Richard Oka

Biomechanical evaluation of clavicle fracture plating techniques: does a locking plate provide improved stability?

Objectives: To evaluate the biomechanical properties of both plate location (superior versus anterior-inferior) and plate type Small Fragment Contourable Dual Compression Plate (CDCP) versus 3.5 mm Universal Locking System Contourable Dual Compression Plate (Locked CDCP) in a synthetic midshaft transverse clavicle fracture model. Methods: Twenty-four pre-osteomized synthetic clavicles were repaired with either CDCP or locked CDCP technology 3.5 mm plates in either the superior or anterior-inferior position to form 4 groups of 6 clavicles. These were subsequently tested to evaluate torsional and axial construct stiffness, as well as bending load to failure, bending failure stiffness, and method of failure. Results: In axial compression, locked CDCP constructs were significantly more stiff than CDCP constructs (p < 0.001), but no statistically significant effect of plate location was observed. Torsional tests demonstrated a significant 2-way interaction favoring locked CDCP plates in the superior position and standard CDCP plates in the anterior-inferior position (p < 0.001). Bending failure testing revealed that the superior plate location had higher load to failure and bending failure stiffness than the anterior-inferior location (p < 0.0001). In addition, the superior locked CDCP plates demonstrated significantly greater bending failure stiffness than superior CDCP plates (p < 0.0001). Conclusions: Biomechanically, repairing a midshaft clavicle fracture with a superior plate was more favorable compared to anterior-inferior plating in terms of both load to failure and bending failure stiffness. Furthermore, superior locked CDCP plates show improved bending failure stiffness over superior CDCP plates.

The mechanical properties of the ligamentum teres: a pilot study to assess its potential for improving stability in children's hip surgery.

The anatomic and histological characteristics of the ligamentum teres and its vascular contributions to the femoral head have been well described. The function of the ligamentum teres remains poorly understood. Although excision is the current standard in treating complete developmental hip dysplasia, we developed an interest in maintaining, shortening, and reattaching the ligament to assure early postoperative stability in developmental hip dysplasia. To analyze its potential for providing hip joint stability, we investigated the biomechanical properties of the ligamentum teres in an in vitro porcine model. Six immature porcine hips were dissected, with the proximal femur and acetabular anatomy kept intact, isolating the ligamentum teres. Specimens were loaded in tension using custom fixation rigs at 0.5 mm/s in line with the fibers. Data for displacement and force were collected and sampled at 10 Hz for duration of each test. The ligamentum teres failed in a stepwise fashion. The mean ultimate load to failure was 882 ± 168 N. Mean stiffness and failure stress were calculated as 86 ± 25 N/mm and 10 ± 2 MPa, respectively. The biomechanical function of the ligamentum teres is not inconsequential. We found the ultimate load of the ligamentum teres in the porcine model to be similar to those reported for the human anterior cruciate ligament. The strength of the ligamentum teres may confirm its potential for providing early stability in childhood hip reconstructions. In the setting of dysplasia, the preservation and the transfer of the ligamentum teres to augment stability should be considered as an adjunct to open reduction.

Spinal growth modulation with use of a tether in an immature porcine model.

BACKGROUND Spinal growth modulation by tethering the anterolateral aspect of the spine, as previously demonstrated in a nonscoliotic calf model, may be a viable fusionless treatment method for idiopathic scoliosis. The purpose of the present study was to evaluate the radiographic, histologic, and biomechanical results after six and twelve months of spinal growth modulation in a porcine model with a growth rate similar to that of adolescent patients. METHODS Twelve seven-month-old mini-pigs underwent instrumentation with a vertebral staple-screw construct connected by a polyethylene tether over four consecutive thoracic vertebrae. The spines were harvested after six (n = 6) or twelve months (n = 6) of growth. Monthly radiographs, computed tomography and magnetic resonance imaging scans (made after the spines were harvested), histologic findings, and biomechanical findings were evaluated. Analysis of variance was used to compare preoperative, six-month postoperative, and twelve-month postoperative data. RESULTS Radiographs demonstrated 14 degrees +/- 4 degrees of coronal deformity after six months and 30 degrees +/- 13 degrees after twelve months of growth. Coronal vertebral wedging was observed in all four tethered vertebrae and progressed throughout each animals survival period. Disc wedging was also created; however, in contrast to the findings associated with vertebral wedging, the tethered side was taller than the untethered side. Magnetic resonance images revealed no evidence of disc degeneration; however, the nucleus pulposus had shifted toward the side of the tethering. Midcoronal undecalcified histologic sections showed intact bone-screw interfaces with no evidence of implant failure or loosening. With the tether cut, stiffness decreased and range of motion increased in lateral bending away from the tether at both time-points (p < 0.05). CONCLUSIONS In this porcine model, mechanical tethering during growth altered spinal morphology in the coronal and sagittal planes, leading to vertebral and disc wedging proportional to the duration of tethering. The resulting concave thickening of the disc in response to the tether was not anticipated and may suggest a capacity for the nucleus pulposus to respond to the compressive loads created by growth against the tether.

Pullout strength of thoracic pedicle screw instrumentation: comparison of the transpedicular and extrapedicular techniques.

Study Design. In vitro biomechanical comparison of two methods of pedicle screw placement in cadaveric thoracic spine vertebrae. Objective. Compare the biomechanical integrity of extrapedicular and transpedicular screw fixation under axial and sagittal pullout loads. Summary of Background Data. Extrapedicular screw placement has been advocated as a safe and effective alternative to the transpedicular screw in thoracic vertebrae. Rigorous biomechanical comparison of these two techniques is presently lacking in the literature. Methods. Thirty-seven vertebral bodies were dissected from six cadaveric thoracic spines. Each body had two polyaxial 5.0-mm screws placed: one transpedicular and one extrapedicular. The 62 screws were randomly designated for one of two loading methods: axial or sagittal. Failure load (N), taken as maximum force on the load-deformation curve, and stiffness (N/mm), calculated between 50 N and 400 N, were measured. Results. Transpedicular screws were statistically stronger in both testing methods (P = 0.008). Load direction, whether axial or sagittal, had no bearing on pullout strength (P = 0.6). Conclusions. These data indicate that transpedicular screws are biomechanically superior to extrapedicular screws. This difference is small, however, and we think that extrapedicular screws offer an excellent alternative when anatomy dictates their use with other screws in segmental spinal constructs.

Biomechanical comparison of four different fixation techniques for pediatric tibial eminence avulsion fractures.

Background: Several different methods have been used to repair tibial eminence avulsion fractures. It is not clear which is the best stabilization method. The purpose of this study was to compare the biomechanical stability of tibial eminence avulsion fractures using suture, resorbable screw, resorbable nail, and metal screw techniques. Methods: Sixteen immature bovine knees were dissected leaving just the anterior cruciate ligament. A fracture was created using a curved osteotome, The knees were randomly stabilized with either 2 single-armed #2 Ethibond sutures, 3 bioabsorbable nails, a single resorbable screw, or a single metal screw. Femurs were tested with the knee flexed to 35 degrees to simulate anterior tibial translation. Tests involved loading between 5 N and 150 N for 200 cycles, then a tensile failure test at 0.5 mm/sec. Cyclic fragment deformation, initial fragment stiffness, and failure load were compared using a 1-way analysis of variance (p < 0.05). Results: There were no significant mechanical differences across groups. The variability in performance was much greater for both the suture and resorbable screw repairs. Both sutures and resorbable screw constructs resulted in a deformation that was 1 mm greater than that of the resorbable nails or a metal screw. Conclusions Increased fracture separation for sutures and resorbable screw groups indicates a potential loss in reduction during cyclic, physiologic loads. Each group could withstand up to 85 lb of tensile force before failure, but it is unlikely that this force would occur with incidental loads during the early rehabilitation period. Clinical Relevance There was not a clear biomechanical advantage to performing any particular fixation method in this study. This suggests that the surgeon can use their clinical judgment and experience to determine the fixation technique.

Biomechanical comparison of a novel percutaneous transfacet device and a traditional posterior system for single level fusion.

Posterior spinal fusions are indicated for a variety of spinal disorders. Transfacet fixation minimizes soft tissue disruption and preserves the adjacent facet joint. This technique is uncommon due to concerns with biomechanical stability and proper implant placement. For these reasons, a length adjustable implant may obviate the clinical concerns but necessitates biomechanical study. This study evaluated the in vitro biomechanical stability between a novel transfacet fixation device compared with standard pedicle screws during cyclic physiologic loading in a human cadaveric model. Cadaveric L4-L5 lumbar motion segments from 16 human spines were tested in cyclic flexion/extension, lateral bending, and torsion after insertion of either transfacet fixation devices or 5.5 mm pedicle screw instrumentation. A load cell was used to measure the compressive forces on the anterior column during testing. Motion segment stiffness and anterior column compression were analyzed with a 1-way analysis of variance (P<0.05). The transfacet device demonstrated a statistically similar stiffness when compared with the pedicle screw system for each test direction. For anterior column loading during physiologic testing, there were no biomechanical differences between stabilization systems. Percutaneous transfacet fixation is an attractive surgical option for single-level spinal fusions. A biomechanical evaluation of a novel device for this application demonstrated similar stability to a pedicle screw system. The length adjustability of the device may alleviate concerns for precise device placement and the biomechanical stability may produce similar rates and quality of posterior spinal fusions.

Biomechanical comparison of lumbosacral fixation using Luque-Galveston and Colorado II sacropelvic fixation: advantage of using locked proximal fixation.

Study Design. Biomechanical evaluation of sacropelvic fixation strategies as they apply to neuromuscular scoliosis. Objectives. The primary objective was to compare the rigidity of 2 methods of sacropelvic fixation (Galveston vs. Colorado II). The secondary objective was to evaluate the effect on construct rigidity by adding a pair of L1 pedicle screws to a Luque wire construct. Summary of Background Data. The Galveston modification to the Luque rodding system has become standard for treating childhood and adolescent neuromuscular scoliosis. The Galveston method provides reasonable lumbo-pelvic fixation with a relatively simple method of insertion. Clinical reviews of sagittal plane stability in neuromuscular patients with Galveston fixations performed at our institution have led to concerns regarding the technique’s ability to maintain proper lumbar lordosis. This concern has generated interest in evaluating biomechanical stability of more rigid fixation methods in these long spino-pelvic constructs. As such, the following biomechanical study evaluated lumbosacral stability of 2 sacropelvic fixation methods: the standard Luque-Galveston method and the Colorado II sacropelvic fixation method using the Chopin plate-screw block. As a secondary interest, evaluations of the rigidity of the proximal construct when using pedicle screw fixation were completed. It was hypothesized that one additional point of rigid fixation at the thoracolumbar junction may make substantial improvement in rigidity to the otherwise Luque construct. Methods. Lumbo-pelvic segments of human cadaveric specimens were instrumented with L1 pedicle screws, sublaminar wires between L2 and L5, and sacropelvic fixation with either Galveston rods or Colorado II sacropelvic plates using S1 screws, S2 alar screws, and iliac screws. Tests were conducted for physiologic flexion-extension and torsional loading. Construct stiffness between L1–S1 was determined for each specimen. Motion measurement data were collected between L1–L5 and L5–S1 using a noncontact marker system. Statistical analysis included a 2 -way analysis of variance (dependent variables: construct/locked screw) with the Tukey post hoc correction for multiple comparisons (P < 0.05). Results. The flexion and extension bending stiffness of the construct was similar between the Galveston and Colorado II constructs. Both constructs were stiffer when the L1 screws were locked rigidly to the rod. Torsional stiffness followed similar trends with no significant difference between the systems, although more rigid in more cases when the L1 screws were locked to the rod. Regarding limiting L5–S1 motion during flexion and extension loading, the Colorado II construct did so to a higher degree. There was no difference in torsional motion between the 2 constructs. Locking the L1 pedicle screws reduced torsional motion but had no effect on flexion-extension motion at L5–S1. Conclusion. The 2 methods of sacropelvic fixation provided similar construct stiffness, although the Colorado II method had less L5–S1 motion on flexion-extension testing, and the Galveston construct tended (although not statistically) to be stiffer in torsional loading. The addition of a pair of L1 pedicle screws increased the construct stiffness for both constructs by approximately 50%.

Biomechanical analysis of single screw fixation for slipped capital femoral epiphysis: are more threads across the physis necessary for stability?

Purpose: To evaluate single screw fixation stability, in the treatment of slipped capital femoral epiphysis, as a function of screw thread distribution across the physis. Study Design: In vitro biomechanical study. Methods: Thirty porcine proximal femurs were sectioned through the physeal line and stabilized with a cannulated 7.3-mm stainless steel AO screw. The distal 16 mm of each screw was threaded (5 threads). The femurs were randomized into 5 groups (1, 2, 3, 4, or 5 threads across the physis) and biomechanically tested to determine failure load (N) and stiffness (N/mm). Results: Femurs with 2 or 3 threads across the physis had a significantly greater load to failure and stiffness compared with femurs with 1, 4, or 5 threads across the physis (P < 0.05). Conclusions: Thread distribution across the physis seems to be important. When using screws with a 16-mm thread, greatest strength and stiffness are achieved when 40%-60% of threads engage the epiphysis, with a significant decrease when greater than 80% of threads cross the physis. Too few threads in the epiphysis as well as too few in the metaphysis both lead to decreased stability. Clinical Relevance: This study challenges the belief that compression across the physis maximizes slipped capital femoral epiphysis fixation stability. We recommend equal distribution of threads across the physis when using 16-mm thread screws, and we postulate that screws with a greater thread length (32 mm or fully threaded) would increase fixation stability even further. Optimizing purchase may decrease the incidence of slip progression, especially as the prevalence of obesity increases in the adolescent population.

Biomechanical Comparison of Locking versus Nonlocking Volar and Dorsal T-plates for Fixation of Dorsally Comminuted Distal Radius Fractures

Objectives: The purpose of this study was to gain insight into the effect of plate location and screw type for fixation of extra-articular distal radius fractures with dorsal comminution (Orthopaedic Trauma Association Type 23-A3.2). Methods: Sixteen pairs of cadaver radii were randomized to four plating configurations: dorsal locking, dorsal nonlocking, volar locking, and volar nonlocking. A standard 1-cm dorsal wedge osteotomy was used. Cyclic axial loads were applied for 5000 cycles. Stiffness and fragment displacement were recorded at 500 cycle-intervals. Pre- and postcyclic loading radiographs were analyzed. An axial load to failure test followed and construct stiffness and failure strength recorded. Biomechanical data were analyzed using a two-way analysis of variance (P < 0.05). Failure modes were descriptively interpreted. Results: Cyclic testing data revealed no difference between constructs at any interval. Within all construct groups, displacement that occurred did so within the first 500 cycles of testing. Pre- and postcyclic loading radiographic analysis showed no differences in construct deformation. Load to failure testing revealed no differences between groups, whereas volar constructs approached significance (P = 0.08) for increased failure strength. Dorsal constructs failed primarily by fragment subsidence and fragmentation, whereas volar constructs failed by plate bending. Conclusions: No difference in all measured biomechanical parameters supports equivalence between constructs and surgeon discretion in determining operative method. Minimal fragment displacement and construct deformation during physiological testing support previous data that early postoperative motion can be recommended. Fragment displacement that occurs does so in the early periods of motion.

Does the direction of pedicle screw rotation affect the biomechanics of direct transverse plane vertebral derotation

Study Design. In vitro biomechanical investigation using human cadaveric vertebrae. Objective. Evaluate the biomechanical differences in transverse plane vertebral body derotation maneuvers of thoracic pedicle screws in both medial and lateral directions. Summary of Background Data. Thoracic pedicle screws are thought to have better vertebral rotation control and better segmental scoliosis correction compared to hooks and wires. Little data exists regarding the biomechanical stability of pedicle screws when derotated in either medial or lateral directions. Methods. Vertebral bodies (T4–L5) from 12 cadavers were instrumented with appropriate length pedicle screws while measuring insertion torque. Each body was anchored for independent loading in medial or lateral directions. Each screw was rotated around a rod using a constant length lever arm (30.5 cm) rigidly attached to the screw head simulating the posterior vertebral derotation maneuver. Yield torques (Nm) were analyzed using a one-way analysis of variance (P < 0.05). Results. Yield torques for both directions were significantly related to screw insertion torque (both P < 0.01). There were no statistical differences in yield torque between medial (12.0 ± 4.9 Nm) or lateral (11.5 ± 5.1 Nm) directions. There were no significant differences after normalization for insertion torque or screw length. Tests rotating the screw tip laterally demonstrated structural failure in the following percentages (anterolateral failure = 67%, posterior element failure = 33%, additional screw bending = 42%). Rotation medially demonstrated structural failures in the following percentages (canal penetration = 51%, posterior element failure = 49%, additional screw bending = 44%). Conclusion. From these data, a surgeon performing a direct vertebral derotation using a 30 cm (12 in) lever would need to apply roughly 40 N (9 lbs) to causeanatomic failure. Adolescent patients would likely tolerate a greater force without bone failure given a greater bone density, yet, extreme caution is still recommended to prevent screw rotation either medially into the spinal canal or laterally into the chest.