Aim

It has been suggested that the use of a pilot-hole may reduce the risk of fracture to the lateral cortex. Therefore the purpose of this study was to determine the effect of a pilot hole on the strains and occurrence of fractures at the lateral cortex during the opening of a high tibial osteotomy (HTO) and post-surgery loading.

The torsional strength of cemented implants is likely influenced by stem geometry. Five straight stems with different cross-sectional shapes (circular, oval, triangular, round-rectangular, sharp-rectangular) were custom-machined. The stems were cemented into tubes using bone cement, and subjected to torsion (2.5deg/min)(n=7). At initial failure (crack through the cement mantle or loss of cement-stem adhesion), the sharp-rectangular stem resisted over 33% more torque than the other four stems (p=0.13). At ultimate failure (5° stem rotation), the resistance provided by the circular stem was less than 12% of either rectangular stem (p< 0.05). Additional studies are needed to determine the effects of long-term loading.

To determine the influence of cross-sectional implant stem shape on cement failure under torsional loading.

The sharp-rectangular stem provided the greatest torsional resistance against initial failure. At ultimate failure, the two rectangular stems performed better than the other stems, with the circular stem providing the least torsional resistance.

A stem design that provides increased resistance to torsion will, in all likelihood, improve the longevity of cemented stemmed implants.

Five straight stems with different cross-sectional shapes (circular, oval, triangular, round-rectangular, sharp-rectangular) were custom-machined. Each stem was cemented in a square aluminum tube using Simplex® cement (Stryker, Michigan, USA). A materials testing machine was used to apply torsion to the stem at 2.5 deg/minute until failure. ‘Initial failure’ was defined as the appearance of a crack through the cement mantle and/or the loss of cement-stem adhesion. ‘Ultimate failure’ was defined as 5° of stem rotation. Results (n=7) were compared using one-way ANOVAs with post-hoc Student-Newman-Keuls tests (p=0.05). The sharp-rectangular stem withstood over 33% more torque at initial failure than the other stems (p=0.13). At ultimate failure, the circular stem provided significantly less torsional resistance than the other four stems (p< 0.05), and was able to resist less than 12% of the torque applied to either rectangular stem. These results suggest shape may play a role in the onset of implant loosening due to torsion. Further studies are required to explore other shapes and to examine the effects of cyclic loading and cement soaking.

Funding: Natural Sciences and Engineering Research Council, University of Western Ontario

Purpose: To develop an experimental testing method to measure bone strains as a function of multiple implant stem designs in a single specimen, and to show the efficacy of this method with an application in the distal ulna.

Methods: Twenty-four strain gauges were applied to the surface of an isolated cadaveric ulna to measure anterior-posterior (AP) and medial-lateral (ML) bending loads at six locations along its length. The bone was potted in a custom-designed jig and positioned in a materials testing machine. Loads (5-25N) were applied to the ulnar head while strains were recorded. The ulnar head was removed and an 8cm threaded rod (diameter=5.8mm) was cemented into the canal, and subsequently removed after cement curing. This established a threaded cement mantle that would accept various threaded stem designs. To show the efficacy of this technique, testing was repeated with 5 and 7cm stems. The entire canal was then filled with cement and testing repeated to determine the effect of the residual cement void.

Results: All 24 strain gauges provided quality signals throughout the testing period. Strain varied linearly with load (R-squared=0.94–0.99). The initial threaded rod was easily removed, and there was no difficulty in placing subsequent stems within the mantle. Comparing the 5 and 7cm stems, little difference in strains was observed for the most proximal gauges (2%), with higher variations in the stem exit regions (17%). The cement-filled canal exhibited distal strains similar to the intact baselines (average 2% difference at 25N).

Conclusions: A reliable method has been developed that allows multiple stems to be tested in a single bone. Observed strain differences are therefore a function of implant parameters only (such as stem length), and are not influenced by differences in bone properties as occurs when testing multiple specimens. The layer of threaded bone cement did not impact the native bone strains. This experimental method will be useful to compare stem designs in a variety of bones, avoiding the need for large numbers of specimens due to the repeated measure experimental protocol.

Purpose: This in-vitro study was conducted to assess the effect of a computer-assisted method of performing shoulder hemiarthroplasty, in comparison to traditional techniques, on passive glenohumeral joint kinematics during abduction.

Methods: Seven pairs of fresh-frozen cadaveric shoulders were tested. One specimen from each pair was randomized to the computer-assisted technique, while the contralateral shoulder underwent a traditional hemiar-throplasty using standard surgical guides by an experienced shoulder surgeon. A simulated four-part proximal humerus fracture was created in each shoulder and was reconstructed using a modular shoulder hemiarthroplasty system (Anatomical Shoulder Hemiarthroplasty System, Centrepulse Orthopaedics Inc, Austin, TX). CT data and computerized simulations of anatomical characteristics were used in the computer-assisted technique. An electromagnetic tracking device (Flock of Birds, Ascension Technologies, Burlington, VT) in conjunction with custom-written software (LabVIEW, National Instruments, Austin, TX) enabled real-time intra-operative feedback.||Passive abduction of the glenohumeral joint was conducted and the resulting motion was quantified using the aforementioned tracking device. Coordinate systems, created on both the humerus and scapula from digitized anatomical landmarks, were used to transform the kinematic data into clinically relevant parameters. Statistical analyses were performed using one-way Analyses of Variance (ANOVAs) followed by post-hoc Student-Newman-Keuls multiple comparisons (p< 0.05).

Results: In the superior-inferior direction, a significant difference in joint kinematics (p=0.011) was found between the computer-assisted and the traditional technique, with the traditional technique resulting in a more inferiorly positioned humeral head at all angles of elevation. There was no difference in translation between the native shoulders and the computer-assisted hemiarthroplasty (p> 0.05). In the anterior-posterior direction there was no difference measured in the position of the humeral head between the two surgical techniques, which were both similar to the native shoulder (p> 0.05).

Conclusions: This is the first known study to examine the effects of a computer-assisted method for performing shoulder hemiarthroplasty. Our results show that the computer-assisted approach should allow improved restoration of glenohumeral joint kinematics relative to conventional techniques, potentially resulting in improved patient outcomes and implant durability.

Purpose: Anterior flanges have been added to the humeral components of some total elbow arthroplasty systems. Surgeons have the option of placing a wedge of bone or bone cement between the anterior surface of the humerus and the flange in an effort to improve implant stability and load transfer. The purpose of this study was to quantify the cortical strains in the humerus after arthroplasty for different materials placed behind the flange.

Methods: Five fresh-frozen cadaveric distal humeri were thawed and cleaned of all soft tissues. Strain gauges were applied to the anterior and posterior surfaces to record bending and axial strains. The bending gauges were positioned just proximal to the location of the flange tip. Cantilever bending and axial compression were applied using a materials testing machine. Following intact testing, the humeral component of a total elbow was implanted by an experienced surgeon and fixed using bone cement. Testing was repeated three times, each with a different material behind the flange: no graft (simulating a humeral component without an anterior flange), cancellous bone graft, and cement graft. Strains were normalized to the intact state and for the applied moments. Data were analyzed using repeated-measure ANOVAs (p< 0.05).

Results: For bending, the strain values were approximately 80% of the intact values with no graft material, 80% with the bone graft, and 87% with the cement graft. These differences among the graft materials were not significant (p=0.5). Similar results were found for the axial strains (p=0.3).

Conclusions: The intention of the anterior flange is to transfer a portion of the load carried by the implant stem to the distal humerus, thereby reducing stress-shielding and improving strength of the construct. In this investigation that employed bending and axial loads, the presence of an anterior flange had no significant effect on load transfer through the distal humerus regardless of graft material used. This would suggest that for the humeral component employed in this study, the flange might not be fulfilling its intended purpose.

The strength of the intact and four reconstruction techniques (figure-eight, docking, single strand utilizing interference screws, and a single strand) of the medial collateral ligament (MCL) of the elbow were compared. Twenty cadaveric specimens were tested with a cyclic valgus loading protocol. The peak loads to failure of the MCL reconstructions were inferior compared to the intact ligament (p< 0.05). The docking and single strand reconstruction utilizing an endobutton for ulnar fixation were equivalent and had greater initial strength than the interference screws or figure-eight technique. It is suggested that improved interference screws are required for this repair.

The purpose of this study was to compare the initial strength of the intact medial collateral ligament (MCL) of the elbow and four reconstruction techniques.

The docking and endobutton reconstructions showed equivalent peak load to failure.

Improved interference screws are required before they are employed clinically.

The average peak load to failure or 5mm of joint gapping was 142.5±39.4N for the intact, 53.0±9.5N for the docking, 52.5±10.4N for the endobutton, 41.0±16.0N for the interference screw, and 33.3±7.1N for the figure-eight reconstructions. The peak load to failure was higher for the intact specimens compared to any of the reconstructions (p< 0.001). The docking reconstruction showed higher peak loads than the figure-eight or interference screw reconstruction, and the endobutton reconstruction showed higher peak loads than the figure-eight reconstruction (p< 0.004). There was no difference in peak loads between the docking and endobutton reconstructions (p> 0.05).

Twenty (ten pairs) unpreserved cadaveric upper extremities were mounted in a custom jig with the elbow at 90°, and a valgus force was applied 12cm from the elbow joint. The specimens were loaded starting at 20N with the load increased in increments of 10N (200 cycles at each load), until either complete ligament failure or a 5mm increase in the distance between the attachment sites of the MCL. The results support that a single strand or multistrand ligament reconstruction can be equivalent with respect to maximal peak loads and cyclic loading. There are concerns with regard to the use of interference screw fixation in the clinical situation.

Passive and active elbow flexion was performed in eight cadaveric arms to determine the effect of Type 1 coronoid fractures and suture repair on kinematics. Testing was performed in ligamentously intact and MCL deficient elbows; with radial head arthroplasty (RHA); with an intact coronoid, following a Type 1 fracture, and with suture repair of the coronoid. There was an alteration in elbow kinematics and stability following Type 1 coronoid fractures that was not corrected with coronoid repair. Suture fixation of the coronoid is probably unnecessary if the lateral ligaments are repaired and the radial head is repaired or replaced.

To determine the effect of fixation of Type 1 coronoid fractures on elbow stability and kinematics in ligamentously intact and medial collateral ligament (MCL) deficient elbows with radial head arthroplasty (RHA).

Type 1 coronoid fractures cause changes in elbow kinematics and stability that are not corrected with suture repair.

Suture fixation of Type 1 coronoid fractures is probably unnecessary if the lateral ligaments are repaired and the radial head is repaired or replaced.

With intact ligaments, there was an increase in valgus angulation following a Type 1 coronoid fracture (p< 0.05) that was not corrected with fixation. With MCL deficiency, there was no change in valgus angulation for all coronoid states. For both ligament states, there was an increase maximum varus-valgus laxity after a Type 1 coronoid fracture with forearm pronation (p=0.03) that was not corrected with fixation (p=0.4). Kinematic data was collected from eight cadaveric arms during passive and simulated active elbow motion. The protocol was performed in stable and MCL deficient elbows with RHA. Testing occurred with the coronoid intact, following Type 1 coronoid fracture, and with suture repair of the fracture. Valgus angulation and maximum varus-valgus laxity were measured.

With intact ligaments, Type 1 coronoid fractures cause an alteration in elbow kinematics and laxity that is not corrected with suture fixation. With MCL disruption, Type 1 coronoid fractures have no effect on elbow kinematics and a small effect on laxity that is not corrected with coronoid repair.

Funding: Research and Institutional Support received from Wright Medical Technologies.

Please contact author for graphs and/or diagrams.

Purpose: To develop a computerized inverse dynamic 3D model of the upper limb, focussing on the elbow.

Methods: Anatomic bony landmarks were identified in one cadaveric arm using an electromagnetic tracking device (Flock of Birds, Ascension Technologies, VT). The articular surfaces of the radiohumeral and ulnohumeral joints were digitized, thereby identifying the areas over which the contact forces could act. Attachment sites of the medial collateral (MCL) and lateral collateral (LCL) ligaments and the major muscles (BRA=brachialis, BIC=biceps, BRD=brachioradialis, TRI=triceps) were also digitized to create line-of-action vectors. These data were fed into custom-written software (MATLAB®, The MathWorks Inc., MA) that simulated flexion with gravity as external loading, and calculated the forces exerted by the joint structures. As an indeterminate system, computerized mathematical optimization solved for the internal loads using a cost function that minimized the sum of forces squared.

Results: Model outputs were comparable with results from previous muscle activity and cadaveric studies. Force ratios among the elbow’s prime movers at 30 degrees of flexion agreed quite closely with previous findings (Funk et al, 1987), with percent differences of 11% (BRA), −5% (BIC), −6% (BRD), and −1% (TRI). Overall, the brachialis force was the highest throughout flexion, being the prime mover, while extensor (triceps) activity remained quiet through mid-range. The magnitude of the radiohumeral contact force showed a decreasing pattern through the arc of flexion, similar to the trend found experimentally by others (Morrey et al, 1988). The results also demonstrated stabilizing forces supplied by the MCL, but not the LCL.

Conclusions: Current understanding of upper extremity loading is very limited. Creating an accurate computerized model of the elbow joint, would reduce the need for experimental testing with cadavers, which are always of limited availability. While stability of the elbow has been experimentally investigated, this model will be able to quantify the forces within the stabilizing structures. By establishing a normal baseline of these forces, surgical procedures and joint replacement designs can be validated. Thus, this model can provide a significant contribution to upper extremity biomechanics research and clinical treatments.

Purpose: To compare the torsional stability provided by five implant stems with different cross-sectional geometries under cyclic loading.

Methods: Cemented stems with five different cross-sectional shapes – circular, oval, triangular, rectangular with rounded edges (round rectangular), and rectangular with sharp edges (sharp rectangular) – were custom machined from stainless steel. Stem dimensions were selected to fit within the humeral canal (based on a 6mm x 8mm dimensioning scheme) and shapes were based on commercially available-designs. Seven specimens of each stem shape were tested. ||The stems were potted in square aluminum tubes using bone cement, and allowed to cure for 24 hours prior to testing. A materials testing machine and a custom designed loading fixture were used to apply torsion to the stems. A sine wave loading pattern was applied until ultimate failure (5° of stem rotation) was reached. This loading pattern had a lower bound of 0.9Nm and an upper bound that started at 4.5Nm and was increased in increments of 2.25Nm every 1500 cycles. The load was cycled at 2Hz. Statistical analyses on both the number of cycles and torque to failure were performed using one-way ANOVAs followed by post-hoc Student-Newman-Keuls (SNK) tests (p< 0.05).

Results: Overall, ANOVAs showed an effect of shape on the number of cycles (p< 0.0001) and torque to failure (p< 0.001). SNK tests revealed the sharp rectangular stem provided the greatest resistance to torque (p-cycles< 0.001; p-torque< 0.001) compared to all other stems. Other significant differences resulted in the following ranking of the shapes: sharp rectangular, round rectangular, triangular, and circular = oval.

Conclusions: The results of this study agree with static testing previously conducted on the same set of stem shapes. Although the sizes of the stems were chosen to roughly replicate upper limb implants, these results may be extrapolated to larger stems such as for the hip or knee. To improve implant longevity, it is important that the best fixation possible be obtained through all available avenues, including improved cementing techniques, and optimal implant designs. An alteration in implant stem shape may assist in achieving this goal.

The influence of the supinator and pronator quadratus (PQ) muscles on distal radioulnar joint stability were evaluated using a joint simulator capable of producing forearm rotation, before and after ulnar head excision. Multiple pronation trials were conducted with incremental loading of the PQ relative to the pronator teres; supination trials were similarly conducted with the supinator and biceps. Incremental supinator muscle loading did not alter forearm kinematics. Increased PQ loading did not affect intact kinematics, but did alter joint motion following ulnar head excision. PQ activation will likely aggravate forearm instability following ulnar head excision; suggesting rehabilitation should incorporate immobilization in supination.

The purpose of this study was to study the effect of pronator quadratus (PQ) and supinator loads on forearm kinematics in both an intact distal radioulnar joint (DRUJ) and following ulnar head excision.

The PQ muscle appears to aggravate instability of the DRUJ following ulnar head excision, while incremental loading of the supinator muscle had no effect.

Patients with DRUJ instability and/or who have undergone surgical removal of the ulnar head should be rehabilitated in supination to limit the influence of the PQ muscle.

Eight cadaveric upper extremities were tested in a custom joint simulator employing motion and load-controlled tendon loading to produce forearm rotation. Pronation was achieved via loading of the pronator teres and PQ muscles. Repeated trials were conducted in which the ratio of the PQ load was increased incrementally relative to the pronator teres load. Supination trials were similarly conducted using the biceps and supinator muscles. Testing was conducted in the intact forearm and following ulnar head excision. An electromagnetic tracking device was used to record motion of the radius and ulna. Kinematic data was analyzed with a planar analysis that measured dorsal palmar displacements and diastasis of the DRUJ.

Greater diastasis and dorsal translation of the radius relative to the ulna were noted under increased PQ loading following ulnar head excision (p< 0.05). Increased supinator load had no effect on kinematics before or after ulnar head excision. This effect is likely due to the location of the two muscles. The effect of PQ muscle loading was only noted in neutral to full pronation. These results suggest that rehabilitation of the forearm following ulnar head excision should be conducted with the forearm in supination to minimize joint instability.

Funding: Natural Sciences and Engineering Research Council of Canada, The Arthritis Society (Canada)

Three 3mm transverse slices were sectioned from the distal cancellous region of seven fresh-frozen cadaveric humerii. Each slice was marked with a 3x3mm grid, and subjected to compressive testing using a flat cylindrical indenter (1.6mm diameter). Indentation modulus and strength were calculated for each site, and pooled into nine anatomically-defined regions. The most distal slice had higher moduli values (p< 0.05), and the posterior capitellar region had lower moduli values (p< 0.05). There were no slice or regional differences in strength. This suggests that surgical procedures requiring cancellous fixation utilize the most distal aspect of the humerus while avoiding the posterior capitellum.

To quantify the indentation strength and modulus of distal humeral cancellous bone, and identify any regional variations.

Cancellous bone modulus in the distal humerus decreases from distal to proximal. The posterior capitellum has a lower modulus than the other regions of the distal humerus.

The influence of slice depth emphasizes the importance of minimizing the amount of bone removed during prosthetic replacement. Regional variations in modulus suggest that the posterior capitellum should be avoided during fixation of implants or placement of screws.

Three 3mm transverse cancellous bone slices obtained from the distal end of each of seven fresh-frozen cadaveric specimens were subjected to compressive testing using a materials testing machine with a 1.6mm flat cylindrical indenter. Testing was performed in a 3x3mm grid. The indentation modulus and local strength were calculated for each test site, and then averaged into nine regions defined by the capitellum, medial and lateral trochlea, and anterior, central and posterior sections for each slice. Mean modulus was found to be 309.8±242.0 MPa (range: 2.9–1041.7 MPa). Yield strength averaged 4.4±2.5 MPa (range: 0.6–16.3 MPa). The highest modulus was found in the distal-most slice (p< 0.05). The lowest modulus region was the posterior capitellum (p< 0.05). There were no differences in strength between slices or across the nine regions. A comparison with proximal tibial cancellous bone properties suggests the distal humerus may carry loads approaching 30% of those at the knee, assuming that bone adapts to stress magnitudes.

Funding: Natural Sciences and Engineering Research Council; University of Western Ontario