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.

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 animal's 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.

Biomechanical differences between transfacet and lateral mass screw-rod constructs for multilevel posterior cervical spine stabilization.

STUDY DESIGN: In vitro biomechanical investigation using human cadaveric cervical spines. OBJECTIVE: Evaluate differences in biomechanical stability between typical lateral mass screw + rod constructs compared to transfacet screw fixation with and without rods. SUMMARY OF BACKGROUND DATA: Lateral mass screw + rod constructs have reported efficacious arthrodesis rates/quality but risk damaging the lateral neurovascular structures. Transfacet screw fixation has been studied in the lumbar spine, but little data exists regarding its potential utility in the cervical spine. METHODS: Sixteen human cadaveric cervical spines were stripped of soft tissue leaving the occiput and ligamentous structures intact. Spines were randomized to lateral mass or transfacet groups (n = 8/group). Spines were prepared in typical surgical fashion and instrumented with the appropriate devices. In the case of the transfacet constructs, the occiput was left intact to simulate the potential surgical difficulty of screw insertion. The transfacet screw group was initially instrumented with rods. Once instrumented (C3-C6) for each group, spines were further dissected to isolate the instrumented levels. End vertebral bodies were rigidly fixed and constructs biomechanically tested in flexion/extension, lateral bending, and axial torsion between +/-2 Nm. After testing for the transfacet screw + rod group, rods were removed and spines retested. All instrumentation was then removed and spines tested in their destabilized state as would occur with surgical preparation. Stiffness data were calculated for each test direction for all groups. Raw and normalized data were each compared across techniques with a 1-way ANOVA (P < 0.05). RESULTS: The transfacet screw groups (with and without rods) were found to have statistically similar biomechanical stability to lateral mass screw + rod constructs for each test direction. CONCLUSION: Transfacet screws (without rods) were found to have similar biomechanical stability compared to typical lateral mass screw + rod constructs. However, transfacet fixation eliminates the risk to the neurovascular structures and lowers the overall implant profile.

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.

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 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.

Biomechanical comparison of fully and partially threaded screws for fixation of slipped capital femoral epiphysis.

BACKGROUND: Previous data have shown that an equal number of threads on each side of the physis maximizes stability for slipped capital femoral epiphysis (SCFE) fixation. The purpose of the current study was to determine if a fully threaded cancellous screw provides greater stability compared with a partially threaded screw in a porcine model. METHODS: Twenty skeletally immature porcine femurs were sectioned, and a 30-degree angular wedge was resected from the femoral neck to simulate SCFE. Femora were randomly assigned to partially threaded (16 mm) or fully threaded screw groups (n = 10/group). Kirschner wires were inserted in a retrograde fashion to stabilize the proximal fragment. Each 7.3-mm-diameter screw was placed using fluoroscopic guidance to obtain 3 threads crossing the physis. Specimens were fixed in custom fixation rigs, and the epiphyseal fragment was loaded at 0.5 mm/s in a posterior-inferior direction to simulate slip progression. Data for displacement (in millimeters) and force (in newtons) were collected for the entire test. Forces at 2, 4, 6, and 8 mm of fragment displacement were compared between groups using a 1-way analysis of variance (p < 0.05). RESULTS: Increasing epiphyseal displacements were associated with incremental increases in loading. There were no significant differences between fully threaded or partially threaded screws for loads at each displacement. Each stepwise increase in displacement was associated with approximately 325 N of force. CONCLUSION: There was no biomechanical benefit when using a fully threaded screw for stabilization of an in vitro SCFE model. CLINICAL RELEVANCE: Although there were no differences between screw types in an in vitro model, bone healing around the fully threaded screw may eventually provide greater stability. The use of fully threaded screw remains a reasonable option in the treatment of SCFE, and implant removal may be easier with such a system. Further studies are warranted to verify these 2 points.

Biomechanical comparison of different anchors (foundations) for the pediatric dual growing rod technique.

BACKGROUND CONTEXT: Children with early onset scoliosis benefit from early operative treatment with dual growing rods as they provide an "internal brace" for the spine and allow curve correction and sequential lengthenings to maximize thoracic cage development. The foundations that provide anchor points for the dual growing rods may incorporate hooks, screws, or a hybrid construct. It is unclear how stable different types of foundation constructs are with regard to pullout. PURPOSE: This study was to determine the differences in stability between four foundation configurations used in the pediatric dual growing rod technique. Eight porcine spines were sectioned and randomly assigned to one of four foundation groups: 1) hook-hook with cross-link; 2) hook-screw with cross-link; 3) screw-screw with cross-link; and 4) screw-screw without cross-link. After instrumentation, biomechanical pullout tests were conducted. Ultimate failure loads, modes of failure, and level of instrumentation were analyzed. STUDY DESIGN/SETTING: Controlled in vitro laboratory investigation. RESULTS: Screw-screw with cross-link constructs demonstrated the greatest failure load but this was not statistically significant compared with the screw-screw without cross-link constructs. Both screw-screw constructs were statistically stronger than either construct containing hooks. There was no difference between the hook-screw and hook-hook constructs that was statistically significant. Hook-containing constructs sustained higher failure loads in lumbar versus thoracic vertebrae. CONCLUSIONS: A foundation composed of four pedicle screws implanted in two adjacent vertebral bodies provides the strongest construct in pullout testing. A cross-link does not seem to enhance fixation. Hook constructs are stronger in lumbar versus thoracic laminae.

Single-row suture anchor repair of the rotator cuff is biomechanically equivalent to double-row repair in a bovine model.

PURPOSE: The purpose of this study was to determine biomechanical differences in cyclic elongation and ultimate strength between double-row rotator cuff repair and single-row repair for partial rotator cuff repairs. METHODS: We randomly assigned 18 immature bovine specimens (aged 12 to 16 weeks) to 3 repair groups (6 per group). A 1 x 2-cm defect was created at the infraspinatus tendon insertion site. Two suture anchors were implanted 1 cm apart at the anatomic insertion area for the lateral row. Two suture anchors were implanted 1 cm medial to the lateral row and 1 cm apart from each other for the medial row. Repair groups were constructed as follows: single-row repair with double-loaded suture anchors (group 1), double-row repair with single-loaded medial row and double-loaded lateral row (group 2), and double-row repair with single-loaded medial row and single-loaded lateral row (group 3). Specimens were cyclically loaded from 10 N to 90 N for 500 cycles and then loaded at 0.5 mm/s to failure. Data for cyclic elongation, with loads at 3 mm, 5 mm, and 10 mm, were analyzed via a 1-way analysis of variance (P < .05). RESULTS: There were no significant differences for peak elongation after cyclic loading between groups. There were no significant differences between repair groups for loads at 3 mm, 5 mm, and 10 mm of elongation. Constructs typically failed by knot slippage (83%), with a single sample having tendon-suture failure (17%). CONCLUSIONS: Double-row repair did not show a biomechanical advantage compared with single-row repair. With this result in mind, the theoretic advantage of a potentially larger footprint must be balanced against the added surgical time, complexity, and cost of double-row repair. CLINICAL RELEVANCE: Arthroscopic surgeons should choose the best form of fixation for a given patient, without undue emphasis on single-row repair versus double-row repair. The clinical and biologic impact of footprint restoration was not addressed in this study.

Biomechanical and radiographic analysis of a novel, minimally invasive, extension-limiting device for the lumbar spine.

OBJECT: Biomechanical testing and fluoroscopic imaging were used to study an extension-limiting device that has been developed to support and cushion the facet complex. It is a titanium screw-based system with a polycarbonate-urethane bumper that lies against the inferior articular process and is anchored into the pedicle by the screw for posterior dynamic stabilization (PDS). METHODS: Six human cadaveric spines were dissected from L-2 to L-5, leaving all ligamentous structures intact. The intact spines were first tested in flexion and extension, lateral bending, and axial rotation at 67.5 Nm. The PDS devices were inserted at L3-4 and testing was repeated. Fluoroscopic analysis of posterior disc height and foraminal area of the intact and instrumented spines while loaded was performed. All test data were compared using a one-way analysis of variance (statistical significance was set at p < 0.05). Instrumented spines had 62% less motion during flexion and 49% less motion during extension compared with the intact spines. Neuroimaging analysis showed 84% less compression of the posterior disc of the instrumented spines during extension, and no difference during flexion compared with intact spines. After instrumentation was affixed, the foraminal area was 36% larger than in intact spines during extension and 9% larger during flexion. During axial loading, compression of the posterior disc was decreased by 70%, and analysis showed 10% decompression prior to loading just from implanting the devices. CONCLUSIONS: The PDS system has the benefit of being a completely percutaneous one, which can be used at all levels of the lumbar spine, including S-1. The PDS system limits spinal motion, enlarges the foramina, and achieves discal decompression.

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.

Biomechanical stability of bioabsorbable screws for fixation of acetabular osteotomies.

The purpose of this study was to compare the biomechanical stability of triple innominate osteotomies fixed with either bioabsorbable or stainless steel screws. Triple innominate osteotomies were performed on composite hemipelves and fixed with either three 4.5-mm bioabsorbable screws or three stainless steel 4.5-mm screws. Two screws were placed from the iliac wing into the acetabular fragment, and 1 screw was placed from below the acetabular fragment into the iliac wing. Eight specimens for each screw type were biomechanically tested in an anatomical position (replicating weight bearing) and in a flexed and abducted position (replicating spica cast positioning). Specimens were cyclically loaded between 10 and 450 N to simulate the hip contact force in this population. Lower screws were then removed, and specimens were tested under identical conditions. Fragment displacement (mm) and construct stiffness (N/mm) were compared with a 2-way analysis of variance (P < 0.05). There were no significant differences between materials for fragment displacement or construct stiffness. Anatomical position showed significantly less displacement than spica position for both materials. Initial displacement in the spica position was significantly less during lower loads for stainless steel fixation. Bioabsorbable screws demonstrate comparable biomechanical stability to stainless steel screws in anatomical and spica positions at physiological loads. Flexion and abduction of the femur adversely affect the stability of the construct for both materials. Bioabsorbable screws behave similarly to steel screws when stabilizing triple innominate osteotomies and would have the advantage of not requiring a second surgery for screw removal. Confirmation of biocompatibility should be completed before widespread clinical application.

Biomechanical analysis of femoral tunnel pull-out angles for anterior cruciate ligament reconstruction with bioabsorbable and metal interference screws.

BACKGROUND: Fixation strength of metal and bioabsorbable interference screws has not been evaluated while varying the anterior cruciate ligament graft tension angle. HYPOTHESIS: There is no difference in fixation strength between 2 types of interference screws for anterior cruciate ligament graft fixation while the graft tension angle is varied relative to the femoral tunnel. STUDY DESIGN: Controlled laboratory study. METHODS: Forty-eight anterior cruciate ligament reconstructions were performed using immature porcine femurs stripped of soft tissue and doubled-over porcine flexor digitorum profundus tendon grafts. Specimens were randomized to bioabsorbable or titanium interference screw fixation. Specimens were randomized to one of three pull angles (0 degrees , 30 degrees , 60 degrees ) representing loading at different knee flexion angles (n = 8/group). Reconstructed ligaments were tensioned to 10 N followed by 200 loading cycles between 10 and 150 N and a final failure test. Construct elongation (mm) at 100 and 200 cycles and failure load (N) were analyzed using a 2-way analysis of variance (P < .05). RESULTS: Screw material interacted significantly with graft tension angle, as the bioabsorbable screw specimens demonstrated significantly greater fixation strength when tensioned at greater angles. Specimens fixed with bioabsorbable screws showed significantly less elongation at both 100 and 200 cycles and significantly greater failure load compared with titanium screws. CONCLUSION: Bioabsorbable interference screws acutely have increased fixation strength compared with titanium interference screws for anterior cruciate ligament grafts loaded at greater tension angles. CLINICAL RELEVANCE: The strength of anterior cruciate ligament reconstruction fixation increases with increasing divergence between the tension angle and femoral tunnel, a condition seen when the knee approaches full extension.

Differences in biomechanical stability of femur fracture fixation when using titanium nails of increasing diameter.

PURPOSE: The purpose of this study was to compare the biomechanical stability generated when utilizing increasing sizes of titanium (Ti) flexible nails for fixation of simulated comminuted femur fractures. METHODS: Five synthetic adolescent-sized femur models were reamed to create a 9-mm canal. A 2-cm section was removed in the mid-diaphysis to simulate comminution. Each femur was first stabilized with bilateral, retrograde 3.0-mm titanium elastic nails. Femurs were tested in axial rotation and axial compression. The constructs were removed, and femurs were re-nailed with 3.5-mm nails. Identical testing was conducted. These nails were then removed, and femurs were re-nailed with 4.0-mm nails. This provided data on "canal fill" representing 67, 78 and 89% of the reamed canal diameter. Data for axial rotation (degrees) and failure load (N) required to produce 5 mm of fracture shortening were analyzed with a one-way ANOVA (P < 0.05) and a Tukey's post-hoc test for multiple comparisons. RESULTS: For axial rotation, there were statistically significant improvements in rotational control for each increase in nail size. For axial stability, each increase in nail size resulted in increased axial failure loads to 5 mm, although these data were not statistically different. A specific comparison between 3.0- and 3.5-mm nails for compressive stability found significantly greater stability afforded by using 3.5-mm nails. CONCLUSIONS: Data from this study demonstrate that increasing the amount of canal fill provides significant improvements in rotational control. The largest improvement was seen when increasing from 3.0- to 3.5-mm nails. While increasing the nail size from 3.5 to 4.0 mm again provided greater stability, larger nails may be more difficult to insert. Thus, increasing the nail size for femoral fracture fixation should be considered after measuring the diameter of the canal and evaluating the potential difficulty of insertion as well as specific demands of the fracture pattern.

Biomechanical evaluation of retrograde intramedullary stabilization for femoral fractures: the effect of fracture level.

BACKGROUND: Retrograde stabilization of mid-diaphysis adolescent femur fractures has shown excellent biomechanical stability. However, it is unclear whether adequate stability is maintained for distal femur fractures using the retrograde approach compared with the clinically recommended antegrade approach. The purpose of this study was to evaluate the biomechanical stability of retrograde and antegrade nailing for mid-diaphyseal and distal diaphysis femoral fractures. METHODS: Twenty adolescent-sized synthetic femurs were randomly assigned to fracture location and surgical approach groups. Comminuted fractures were simulated at the mid-diaphysial level and 4 cm proximal to the distal physis. The retrograde approach used 2 c-shaped 3.5-mm titanium nails. The antegrade used c and s 3.5-mm nail configurations. Both techniques achieved maximum nail divergence at the level of the fracture. Biomechanical testing was conducted to determine differences in torsional range of motion (degrees)and failure load (N) at 5 mm. These data were analyzed with a 2-way analysis of variance (p < 0.05). RESULTS: In torsion, there were no differences related to surgical approach or fracture level. For axial compression to 5 mm, the antegrade approach required significantly greater force to achieve 5 mm of compression compared with the retrograde approach. The mid-diaphyseal fracture required significantly greater force to achieve 5 mm of compression compared with the distal diaphysis group. CONCLUSIONS: For maximum stabilization of a distal femur fracture, c- and s-shaped nails placed in the antegrade position is suggested. CLINICAL RELEVANCE: Surgical decision making regarding the use of either the antegrade or retrograde approach will be influenced by both the stability provided (antegrade) and the ease of insertion (retrograde).

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 stability of single-screw versus two-screw fixation of an unstable slipped capital femoral epiphysis model: effect of screw position in the femoral neck.

PURPOSE: To biomechanically evaluate single screw and varying 2 screw fixations for an unstable slipped capital femoral epiphysis (SCFE) model using physiologically relevant loading. STUDY DESIGN: In vitro biomechanical study. METHODS: Twenty-four immature porcine proximal femurs were prepared to simulate a mild-to-moderate unstable SCFE. The femurs were randomized into 4 fixation groups: single screw, 2 screws horizontally placed, 2 screws vertically placed, and 2 oblique screws. Biomechanical testing determined maximum load to failure (N), load (N) at 2, 4, 6, and 8 mm of femoral head displacement, and stiffness (newtons per millimeter) for each group. RESULTS: No significant differences were found among the 3 different 2 screw configurations. The 2 screw constructs were 66% stiffer and 66% stronger than the single screw construct. In addition, whereas there was no difference at 2 mm of femoral head displacement, each subsequent displacement (4, 6, and 8 mm) demonstrated significantly higher failure loads when 2 screws were used for stabilization. CONCLUSIONS: Slipped capital femoral epiphysis stabilization with 2 screws leads to increased stability over single screw fixation; however, none of the 3 configurations/placement patterns of the 2 screw constructs seemed to be superior in fixation stability. CLINICAL RELEVANCE: These data support the use of a 2 screw construct in acute/unstable SCFE fixation. The biomechanical benefit of 2 screws needs to be considered in the face of greater potential for inadvertent penetration into the joint with an increased number of screws.

Biomechanics of cantilever "plow" during anterior thoracic scoliosis correction.

BACKGROUND CONTEXT: Anterior instrumentation is often used for correction of thoracic scoliosis. Loss of spinal correction may occur after failure at the bone-implant interface, and forces on the bone-implant interface during scoliosis correction remain unclear. PURPOSE: Evaluate two different mechanisms of loading associated with anterior scoliosis correction. STUDY SETTING: In vitro biomechanics lab. METHODS: Polyurethane foam and human cadaveric thoracic vertebral bodies were instrumented with transvertebral body screws. Bone-implant interface failure loads were measured during constrained, fixed-angle screw translation, as well as unconstrained translation allowing coronal plane screw rotation. Vertebral body staples were randomly assigned to both conditions. RESULTS: Data were consistent across foam and cadaveric specimens. Failures occurred at significantly lower loads during unconstrained translation (with rotation) compared with constrained translation. Staple usage significantly increased the load to failure in both testing modes. In cadaveric bone, the constrained plowing load to failure was 562N+110N versus 188N+20N in the unconstrained testing. With a staple, these values increased to 694N+53N and 530N+100N, respectively. CONCLUSIONS: The 280% increase in cadaveric failure loads when a staple was added in the unconstrained testing method exceeds previous reports. The unconstrained method of plow simulated anterior scoliosis instrumentation when a rod was cantilevered and compressed into position. Supplemental vertebral body staples may be clinically indicated, particularly at the ends of the construct where residual deforming forces remain the greatest.

Biomechanical comparison of the screw-bone interface: optimization of 1 and 2 screw constructs by varying screw diameter.

STUDY DESIGN: In vitro biomechanical investigation of 1 and 2-screw anterior scoliosis constructs with varying screw diameters. OBJECTIVE: To determine a possible optimal configuration of screw number and diameter at varying levels within the thoracic spine for anterior vertebral body fixation. SUMMARY OF BACKGROUND DATA: Single-rod systems are typical in anterior thoracic and thoracolumbar correction of adolescent idiopathic scoliosis; although dual rod systems may offer more flexural stability. Loss of fixation remains problematic, particularly in the proximal thoracic vertebrae, and it remains unclear how screw diameter or the number of screws within the vertebrae affect fixation. METHODS: Individual vertebral levels from 10 cadaveric thoracic spines were randomly assigned to either 1 or 2 screws of 5, 6, or 7-mm diameter. Bone-screw interface failures were created in coronal plane cantilever plow, and failure loads were compared across vertebral levels for each instrumentation method. RESULTS: Two-screw constructs had significantly higher failure loads than single-screw constructs, while increasing screw diameter also produced significant changes in fixation strength. Two-screws had improved performance in the mid and lower thoracic spine, while a single screw was more stable in the upper thoracic spine. CONCLUSIONS: Failure modes for 1-screw constructs almost entirely (89%) showed gradual plowing through the bone, whereas acute fracture through the vertebral body or pedicles were common forms of failure (85%) for 2-screw constructs.

Biomechanical comparison of two different periarticular plating systems for stabilization of complex distal humerus fractures.

BACKGROUND: Complex intra-articular distal humerus fractures are relatively uncommon injuries but are fraught with poor outcomes such as malunion, elbow stiffness and deformity. Various types of internal fixation screw-plate constructs have been developed to improve fixation. Specifically, a 90 degrees offset periarticular system lowers the profile on the lateral epicondyle, yet it is unclear how this design compares to other plate constructs. This study compared the mechanical stiffness and plate surface strains between two types of constructs for stabilization of complex distal humerus fractures. METHODS: Identical bi-columnar segmental intra-articular fractures were created in ten epoxy composite left humeri. Models were randomly assigned to two groups (n=5/group) with either parallel plates or perpendicular plates. Rosette strain gages were placed at the most distal possible space on the lateral plate for both constructs. Models were mechanically tested with estimates of physiologic loads in flexion, extension, varus, valgus axial compression and axial torsion. Data for mechanical stiffness, transverse plate strain and longitudinal plate strain were compared with a one-way ANOVA (P<0.05). FINDINGS: There was no statistical difference in stiffness in any direction. The longitudinal strain for the 90 degrees construct was significantly lower in axial compression. The 180 degrees system demonstrated significantly lower transverse strains during axial torsion. INTERPRETATION: Both systems demonstrated similar mechanical stiffness theoretically providing similar fracture stabilization. Plate strain differences may affect fragment position, but it is unclear how much plate loading occurs in vivo. Surgeon experience and preference may dictate the choice of a plate construct for this fracture configuration.