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.

Spinal growth modulation with an anterolateral flexible tether in an immature bovine model: disc health and motion preservation.

STUDY DESIGN: An immature bovine model was used to evaluate multilevel anterolateral flexible tethering in a growing spine. OBJECTIVE: To evaluate radiographic, biochemical, histologic, and biomechanical results of tethered spinal growth. SUMMARY OF BACKGROUND DATA: An anterolateral flexible tether has been shown to create a kyphotic and scoliotic spinal deformity in calves. Subsequent disc health and spinal motion has not been analyzed. METHODS: Four consecutive thoracic vertebral bodies (T6-T9) were instrumented anteriorly in 36 1-month-old calves. Seventeen animals (Tether Group) were instrumented with a vertebral staple-two screw construct connected by 2 flexible stainless steel cables. Nineteen animals (Control Group) were instrumented with 1 vertebral body screw with no connecting cable. After a 6-month survival period, the spines were harvest en-bloc and underwent radiographic, computed tomography, biochemical, histologic, and biomechanical analysis. RESULTS: On average, 37.6 degrees +/- 10.6 degrees of coronal and 18.0 degrees +/- 9.9 degrees of sagittal deformity was created in the Tether Group, with significant vertebral wedging toward the tether (P < 0.001). Disc thickness decreased significantly in the Tether Group (P < 0.001), however, disc wedging was not observed. There was no change in gross morphologic disc health or disc water content (P = 0.73). However, proteoglycan synthesis was significantly greater in the tethered discs compared with controls (P < 0.001), and collagen type distribution was different with a trend toward increased type II collagen present on the tethered side of the disc (P = 0.09). Tethers significantly increased spinal stiffness in lateral bending and in flexion/extension (P < 0.05) without affecting torsional stiffness, however, after tether removal range of motion returned to control values. CONCLUSION: Tethering resulted in vertebral wedging while maintaining spinal flexibility. Although changes in proteoglycan synthesis, collagen type distribution, and disc thickness were observed, the tethered discs had similar water content to control discs and did not demonstrate gross morphologic signs of degeneration. Growth modulation is an attractive treatment option for growing patients with scoliosis, avoiding multilevel fusions or brace wear. Strategies for fusionless scoliosis correction should preserve disc health, as adolescent patients will rely on these discs for decades after treatment.

A biomechanical analysis of a novel arthroscopic suture method compared to standard suture knots and materials for rotator cuff repair.

PURPOSE: To biomechanically compare a novel suture welding method to 2 standard suture materials when tied with 3 types of arthroscopic knots. METHODS: Sixty suture loops were tied with Weston, Roeder, and Duncan knots for both No. 2 FiberWire (FW) and No. 2 Ethibond (EB; 10 loops/group). Ten suture loops were created using a novel technique using No. 2 Nylon suture, with welding of the suture ends using sonic energy. All loops were created over the same dowel. Loops were cycled between 10N and 45N for 200 cycles, followed by a failure test. Loop elongation and ultimate failure load were compared between welded sutures and both materials using a 2-way analysis of variance. RESULTS: There were no differences in loop elongation for the welded Nylon sutures compared to EB Roeder and EB Duncan knots, although EB Weston knots had significantly less loop elongation (P < .05). Loop elongation was significantly lower with FW than with EB (P < .001). For maximum failure load, there were no differences between the various EB loops. Welded loops demonstrated similar maximum failure loads to EB Roeder and EB Duncan knots, but were lower than EB Weston knots. Overall, the FW knots were significantly stronger than the EB groups (P < .001 for all comparisons). CONCLUSIONS: A novel method to simplify the knot tying procedure by welding the suture ends demonstrated mechanical properties generally similar to No. 2 Ethibond. This method provides: (1) surgical advantages by eliminating knot tying; (2) a quick and easy locking mechanism; and (3) a low profile that eliminates high-profile knots. However, it is not clear whether the lower stiffness or the lower failure load may detract from these potential benefits compared to constructs with greater stiffness. CLINICAL RELEVANCE: The suture welding technique generally behaved in a manner similar to that of Ethibond suture loops, although the difficulty of tying arthroscopic knots was eliminated.

Short-segment fixation of lumbar burst fractures using pedicle fixation at the level of the fracture.

STUDY DESIGN: Cadaveric biomechanical study and retrospective chart review. OBJECTIVE: Biomechanical comparison of segmental versus nonsegmental fixation of lumbar burst fractures and clinical analysis of short-term radiographic outcomes. SUMMARY OF BACKGROUND DATA: Traditional short nonsegmental posterior fixation of thoracolumbar burst fractures suffers from high rates of failure. Construct stability may be improved by inserting additional screws at the fracture level. METHODS: Six intact human spines (L1-L3) were biomechanically tested in flexion-extension, lateral bending, and axial torsion. The inferior half of the L2 vertebral bodies and L2-L3 discs were resected to mimic an unstable L2 burst fracture with loss of anterior column support. Pedicle screws were inserted in L1 and L3 for the control group (nonsegmental fixation). Screws were inserted at all levels for the experimental group (segmental fixation). Construct stiffness and L1-L2 disc pressure were analyzed. Twelve patients with thoracolumbar burst fractures treated with this type of segmental fixation were reviewed. RESULTS: Construct stiffness during axial torsion was significantly higher for segmental constructs compared with nonsegmental constructs (P < 0.02) with no differences between flexion-extension and lateral bending. Disc pressure fluctuations during flexion-extension were significantly higher for segmental compared with nonsegmental constructs (P < 0.02), with no differences in lateral bending and torsion. Mean preoperative kyphotic deformity was 9 degrees and improved by 15 degrees after surgery. Follow-up films on 9 patients showed 5 degrees of kyphosis correction loss. Mean anterior vertebral body height was 58% of normal before surgery. After surgery height was 89% of normal and at final follow-up, 78%. CONCLUSIONS: Segmental fixation of burst fractures with screws at the level of the fracture offers improved biomechanical stability. Theoretically, segmental fixation provides for additional fixation points that may aid in fracture reduction and kyphosis correction. This specific parameter is not evaluated in this study but will be an important outcome measure for a planned randomized controlled trial.

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.

Effects of a novel sterilization process on soft tissue mechanical properties for anterior cruciate ligament allografts.

BACKGROUND: Allograft anterior cruciate ligament reconstruction provides benefits such as earlier return to activities and less pain, but concerns remain regarding potential infection and biomechanical stability. HYPOTHESIS: There is no difference in biomechanical properties of soft tissue allografts treated with the Biocleanse tissue sterilization process compared with irradiated and fresh-frozen allografts. STUDY DESIGN: Controlled laboratory study. METHODS: Thirty-six tibialis anterior allografts were equally divided between Biocleanse, irradiated, and fresh-frozen groups. Grafts were measured for cross-sectional area and looped over a smooth rod with the free sutured ends of the graft fixed in custom clamps. Specimens were tensioned to 10 N for 2 minutes and then loaded between 50 and 300 N for 1000 cycles followed by a failure test. Data for creep (mm); stiffness (N/mm) at cycles 1, 10, 100, and 1000; failure load (N); and failure stress (MPa) were compared with a one-way analysis of variance (P < .05). RESULTS: There were no statistically significant differences in creep between groups. Sterilized groups (irradiated = 144.7 +/- 17.7 N/mm and Biocleanse = 146.5 +/- 28.2N/mm) were significantly stiffer during the first cycle than the fresh-frozen group (117.8 +/- 15.7 N/mm, P < .005) without statistically significant differences for subsequent cycles. There were no differences between groups for either failure load (fresh-frozen = 1665 +/- 291.3 N, irradiated = 1671.9 +/- 290.2 N, Biocleanse = 1651.6 +/- 377.4 N) or failure stress. CONCLUSION: Data for "time-zero" physiologic stiffness and failure loads indicate that the Biocleanse process does not adversely affect the biomechanical properties of the allograft material. CLINICAL RELEVANCE: This novel sterilization technique may provide surgeons with potential allograft material with similar biomechanical properties to native tissue.

Titanium versus stainless steel for anterior spinal fusions: an analysis of rod stress as a predictor of rod breakage during physiologic loading in a bovine model.

STUDY DESIGN: In vitro biomechanical evaluation of rod stress during physiologic loading of anterior scoliosis instrumentation. OBJECTIVES: To determine effects of material properties and rod diameter on rod stresses in anterior scoliosis instrumentation. SUMMARY OF BACKGROUND DATA: Relationships between instrumentation dimensions, materials, and potential rod failure in anterior scoliosis instrumentation remain unclear. METHODS: Eighteen immature bovine spines were randomized to 3 groups: 1) 4.0-mm stainless steel, 2) 5.0-mm stainless steel, and 3) 4.75-mm titanium alloy. Spines underwent physiologic tests in flexion-extension, lateral bending, and torsion. Rod surface strains were converted to rod stress and normalized to each material's yield stress. Construct stiffness and the normalized rod stresses were compared with a one-way ANOVA (P < 0.05). RESULTS: The 4.0-mm steel and 4.75-mm titanium construct stiffness was similar across all tests. The 5.0-mm steel system was significantly stiffer than 4.0-mm steel (lateral bending/torsion) and 4.75-mm titanium (torsion/flexion) constructs. Rod surface stress was significantly lower for the 4.75-mm titanium rod compared with 4.0-mm and 5.0-mm steel rods for all tests. CONCLUSIONS: The percentage of yield stress was lowest for the 4.75-mm Ti rod for all tests due to titanium's greater yield stress. This suggests the 4.75-mm rod has a lower fatigue failure risk than either steel construct.

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.

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.