimental to inferiorly offset glenospheres and warrants further investigation.

Center of rotation affects abduction range of motion of reverse shoulder arthroplasty.

Although clinical outcomes of the reverse shoulder replacement have noted improvements in pain and function, evaluation of these outcomes reveals concerns regarding progressive scapular notching and variability of functional improvements in range of motion. Therefore, an apparatus was designed to examine differences in abduction range of motion for seven configurations of reverse shoulder arthroplasty. An electronic goniometer was used to measure abduction range of motion, and digital video analysis was used to determine impingement points. Finally, a correlation analysis between range of motion and the effect of changing the center of rotation of the glenosphere was performed. As the center of rotation was moved more lateral from the glenoid, abduction range of motion increased. The greatest range of motion was 97° ± 0.9° using a glenoid component with a center of rotation offset 10 mm ± 0.4 mm from the glenoid. The smallest range of motion was 67° ± 1.8° using a glenosphere with a center of rotation offset 0.5 mm ± 0.1 mm from the glenoid surface. Range of motion always was limited by impingement points on the scapula. Inferiorly, adduction was limited by impingement on either the inferior scapular border or the glenoid. Superiorly, abduction was limited by impingement on the acromion. A positive linear correlation was found between abduction range of motion and center of rotation offset relative to the glenoid.

Arc of motion and socket depth in reverse shoulder implants

BACKGROUND Reverse shoulder arthroplasty relies on its congruent ball/socket joint to restore shoulder function. For a simple ball/socket joint, as shown in total hip arthroplasty, range of motion decreases with the increase of articular constraint. We challenge here that this intuitive concept might not be held in reverse shoulder arthroplasty because of the effect of multiple concurrent factors. METHODS Abduction impingement-free arc of motion in reverse shoulder arthroplasty was examined with a virtual computer model. Six articular constraints, defined by normalized socket depths, were simulated. Four concurrent factors: glenosphere diameter, lateral offset of glenosphere from the glenoid surface, humeral neck-shaft angles, and locations of the glenosphere on the glenoid surface, were also studied, which composed a total of 81 combinations and 486 individual conditions. FINDINGS Three distinct classes of arc of motion relative to the articular constraint were revealed: I--arc of motion decreased with increased constraint (57%), II--arc of motion with a complex relationship to constraint (37%), and III--arc of motion increased with increased constraint (6%). INTERPRETATION Classes II and III were counter-intuitive which could be caused by impingement on the acromion associated primarily with superior positioning. Surgeons may need to be aware of it when the glenoid component has to be placed superiorly. The detailed motion/constraint relationship will further help engineers improve the design in reverse shoulder arthroplasty.

Reverse shoulder arthroplasty components and surgical techniques that restore glenohumeral motion

BACKGROUND Modifications in reverse shoulder arthroplasty (RSA) have been made with the intent of maximizing motion, although there is little objective evidence outlining their benefit. This study investigated the RSA component combinations that impart the greatest effect on impingement-free glenohumeral motion. METHODS A previously validated virtual shoulder model was implanted with RSA components that varied by humeral implant type (inset/onset), glenosphere diameter (30, 36, and 42 mm), glenosphere placement (inferior/neutral), glenosphere center-of-rotation offset (0, 5, and 10 mm), humeral neck-shaft angle (130° and 150°), and humeral offset (zero, five, and ten mm). Motion was simulated in all technique combinations until the point of impingement in abduction, flexion/extension (F/E), and internal/external rotation (IR/ER). Regression analysis was used to rank combinations based on motion. RESULTS Of 216 possible study combinations, 126 constructs (58%) demonstrated no arm-at-side impingement and were included for analysis. Models with the largest motion in abduction, F/E, and IR/ER, respectively, were inset-42-inferior-10-150-zero (107°), inset-36-inferior-10-130-five (146°), and inset-42-inferior-10-130-ten (121°). Humeral neck-shaft angle, glenosphere center-of-rotation offset, glenosphere placement, and glenosphere diameter had a significant effect on motion in all planes tested. Of these variables, humeral neck-shaft angle was most predictive of a change in abduction and F/E motion, whereas glenosphere placement was most predictive of a change in IR/ER motion. CONCLUSION Higher glenosphere center-of-rotation offsets led to an increase in motion in all planes. To maximize motion in abduction, a valgus humeral component should be selected; to maximize F/E, a varus humeral component should be selected; and, to maximize IR/ER, the glenosphere should be placed inferiorly.

Biomechanics of lateral plate and pedicle screw constructs in lumbar spines instrumented at two levels with laterally placed interbody cages

BACKGROUND CONTEXT The lateral transpsoas approach to interbody fusion is gaining popularity because of its minimally invasive nature and resultant indirect neurologic decompression. The acute biomechanical stability of the lateral approach to interbody fusion is dependent on the type of supplemental internal fixation used. The two-hole lateral plate (LP) has been approved for clinical use for added stabilization after cage instrumentation. However, little biomechanical data exist comparing LP fixation with bilateral pedicle screw and rod (PSR) fixation. PURPOSE To biomechanically compare the acute stabilizing effects of the two-hole LP and bilateral PSR fusion constructs in lumbar spines instrumented with a lateral cage at two contiguous levels. STUDY DESIGN Biomechanical laboratory study of human cadaveric lumbar spines. METHODS Eighteen L1-S1 cadaveric lumbar spines were instrumented with lateral cages at L3-L4 and L4-L5 after intact kinematic analysis. Specimens (n=9 each) were allocated for supplemental instrumentation with either LP or PSR. Intact versus instrumented range of motion was evaluated for all specimens by applying pure moments (±7.5 Nm) in flexion/extension, lateral bending (LB) (left+right), and axial rotation (AR) (left+right). Instrumented spines were later subjected to 500 cycles of loading in all three planes, and interbody cage translations were quantified using a nonradiographic technique. RESULTS Lateral plate fixation significantly reduced ROM (p<.05) at both lumbar levels (flexion/extension: 49.5%; LB: 67.3%; AR: 48.2%) relative to the intact condition. Pedicle screw and rod fixation afforded the greatest ROM reductions (p<.05) relative to the intact condition (flexion/extension: 85.6%; LB: 91.4%; AR: 61.1%). On average, the largest interbody cage translations were measured in both fixation groups in the anterior-posterior direction during cyclic AR. CONCLUSIONS Based on these biomechanical findings, PSR fixation maximizes stability after lateral interbody cage placement. The nonradiographic technique served to quantify migration of implanted hardware and may be implemented as an effective laboratory tool for surgeons and engineers to better understand mechanical behavior of spinal implants.

Improving glenoid-side load sharing in a virtual reverse shoulder arthroplasty model

BACKGROUND The goal of glenoid fixation in reverse shoulder arthroplasty (RSA) is to provide a stable environment to allow bony ingrowth into the baseplate. When this does not occur, eventual baseplate failure is likely. This study aims to determine the additional implant-bone contact achieved when the glenosphere undersurface is in contact with the glenoid and if this increase in implant-bone contact improves stability through load sharing with respect to baseplate fixation. We hypothesize that substantial increases in contact area are possible and that this increased contact area will improve baseplate stability through load sharing. METHODS A computer-assisted design program was used to create 3-dimensional models of 7 currently available RSA devices. Total implant-bone contact area was compared in 2 conditions: (1) baseplate flush with bone and no additional glenosphere contact, or (2) baseplate and glenosphere undersurface in contact with bone. Next, finite element models were created from a commercially available system. Micromotion and stress were computed for each size of implant in the 2 conditions. RESULTS All devices tested can achieve increased total contact area when the glenosphere is in contact with bone. Stress and micromotion were reduced when comparing condition 2 with condition 1 in all sizes of one commercially available system. The average micromotion decreased 37%, from 98.04 to 61.97 μm. Larger glenospheres experienced a greater reduction in micromotion. Likewise, average von Mises stress decreased 26%, from 3.29 to 2.42 MPa. CONCLUSION Increasing glenosphere size and allowing glenosphere undersurface contact increased overall implant-bone contact area and baseplate stability.

Biomechanical analysis of impending femoral neck fractures: the role of percutaneous cement augmentation for osteolytic lesions

BACKGROUND Management of impending pathologic femoral neck fractures includes internal fixation, arthroplasty and megaprostheses. The study aim was to determine the augmentative effect of cement injection for minimally invasive treatment of femoral neck lesions. METHODS Twenty-seven cadaveric femora received a simulated osteolytic lesion previously shown to decrease the femurs failure load by 50%. Specimens were allocated to three groups of nine and loaded to failure in simulated single-leg stance: (1) percutaneous cementation + internal fixation (PCIF); (2) percutaneous cementation (PC); and (3) internal fixation (IF). Lesion-only and augmented finite element models were virtually loaded and stresses were queried adjacent to the lesion. FINDINGS PCIF resulted in the largest failure load though the increase was not significantly greater than the PC or IF groups. Inspection of the PC and PCIF specimens indicated that the generation of a cement column that spanned the superior and inferior cortices of the femoral neck increased failure loads significantly. Finite element analysis indicated that IF and PCIF constructs decreased the stress adjacent to the lesion to intact femur levels. Cementation without superior-to-inferior femoral neck cortical contact did not restore proximal femoral stress toward the intact condition. INTERPRETATION Internal fixation alone and internal fixation with or without cementation produce similar levels of mechanical augmentation in femora containing a high-risk lesion of impending fracture. A cement injection technique that produces a cement column contacting the superior and inferior femoral neck cortices confers the highest degree of biomechanical stability, should percutaneous cementation alone be performed.

Biomechanical model of a high risk impending pathologic fracture of the femur: Lesion creation based on clinically implemented scoring systems

BACKGROUND Multiple classifications combine objective and subjective measures to predict fracture risk through a metastatic lesion. In our literature review, no studies have attempted to validate this predicted fracture risk from a biomechanical perspective. The study goal was to evaluate proximal femur strength after creating osteolytic defects. We report a standardized technique to re-create a metastatic lesion. METHODS Eight femoral matched pairs were procured and a standardized technique was used to create an osteolytic femoral neck defect in one femur with the contralateral specimen serving as the control. Femurs were loaded to failure in a material testing machine at 2 mm/s. Failure load (N) and location of failure were documented. 3D finite element (FE) femur models with and without the lesions were developed to predict von Mises stresses in the femoral neck and compare between the two models. FINDINGS Femurs containing the osteolytic defect failed at significantly lower loads than the intact specimens in a reproducible manner (intact: 10.69 kN (3.09 SD); lesion: 5.56 kN (2.03 SD), p<0.001). The average reduction in failure load was 48%, and the fracture pattern was consistent in all specimens. FE model comparison similarly predicted significantly higher von Mises stress at the lesion. INTERPRETATION Our methods and pathologic fracture model represent the clinical parameters of metastatic bone disease and suggest a significant reduction in structural integrity of the lesion-containing femur. Prophylactic surgical fixation may be warranted clinically to reduce the risk of pathologic fracture. Our model technique is reproducible and may be used in future studies.