Normal digital flexion relies on flexor tendon pulleys to transmit linear muscular force to angular digital motion. Despite the critical role these pulleys play, there is a growing trend among surgeons to partially sacrifice or “vent” them during flexor tendon repair to improve surgical exposure. Although this new practice is reported to improve outcomes after flexor tendon repair, there is concern for the long-term effects of bowstringing, reduced finger range of motion (ROM) and altered tendon biomechanics. The objective of this study was to examine the effects of the application of a thermoplastic ring, acting as an “external” pulley, on flexor tendon biomechanics and finger ROM. We hypothesized that the application of an external thermoplastic ring would produce a centripetal force over the tendon to reduce bowstringing, improve finger ROM, and restore tendon loads following pulley venting Twelve digits comprised of the index, long, and ring fingers from four cadaveric specimens were tested using a novel in-vitro active finger motion simulator. Servo-motors were used to generate motion. Loads induced by flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP), and joint range of motion were measured with each sequential sectioning of the A2, A3, and A4 flexor pulley, in comparison to a native healthy finger condition. At each finger condition, A2 and A4 external thermoplastic pulley rings were applied over the proximal phalanx and middle phalanx, respectively, to recreate A2 and A4 function. Results were recorded and analyzed using a one way repeated-measures ANOVA. Following venting of the A2, A3 and A4 pulley, proximal interphalangeal joint (PIPJ) ROM significantly decreased by 17.02 ± 8.42 degrees and distal interphalangeal joint (DIPJ) range of motion decreased by 17.25 ± 8.68 degrees compared to intact pulleys. Application of the external rings restored range of motion to within 8.14 ± 8.17 degrees at the PIPJ and to within 7.72 ± 8.95 degrees at the DIPJ. Similarly, pulley venting resulted in a 36% reduction in FDS load and 50% in FDP load compared to intact pulleys. Following application of the external rings, loads were almost restored to normal at 7% reduction for FDS load and 13% reduction for FDP load. Venting of flexor tendon pulleys significantly alters flexor tendon biomechanics and digit range of motion. The application of thermoplastic rings acting as external pulleys over the proximal and middle phalanges is an effective, inexpensive, non-invasive and reproducible therapeutic method to restore flexor tendon biomechanics and digit range of motion.
Glenoid replacement is a manual bone removal procedure that can be difficult for surgeons to perform. Surgical robotics have been utilized successfully in hip and knee orthopaedic procedures but there are no systems currently available in the shoulder. These robots tend to have low adoption rates by surgeons due to high costs, disruption of surgical workflow and added complexity. As well, these systems typically use optical tracking which needs a constant line-of-sight which is not conducive to a crowded operating room. The purpose of this work was developing and testing a surgical robotic system for glenoid replacement. The new surgical system utilizes flexible components that tether a Stewart Platform robot to the patient through a patient specific 3D printed mount. As the robot moves relative to the bone, reaction loads from the flexible components bending are measured by a load cell allowing the robot to “feel” its way around. As well, a small bone burring tool was attached to the robot to facilitate the necessary bone removal. The surgical system was tested against a fellowship-trained surgeon performing standard surgical techniques. Both the robot and the surgeon performed glenoid replacement on two different scapula analogs: standard anatomy and posterior glenoid edge wear referred to as a Walch B2. Six of each scapula model was tested by the robot and the surgeon. The surgeon created a pre-operative plan for both scapula analogs as a target for both methodologies. CT scans of the post-operative cemented implants were compared to the pre-operative target and implant position and orientation errors were measured. For the standard shoulder analogs the net implant position and orientation errors were 1.47 ± 0.48 mm and 2.57 ± 2.30° for the robot and 1.61 ± 0.29 mm and 5.04 ± 1.92° for the surgeon respectively. For the B2 shoulders, the net implant position and orientation errors were 2.16 ± 0.36 mm and 2.89 ± 0.88° for the robot and 3.01 ± 0.42 mm and 4.54 ± 1.49° for the surgeon respectively. The new tracking system was shown to be able to match or outperform the surgeon in most metrics. The surgeon tended to have difficulty gauging the depth needed as well as the face rotation of the implant. This was not surprising as the reaming tool used by the surgeon obscures the view of the anatomy and the spherical cutter hinders the ability to index the tool. The robot utilized only one surgical tool, the bone burr, precluding the need for multiple instruments used by the surgeon to prepare the glenoid bone bed. The force-space navigation method can be generalized to other joints, however, further work is needed to validate the system using cadaveric specimens.
Subject-specific finite element models (FEMs) allow for a variety of biomechanical conditions to be tested in a highly repeatable manner. Accuracy of FEMs is improved by mapping density using quantitative computed tomography (QCT) and choosing a constitutive relationship relating density and mechanical properties of bone. Although QCT-derived FEMs have become common practice in contemporary computational studies of whole bones, many density-modulus relationships used at the whole bone level were derived using mechanical loading of small trabecular or cortical bone cores. These cores were mechanically loaded to derive an apparent modulus, which is related to each core's mean apparent or ash density. This study used these relationships and either elemental or nodal material mapping strategies to elucidate optimal methods for scapular QCT-FEMs. Six cadaveric scapulae (3 male; 3 female; mean age: 68±10 years) were loaded within a micro-CT in a custom CT-compatible hexapod robot Pre- and post-loaded scans were acquired (spatial resolution = 33.5 µm) and DVC was used to quantify experimental full-field displacements (BoneDVC, Insigneo) (Figure 1).. Experimental reaction forces applied to the scapulae were measured using a 6-DOF load cell. FEMs were derived from corresponding QCT scans of each cadaver bone. These models were mapped with one of fifteen density-modulus relationships and elemental or nodal material mapping strategies. DVC-derived BCs were imposed on the QCT-FEMs using local displacement measurements obtained from the DVC algorithm. Comparisons between the empirical and computational models were performed using resultant reaction loads and full-field displacements (Figure 2).Introduction
Methods
Mechanical properties mapping based on CT-attenuation is the basis of finite element (FE) modeling with heterogeneous materials and bone geometry defined from clinical-resolution CT scans. Accuracy between empirical and computational models that use constitutive equations relating CT-attenuation to bone density are well described, but material mapping strategy has not gained similar attention. As such, the objective of this study was to determine variations in the apparent modulus of trabecular bone cores mapped with various material mapping strategies, using a validated density-modulus relationship and co-registered µFEMs as the gold standard. Micro-CT images (isotropic 32 µm) were used to create µFEMs from glenoid trabecular bone cores of 14 cadaveric scapula. Each µFEM was loaded in unconstrained compression to determine the trabecular core apparent modulus (Eapp). Quantitative CT (QCT) images (isotropic 0.625 mm) were subsequently acquired and co-registered QCT-FEMs created for each of the 14 cores. The QCT-FEMs were meshed with either linear hexahedral (HEX8), linear tetrahedral (TET4), or quadratic tetrahedral (TET10) elements at 3 mesh densities (0.3125 mm, 0.46875 mm, 0.625 mm). Three material mapping strategies were used to apply heterogeneous element-wise (element-averaging of the native HU field (Mimics V.20, Materialise, Leuven BE)) or nodal (tri-linear interpolation of HU Field or E Field (Matlab V. R2017a, Natick, RI, USA)) material properties to the QCT FEMs. Identical boundary conditions were used and Eapp between the µFEMs and QCT-FEMs was compared (Figure 1). The QCT density of each hexahedral mesh with element size equal to voxel dimensions was used to compare the QCT density mapping between tetrahedral meshes and material mapping strategy.INTRODUCTION
METHODS
Trabecular bone transmits loads to the cortical shell and is therefore most active in bone remodeling. This remodeling alters trabecular material strength thereby changing the bending stiffness. Accounting for trabecular material heterogeneity has been shown to improve empirical-µFEM correlations by allowing for more realistic trabecular bending stiffness. In µFEMs to reduce computation time, region averaging is often used to scale image resolution. However, region averaging not only alters trabecular architecture, but inherently alters the CT-intensity of each trabeculae. The effect of CT-intensity variations on computationally derived apparent modulus (Eapp) in heterogenous µFEMs has not been discussed. The objectives of this study were to compare trabecular Eapp among i) hexahedral and tetrahedral µFEMs, ii) µFEMs generated from 32 µm, 64 µm, and 64 µm down-sampled from 32 µm µ-CT scans, and iii) µFEMs with homogeneous and heterogeneous tissue moduli. Fourteen cadaveric scapulae (7 male; 7 female) were micro-CT scanned at two spatial resolutions (32 µm & 64 µm). Virtual bone cores were extracted from the glenoid vault, maintaining a 2:1 aspect ratio, to create µFEMs from the 32 µm, 64 µm, and down-sampled 64 µm scans. Custom code was used to generate µFEMs with 8-node hexahedral elements (HEX8), while maintaining the bone volume fraction (BV/TV) of each HEX8 32 µm model (BV/TV=0.24±0.10). Each virtual core was also generated as a 10-node tetrahedral (TET10) µFEM. All µFEMs were given either a homogeneous tissue modulus of 20 GPa, or a heterogeneous tissue modulus scaled by CT-intensity. All FEMs were constrained with identical boundary conditions and compressed to 0.5% apparent strain. The apparent modulus of each model was compared.Introduction
Methods
The Walch Type B2 glenoid has the hallmark features of posteroinferior glenoid erosion, retroversion, and posterior humeral head subluxation. Although our understanding of the pathoanatomy of bone loss and its evolution in Type B's has improved, the etiology remains unclear. Furthermore, the morphology of the humerus in Walch B types has not been studied. The purpose of this imaging based anthropometric study was to examine the humeral torsion in Walch Type B2 shoulders. We hypothesized that there would be a compensatory decrease in humeral retroversion in Walch B2 glenoids. Three-dimensional models of the full length humerus were generated from computed tomography data of normal cadaveric (n = 59) and Walch Type B shoulders (n = 59). An anatomical coordinate system referencing the medial and lateral epicondyles was created for each model. A simulated humeral head osteotomy plane was created and used to determine humeral version relative to the epicondylar axis and the head-neck angle. Measurements were repeated by two experienced fellowship-trained shoulder surgeons to determine inter-rater reliability. Glenoid parameters (version, inclination and 2D critical shoulder angle) and posterior humeral head subluxation were calculated in the Type B group to determine the pathologic glenohumeral relationship. Two-way ANOVAs compared group and sex within humeral version and head-neck angle, and intra-class correlation coefficients (ICCs) with a 2-way random effects model and absolute agreement were used for inter-rater reliability.Introduction
Methods
Mechanical property relationships used in the computational modeling of bones are most often derived using mechanical testing of normal cadaveric bone. However, a significant percentage of patients undergoing joint arthroplasties exhibit some form of pathologic bone disease, such as osteoarthritis. As such, the objective of this study was to compare the micro-architecture and apparent modulus (Eapp) of humeral trabecular bone in normal cadaveric specimens and bone extracted from patients undergoing total shoulder arthroplasty. Micro-CT scans were acquired at 20 µm spatial resolution for humeral heads from non-pathologic cadavers (n=12) and patients undergoing total shoulder arthroplasty (n=10). Virtual cylindrical cores were extracted along the medial-lateral direction. Custom-code was used to generate micro finite element models (µFEMs) with hexahedral elements. Each µFEM was assigned either a homogeneous tissue modulus of 20 GPa or a heterogeneous tissue modulus scaled by CT- intensity. Simulated compression to 0.5% apparent strain was performed in the medial-lateral direction. Morphometric parameters and apparent modulus-bone volume fraction relationships were compared between groups.Introduction
Methods
Density-modulus relationships are often used to map the mechanical properties of bone based on CT- intensity in finite element models (FEMs). Although these relationships are thought to be site-specific, relationships developed for alternative anatomic locations are often used regardless of bone being modeled. Six relationships are commonly used in finite element studies of the shoulder; however, the accuracy of these relationships have yet to be compared. This study compares each of these six relationships ability to predict apparent strain energy density (SEDapp) in trabecular bone cores from the glenoid. Quantitative-CT (QCT) (0.625 mm isotropic voxels), and µ-CT scans (0.032 mm isotropic voxels) were obtained for fourteen cadaveric scapulae (7 male, 7 female). Micro finite element models (µ-FEMs) were created from 98 virtual ‘cores’ using direct conversion to hexahedral elements. Two µ-FEM cases were considered: homogeneous tissue modulus of 20 GPa, and heterogeneous tissue modulus scaled by CT intensity of the µ-CT images (196 models). Each µ- FEM model was compressively loaded to 0.5% apparent strain and apparent strain energy density (SEDapp) was calculated. Additionally, each of the six density-modulus relationships were used to map heterogeneous material properties to co- registered QCT-derived models (588 models in total). The loading and boundary conditions were replicated in the QCT-FEMs and the SEDapp was calculated and compared to the µ-FEM SEDapp. To account for more samples than donors, restricted maximum likelihood estimation (REML) linear regression compared µ-FEM SEDapp and QCT-FEM SEDapp for each relationship.Introduction
Methods
Stability of the glenoid component is essential to ensure successful long-term outcomes following Total shoulder arthroplasty (TSA), and may be improved through better glenoid component design. As such, this study assessed identical all-polyethylene glenoid components stability, having various fixation types, using component micromotion under simulated joint loading in an osteoarthritic patient cohort. Five all-polyethylene glenoid component designs were compared (Keel, Central-Finned 4-Peg, Peripheral 4-Peg, Cross-Keel, and Inverted-Y). A cement mantle surrounded each fixation type, except the Central-Finned 4-Peg which was surrounded by bone. The humeral component had a non-conforming radius of curvature. Scapular models of six type A1 osteoarthritic male patients (mean: 61 years old, range: 48 to 76 years old) were assigned heterogeneous bone properties based on CT intensity. Each of the 30 scapula models were truncated and fully constrained on the medial scapular border. The bone/cement interface was fully bonded, and the fixation feature/cement interface was frictionally constrained. A ‘worst case’ load magnitude of 125% BW of a 50th percentile male was used. A purely compressive load was applied to the center of the glenoid component, followed by superior, superior-posterior, posterior, inferior-posterior, and inferior loads. Stability of the glenoid component based solely on the fixation type was determined using the mean and maximum normal (liftoff) and tangential (sliding) micromotion in six regions of the glenoid component.BACKGROUND
METHODS
Shoulder arthroplasty is used to treat several common pathologies of the shoulder, including osteoarthritis, post-traumatic arthritis, and avascular necrosis. In replacement of the humeral head, modular components allow for anatomical variations, including retroversion angle and head-neck angle. Surgical options include an anatomic cut or a guide-assisted cut at a fixed retroversion and head-neck angle, which can vary the dimensions of the cut humeral head (height, anteroposterior (AP), and superoinferior (SI) diameters) and lead to negative long term clinical results. This study measures the effect of guide-assisted osteotomies on humeral head dimensions compared to anatomic dimensions. Computed tomography (CT) scans from 20 cadaveric shoulder specimens (10 male, 10 female; 10 left) were converted to three-dimensional models using medical imaging software. An anatomic humeral head cut plane was placed at the anatomic head – neck junction of all shoulders by a fellowship trained shoulder surgeon. Cut planes were generated for each of the standard implant head-neck angles (125°, 130°, 135°, and 140°) and retroversion angles (20°, 30°, and 40°) in commercial cutting guides. Each cut plane was positioned to favour the anterior humeral head-neck junction while preserving the posterior cuff insertion. The humeral head height and diameter were measured in both the AP plane and the SI plane for the anatomic and guide-assisted osteotomy planes. Differences were compared using separate two-way repeated measures ANOVA for each dependent variable and deviations were shown using box plot and whisker diagrams.Introduction
Methods
Humeral version is the twist angle of the humeral head relative to the distal humerus. Pre-operatively, it is most commonly measured referencing the transepicondylar axis, although various techniques are described in literature (Matsumura et al. 2014, Edelson 1999, Boileau et al., 2008). Accurate estimation of the version angle is important for humeral head osteotomy in preparation for shoulder arthroplasty, as deviations from native version can result in prosthesis malalignment. Most humeral head osteotomy guides instruct the surgeon to reference the ulnar axis with the elbow flexed at 90°. Average version values have been reported at 17.6° relative to the transepicondylar axis and 28.8° relative to the ulnar axis (Hernigou, Duparc, and Hernigou 2014), although it is highly variable and has been reported to range from 10° to 55° (Pearl and Volk 1999). These studies used 2D CT images; however, 2D has been shown to be unreliable for many glenohumeral measurements (Terrier 2015, Jacxsens 2015, Budge 2011). Three-dimensional (3D) modeling is now widely available and may improve the accuracy of version measurements. This study evaluated the effects of sex and measurement system on 3D version measurements made using the transepicondylar and ulnar axis methods, and additionally a flexion-extension axis commonly used in biomechanics. Computed tomography (CT) scans of 51 cadaveric shoulders (26 male, 25 female; 32 left) were converted to 3D models using medical imaging software. The ulna was reduced to 90° flexion to replicate the arm position during intra-operative version measurement. Geometry was extracted to determine landmarks and co-ordinate systems for the humeral long axis, epicondylar axis, flexion-extension axis (centered through the capitellum and trochlear groove), and ulnar long axis. An anatomic humeral head cut plane was placed at the head-neck junction of all shoulders by a fellowship trained shoulder surgeon. Retroversion was measured with custom Matlab code that analysed the humeral head cut plane relative to a reference system based on the long axis of the humerus and each elbow axis. Effects of measurement systems were analyzed using separate 1-way RM ANOVAs for males and females. Sex differences were analyzed using unpaired t-tests for each measurement system.Background
Methods
Glenoid baseplate fixation for reverse shoulder arthroplasty relies on the presence of sufficient bone stock and quality. Glenoid bone may be deficient in cases of primary erosions or due to bone loss in the setting of revision arthroplasty. In such cases, the best available bone for primary baseplate fixation usually lies within the three columns of the scapula. The purpose of this study was to characterise the relationship of the three columns of the scapula independent of glenoid anatomy and to establish the differences between male and female scapular anatomy. Fifty cadaveric scapulae (25 male, 25 female) were analysed using CT-based imaging software. The surface geometries of the coracoid, scapular spine and inferior scapular column were delineated in the sagittal plane. A linear best-fit line was drawn to establish the long axis of each column independent of the glenoid. The width of the glenoid was measured and points marked at the midpoint of each measurement. A best-fit line starting at the supra glenoid tubercle passing through the midpoints was chosen as the superior inferior (SI) axis of the glenoid. An orthogonal plane to the scapular plane was developed parallel to the glenoid face. The axis representing each of the three columns of the scapula and the SI axis of the glenoid, were projected onto this plane. The relationship between each column was analysed with respect to each other and with respect to the SI glenoid axis. Thus, measurements obtained gave the relationships of the three columns of the scapula (independent of the glenoid) and their relationships to the long axis of the glenoid (dependant on the glenoid). Comparisons were made between males and females using the independent t-tests.Background
Methods
Shoulder arthroplasty is used to treat osteoarthritis, post-traumatic arthritis, and avascular necrosis. Modular components allow for natural variability in shoulder anatomy, including retroversion and head-neck angles. Surgical options include anatomic or guide-assisted cut at a fixed retroversion and head-neck angle. The purpose of this study was to determine the variability between head height (HH) and anteroposterior (AP) and superoinferior (SI) diameters using anatomic and guide-assisted humeral head cuts. Computed tomography scans of 10 cadaveric shoulder specimens (5 male, 5 female) were converted to 3D models. An anatomic humeral head cut plane was placed at the anatomic head–neck junction maintaining the posterior cuff insertion for all shoulders by a fellowship trained shoulder surgeon. Cut planes were generated for standard implant head neck angles (125°,130°,135°, and 140°) and retroversion angles (20°,30°, and 40°) in commercial cutting guides, for a combination of 12 repeated cut conditions per specimen. The humeral HH and the head diameter were measured in the AP and the SI planes for anatomic and guide-assisted osteotomy planes. Differences were compared using a separate two-way repeated measures ANOVA for each dependent variable. Guide-assisted cuts showed no significant effect on HH due to head-neck (p=0.205) or retroversion angles (p=0.190). These results persisted by gender (male: head-neck p=0.659 and retroversion p=0.386; female: head-neck p=0.204 and retroversion p=0.190). SI diameter increased by 1.3 mm with increasing head-neck angle (p<0.001), but there was no effect due to retroversion (p=0.148). A head-neck angle of 125° caused the greatest decrease in SI diameter of −2.8 mm compared to the anatomic cut, averaged over the retroversion range. The greatest reduction of SI diameter, −3.4 mm compared to anatomic, occurred with 125° head-neck angle and 20° retroversion. By gender, males showed a significant effect from head-neck angle (p=0.008), but females did not (p=0.077). There was no significant difference from retroversion in either gender group (male p=0.792; female p=0.057). There was no significant difference in AP diameter by head-neck (p=0.192) or retroversion angles (p=0.168). These results persisted in the males (head-neck p=0.420 and retroversion p=0.780). In females, there was no effect from head-neck angle (p=0.232); however, retroversion angle trended toward significance (p=0.050). For patients whose natural anatomy falls outside the range of the commercial cut guides, templated resection may result in deviation from natural humeral head dimensions. Due to the large variability in anatomic retroversion and head-neck angles in the subjects of this study, further study with a larger sample size is needed to investigate observed trends. These preliminary results have implications for manufacturers to create guides to represent a larger segment of the population, and surgeons' intra-operative choice.
We compared the initial strength of two techniques for repair of rotator cuff tears. Eight paired cadaveric shoulders with a standardized supraspinatus defect were studied. A transosseous suture and anchor repair was conducted on each side. Specimens were tested under cyclic loading, while fixation was monitored with an optical tracking technique. Mode of failure, number of cycles and load to failure were measured for 50% (5 mm) and 100% (10 mm) loss of repair. Anchors provide improved repair strength at 50% repair loss, in comparison to sutures (p<
0.05). Strength was unaffected by bone mineral density, age and gender. The purpose of this study was to compare the initial strength of two rotator cuff repair techniques. Repair strength with anchors was superior to sutures. Strength was unaffected by bone quality. Anchors, enabling a quicker, less invasive arthroscopic repair, offer improved fixation over sutures, which are more time consuming and invasive. Eight paired shoulders with a standardized supra-spinatus defect were randomized to anchor or suture repair, and subjected to cyclic loading. Repair migration was measured using a digital camera. Failure mode, cycles and load were measured for 50% and 100% loss of repair. Results were correlated with bone mineral density, age and gender. The anchors failed at the anchor-tendon interface, whereas the sutures failed through the sutures. Mean values for 50% loss of repair were 205.6 ± 87.5 cycles and 43.8 ± 14.8 N for the sutures, and 1192.5 ± 251.7 cycles and 156.3 ± 19.9 N for the anchors (p<
0.05). The corresponding values for 100% loss of repair were 2457.5 ± 378.6 cycles and 293.8 ± 27.4 N for the sutures, and 2291.9 ± 332.9 cycles and 262.5 ± 28.0 N for the anchors (p>
0.05). These results did not correlate with bone quality. This study has demonstrated that anchors provide improved repair strength, in comparison to sutures. This may be due to the relative less deformability of the anchors. Repair strength did not correlate with bone quality. This may be attributed to each repair failing primarily through the repair construct or at the anchor-tendon interface, and not through bone.
To determine the relationship between forearm muscle activity and joint reaction force (JRF) in the distal radioulnar joint (DRUJ). The DRUJ reaction force is linearly related to the muscle activity of the PT and biceps, but not necessarily to the activity of the supinator and PQ. This work has implications for biomechanical modelling, implant design, fixation and rehabilitation protocols following DRUJ arthroplasty. JRFs were found to increase linearly with muscle load for all muscles simulated (biceps, pronator teres (PT), pronator quadratus (PQ), supinator) in all forearm positions tested (supination, neutral and pronation) (correlation coefficient r>
0.85, p<
0.01) with two exceptions; simulation of the PQ in the neutral position (r=−0.65, p=0.2), and the supinator in the pronated position (r=0.72, p=0.2). Biceps simulation generated larger JRF magnitudes in all positions compared to other muscles (p<
0.001), and the PQ generated larger JRF magnitudes compared to the supinator (p=0.05). Ulnar head arthroplasty was performed with a replacement ulnar head implant instrumented with strain gauges to allow measurement of the DRUJ reaction force. An upper extremity joint simulator applied muscle loads in seven fresh frozen cadaveric upper extremities through computer-controlled pneumatic actuators. Load was varied in 10N increments from 10-80N (biceps and PT) and from 10-50N (PQ and supinator). A hand clamp was used to restrain the forearm in varying positions. The results illustrate that broad insertion and non-linear muscles may not be linearly correlated to joint reaction force in the DRUJ. Please contact author for diagrams and graphs.
Quantitative measurements of load transfer through the distal radioulnar joint (DRUJ) are limited. An instrumented ulnar head prosthesis was developed to measure bending and torsion moments about the three anatomic axes of the ulna. This device has shown repeatable loading data following insertion in a cadaveric specimen during active forearm rotation trials conducted in an To develop a system to quantify A load-measuring system was developed that was easily surgically inserted, and produced repeatable loading data. The instrumented implant developed in this study will contribute to the optimization of surgical procedures and implant design parameters related to distal ulnar arthroplasty. Four pairs of strain gauges were applied to the stem of an ulnar head prosthesis to measure bending and torsion moments about the three anatomic axes of the ulna. Three ulnar heads were machined with varying eccentricities (axisymmetric, 1.5 mm offset and 3.0 mm offset). The implant was inserted in one unpreserved cadaveric upper extremity and active forearm rotation induced using a computer controlled joint simulator. Repeatability (assessed using the maximum standard deviation over 5 trials of pronation and supination) was less than 9% of the output range for all loads. Bending and torsion moments between −0.4 and 0.5 Nm, correlating to joint loads between 0 and 50 N, were measured. The measured loads followed a consistent pattern with forearm position. Higher loads were noted for the eccentric implant heads compared to the axisymmetric head, especially at the extreme ranges of rotation. Clinical interpretation of these findings is difficult since the optimal loading scenario for distal ulnar implant longevity remains unknown. Please contact author for diagrams and graphs.
