Biomechanical analysis of supracondylar humerus fracture pinning for slightly malreduced fractures.

BACKGROUND: The most common position of malreduced type III supracondylar humerus (SCH) fractures is internal rotation and medial collapse of the distal fragment. The purpose of this study was to determine the effect of SCH fracture rotational deformity on stability with various pin configurations. Specifically, is the biomechanical stability lost when an SCH fracture is pinned in slight malreduction (compared with anatomical pinning) improved by adding a third pin? METHODS: Sixty-four synthetic humeri were sectioned in the mid-olecranon fossa to simulate an SCH fracture. Specimens were randomized to an anatomically reduced group or a group with 20 degrees of distal fragment internal rotation (n = 32 per group). Each was randomized to one of 4 pin configurations: 2 laterally divergent pins, 2 crossed pins, 3 laterally divergent pins, or 2 lateral with 1 medial pin (n = 8 per group). All fractures were stabilized with 1.6 mm (0.062 in) Kirschner wires. Models were tested in extension, varus, and valgus for 10 cycles between 5 N and 50 N. Internal and external rotations were tested between +/-1 Nm. Data for fragment stiffness (newtons per millimeter or newton millimeters per degree) were analyzed with a 2-way analysis of variance (p < 0.05). RESULTS: Internally rotated fractures were significantly less stable than the anatomically reduced group for external rotation, internal rotation, and varus loading regardless of pin configuration. Within the malreduced group, 3-pin configurations were more stable than 2-pin configurations in internal rotation, varus, and extension loading. Two lateral divergent pins were similar to 2 crossed pins, except in extension, where 2 lateral pins had greater stiffness. CONCLUSIONS: Construct stiffness for malreduced specimens after pinning was less than those pinned with an anatomical reduction when loaded in varus, internal rotation, and external rotation. For simulated fractures with residual internal rotation, the addition of a third Kirschner wire compared with an anatomically reduced 2-crossed-pin configuration resulted in increased stiffness of the model for all loading directions. CLINICAL RELEVANCE: Consider a 3-pin pattern, either 3 laterally divergent pins or 2 lateral pins and 1 medial pin, for SCH fractures when a less than complete anatomical reduction is obtained.

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.

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.

Comparison and performance characteristics of 3 different knots when tied with 2 suture materials used for shoulder arthroscopy.

PURPOSE: To compare the performance of a standard suture material with that of a new material across several arthroscopic knot configurations. METHODS: Three knots were evaluated (Duncan loop, Weston, and San Diego knots) with the use of 2 suture materials (No. 2 Ethibond [Ethicon, Somerville, NJ] and No. 2 Force Fiber [Stryker Endoscopy, San Jose, CA]). Ten samples were tested for each knot and suture configuration. Samples were pretensioned to 10 N and were mechanically loaded from 10 to 45 N for 1,000 cycles. The number of cycles to 3 mm of loop elongation was recorded as "early" slippage. Intact knots with no evidence of early slippage were then subjected to a load-to-failure test so that the ultimate failure strength of the knot/material could be determined. RESULTS: Force Fiber knots required a statistically greater number of cycles to reach 3 mm of slippage compared with Ethibond knots (P < .0001). A single Force Fiber knot experienced 3 mm of displacement during cyclic loading; all others survived to 1,000 cycles. One third of all Ethibond knots were displaced to 3 mm during cyclic loading. For Ethibond sutures, no significant differences in ultimate failure strength were observed between knots (average, 143 N across knot types). For Force Fiber sutures, the San Diego knot (279 +/- 41 N) was statistically similar in ultimate failure strength to the Weston knot (254 +/- 41 N), but it was significantly stronger than the Duncan knot (224 +/- 70 N) (P < .03). CONCLUSIONS: The higher failure loads associated with Force Fiber may allow the surgeon to increase suture tension during knot tying, thereby creating a tighter knot. Loop elongation with Force Fiber occurred at loads that were greater than the typical breakage load for No. 2 Ethibond. Force Fiber is a new material that may be useful for various arthroscopic procedures. CLINICAL RELEVANCE: Force Fiber provided increased biomechanical stability compared with Ethibond regardless of knot type. The cost benefit of using very strong yet very stiff sutures must be clinically evaluated.

A titanium expandable pedicle screw improves initial pullout strength as compared with standard pedicle screws.

BACKGROUND CONTEXT: Pedicle screws are now standard for spinal arthrodesis as they provide three-column spinal stabilization. Decreased vertebral body bone density because of aging reduces the stability of the bone-screw interface, potentially increasing screw pullout or pseudarthrosis. Modifications to standard pedicle screw designs to improve screw stabilization may help to compensate for the detrimental effects of decreased vertebral bone density. PURPOSE: To evaluate differences in initial pullout strength of an expandable titanium pedicle screw as compared with a standard titanium pedicle screw. STUDY DESIGN: In vitro human cadaveric biomechanical investigation. METHODS: Fresh thoracolumbar spines from four human cadavers were imaged using quantitative computed tomography to obtain standard lumbar osteoporosis (Dual-energy X-ray absorptiometry [DXA]) T scores. Six bodies were sectioned per spine, and standard titanium 6.5-mm diameter pedicle screws and expandable 6.5-mm diameter titanium screws (maximum expanded diameter=10 mm) were randomized to right and left sides. Screw testing, in axial pullout at 25 mm/min, was randomized to reduce the effects of testing order. Data for stiffness (N/mm), yield load (N), ultimate load (N), and energy (N mm) (area under the load-displacement curve) were analyzed using a one-way analysis of variance (p<.05). RESULTS: Lumbar DXA scores averaged -3.6. There were no statistical differences between screw types for stiffness. Yield load was not statistically different between groups, although the expandable screw yield load was nearly 25% greater than that of the standard screw. Ultimate load was found to be statistically greater (∼30%) for the expandable screw compared with the standard screw (p<.05). The energy required to cause bone-implant failure was also statistically greater for the expandable screw compared with the standard screw (p<.0001). CONCLUSIONS: Expandable titanium pedicle screws demonstrated improved screw pullout stability compared with standard titanium screws in osteopenic or osteoporotic bone. Further studies are warranted examining other loading methods to evaluate the stability provided by an expandable pedicle screw.

Biomechanical comparison of kyphoplasty versus a titanium mesh implant with cement for stabilization of vertebral compression fractures.

STUDY DESIGN: In vitro biomechanical investigation. OBJECTIVE: To evaluate differences in biomechanical stability of vertebral compression fractures repaired using balloon kyphoplasty versus a titanium mesh implant. SUMMARY OF BACKGROUND DATA: Vertebral compression fractures may be stabilized using an expandable balloon followed by cement injection. There are small but finite risks of endplate fracture and cement extravasation with this procedure. Alternative techniques may affect cement injection volumes, height maintenance, and biomechanical stability but require investigation. METHODS: Four male human cadaveric spines from T2 to L5 were used in this study. After determining bone mineral density, individual vertebral bodies were dissected and inspected for previous fractures or additional exclusion criteria. In the remaining vertebral bodies (n=48) anterior wedge fractures were created using a materials testing machine. Fractured vertebral bodies were systematically randomized to be repaired either with balloon kyphoplasty or with titanium mesh implant and polymethylmethacrylate bone cement, using image intensified fluoroscopy. Anterior vertebral body height (cm) was measured initially, after mechanically creating an anterior wedge fracture, after repairing the compression fracture with either technique, and after recompressing the vertebral body following a 24-hour cement polymerization period. Data for cement injection volume (mL) and height maintained following testing (cm) were compared between repair groups using a 1-way analysis of variance (P<0.05). Data for stiffness (N/mm), yield load (N), and ultimate load (N) were compared between intact bodies and repaired bodies using a 2-way analysis of variance (P<0.05). RESULTS: There was significantly less cement injected (P<0.001) and significantly greater height maintained (P<0.025) with the titanium implant group compared to the kyphoplasty group. There were no significant differences in biomechanical stability between the 2 groups (P>0.05). CONCLUSION: The titanium implant was biomechanically equivalent to the kyphoplasty repair while necessitating less cement and providing greater height maintenance in vitro. Improvements in pain and function could not be specifically addressed in this in vitro study and should be evaluated in a clinical case series.

Pedicle screw surface coatings improve fixation in nonfusion spinal constructs.

STUDY DESIGN: Biomechanical and histologic analysis. OBJECTIVE: To compare the strength of the bone-screw interface of standard uncoated pedicle screws with screws treated with hydroxyapatite (HA), titanium plasma spray (TPS), and a composite HA-TPS coating. SUMMARY OF BACKGROUND DATA: Transpedicular screw fixation has become the gold standard in the treatment of various thoracolumbar spinal conditions. Pedicle screw loosening, however, has been reported, especially in mechanically demanding constructs or in vertebrae with low bone mineral density. METHODS: Six mature porcine were instrumented with 4 types of titanium monoaxial pedicle screws (uncoated, HA-only coated, TPS-only coated, and HA-TPS composite coated) in a systematically varied, single-blinded fashion. After a 3-month survival period, the spines were harvested en-bloc and "time zero" control screws were instrumented in adjacent vertebrae. Screw placement and bone mineral density were evaluated with a postharvest computed tomography, and the strength of the tissue-implant interface was evaluated with a torsional screw extraction analysis (60 screws) and a nondecalcified histologic analysis (16 screws). RESULTS: At 3 months postoperative, peak torque increased for all 3 types of coated screws (increased fixation) and decreased significantly for the uncoated screws (P < 0.001). Although 3-month peak torque was not statistically different between the 3 screw coatings, 4 of 10 TPS-only coated screws had a peak torque that was nearly 0 (<0.1 N m) versus only 1 of 10 HA-only screws and 0 of 10 HA-TPS composite screws. Histologic analysis confirmed the biomechanical findings with improved osseointegration in the HA-only and HA-TPS composite screws. CONCLUSION: Pedicle screw coatings that promote mechanical interlocking, TPS, or direct osteoblast bonding(HA) increased screw fixation in this nonfusion model. More non-HA coated screws, however, were thought to be "loose" with a nearly zero peak extraction torque and fibrous encapsulation. Increased osseointegration with HA may result in a decreased incidence of screw loosening and improved outcomes of transpedicular spinal instrumentation in nonfusion procedures.

Displaced olecranon fractures in children: a biomechanical analysis of fixation methods.

BACKGROUND: Wire and suture methods have been used to stabilize pediatric olecranon fractures. This study (1) compared differences in simulated intraoperative compression during fracture reduction, (2) evaluated articular surface compression during cyclic loading of the tension band, and (3) compared fracture stabilization after cyclic physiologic loading at low/high levels. METHODS: Identical olecranon fractures were created in 10 synthetic ulnae and randomized to suture or wire fixation. Compression after fixation and compression during cyclic loading between 10 and 50 N was measured with a load cell at the articular surface and compared with a 1-way analysis of variance (p < 0.05). Twenty-four fractured synthetic ulna were randomly assigned to wire or suture tension band constructs and low- or high-loading groups. The low-load group (12 ulnae) cycled loading from 3 to 10 N for 100 cycles followed by a failure test. The high-load group (12 ulnae) experienced 10 to 100 N before failure testing. Fracture separation (mm) and failure load (N) were compared using a 2-way analysis of variance (p < 0.05). Ten synthetic ulnae were randomized to wire/suture groups and cyclically loaded between 10 to 50 N while measuring loads across the fracture using a load cell. Correlation data were statistically compared with a Fisher transformation and z test (p < 0.05). RESULTS: Residual compression was statistically greater for wire compared with suture. There was no difference in fracture displacement between groups during low loads. Suture had significantly greater displacement compared with wire at high loads. Failure loads were significantly greater for wire at both load settings. Wires transmitted forces across the joint surface more readily than sutures. CONCLUSIONS: Suture tension bands had lower ultimate failure loads and less compression at the fracture site. However, if low loads are expected or if the fracture is reduced easily, the suture tension band may be an appropriate alternative to wire fixation. CLINICAL RELEVANCE: Perhaps, in small children or when using casts in bigger children, a bioabsorbable suture may be used for fracture stabilization avoiding the need for extensive surgery to remove the fixation material.

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.

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.

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.

Screw head impingement after in situ fixation in moderate and severe slipped capital femoral epiphysis.

In situ stabilization remains the standard of care in the treatment of stable slipped capital femoral epiphysis (SCFE). Screw placement perpendicular to the physis has shown satisfactory results with minimal complications. A prominent screw head may produce femoral acetabular impingement and pain after in situ fixation in severe SCFE. We performed a biomechanical study to establish whether screw head impingement occurs after in situ fixation of SCFE and to define the anatomy of slip severity and screw head position that may lead to impingement. A femoral neck dome osteotomy was created in a human cadaveric model simulating 2 conditions: a moderate and severe SCFEs. We tested the specimens after in situ fixation perpendicular to the simulated physis. The simulated SCFEs and normal control were tested through a full arc of motion. Coverage of the femoral head by the labrum was evaluated at 90 degrees of flexion using fluoroscopy. Impingement occurred at 70 degrees of hip flexion in the simulated moderate SCFE, and at 50 degrees of flexion in the severe simulated SCFE. Anteroposterior fluoroscopy revealed that screw heads lateral to the intertrochanteric line were unlikely to impinge on the acetabulum. Screw head impingement occurred with in situ fixation perpendicular to the physis in simulated moderate and severe SCFEs. Anteroposterior radiographs appear helpful in identifying a hip at risk for screw head impingement after in situ fixation. Alternative in situ fixation techniques (screw head resting lateral to the intertrochanteric line on the anteroposterior radiograph) may decrease the rate of screw head impingement in moderate and severe SCFEs.

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 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.

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.

Increasing the insertion depth of suture anchors for rotator cuff repair does not improve biomechanical stability.

The purpose of this study was to evaluate whether deeper-than-recommended insertion of a suture anchor within the rotator cuff footprint of human cadaveric humeri affects fixation characteristics. Metallic 5-mm screw-in anchors loaded with a single No. 2 suture were placed in the infraspinatus footprint of 8 human cadaveric humeri at standard and deep depths. Specimens were cyclically loaded from 10 to 45 N for 500 cycles and then loaded to failure. Cylic displacement, failure load, and failure mode were compared. All deep anchors became flush within a few cycles, and both anchor depths displaced and rotated at the bone surface. Displacement of the deep anchors was significantly greater than that of standard anchors. There was no difference in failure load. Cyclic testing showed significant displacement, regardless of anchor position, possibly leading to gap formation of the repair. Deep placement of suture anchors for increased purchase caused greater displacement and is not recommended.

Strut allograft union and remodeling using rhBMP-2 in a spinal corpectomy model.

STUDY DESIGN: Growth factor in a collagen sponge carrier was compared to autograft, both packed within an allograft strut following corpectomy in a bovine spinal model. OBJECTIVE: To evaluate incorporation of cortical strut allografts after lumbar corpectomy, comparing augmentation with recombinant human bone morphogenetic protein-2 (rhBMP-2) to local autograft. SUMMARY OF BACKGROUND DATA: Autogenous strut grafts are the gold standard for successful fusion in reconstruction following corpectomy; however, significant donor site morbidity can occur. Recent studies describing consistently successful anterior interbody fusions with BMP augmentation suggest an exciting prospect of unlimited and potent grafting material for these difficult fusions. METHODS: Sixteen calves underwent L3 corpectomy with instrumented strut allograft reconstruction. The rhBMP-2 impregnated collagen sponges filled the empty medullary canal of the allograft in 8 animals. Eight animals had the allograft strut filled with local autogenous bone. After 4 months, the lumbar spines were harvested for radiographic, biomechanical, and histologic evaluation. RESULTS: Computerized tomography revealed allograft fusion in 7 of 8 autograft specimens and 8 of 8 BMP specimens. The BMP treated group had denser bone at the ends of the cortical allograft, but a central void persisted. Autograft filled struts maintained a more uniform distribution of less organized bone throughout the strut canal. Histologic assessment verified remodeling and incorporation of the allografts for both groups. Biomechanical testing confirmed no significant difference in fusion strength between groups. CONCLUSIONS: Large cortical strut allografts (after lumbar corpectomy) supplemented with rhBMP-2 had incorporation and fusion strength comparable to allografts enhanced with cancellous autograft.

Thoracic vertebral screw impingement on the aorta in an in vivo bovine model.

STUDY DESIGN: A bovine model was used to evaluate the effects of thoracic vertebral screw impingement of the aorta. OBJECTIVES: To evaluate the histologic and biomechanical changes in aortic wall tissue that was severely impinged by abutting instrumentation. SUMMARY OF BACKGROUND DATA: Case reports of vascular injury associated with spinal instrumentation generally describe intraoperative injury; some report delayed presentation of large vessel damage. Risks associated with placing instrumentation adjacent to large vessels are largely unknown. METHODS: Six 1-month-old calves underwent left-sided thoracotomies, exposing the anterior thoracic spine and aorta. With the heads removed, screws were inserted in reverse fashion into T6 through T11, leaving the screw tips 1 cm proud and abutting the aorta. After 3, 6, or 12 months (2 calves each), the spines were resected with the adjacent aorta and underwent radiographic, histologic, and biomechanical testing. RESULTS: Computed tomography revealed varying degrees of vessel impingement. Although there were no frank ruptures, 96% of aortic specimens showed histopathologic changes, including 52% with wall thinning; 43% were no longer impinged, yet 60% of these had increased collagen (scar). Biomechanical testing of screw-impinged aortas demonstrated a lower failure stress (1.2 +/- 0.5 N/mm vs. 1.8 +/- 0.4 N/mm, P = 0.016) but no difference in failure strain (42 +/- 9% vs. 32 +/- 10%, P = 0.06) than controls. CONCLUSIONS: Major impingement of vertebral screws on the aorta caused acute and chronic histopathologic and biomechanical changes in the vessel wall. This model represents a severe form of vessel penetration by a screw that confirms such a "worst case" scenario results in marked compromise of the vessel wall integrity. The sequelae of less severe impingement are unknown.

Biomechanical analysis of anterior scoliosis instrumentation: differences between single and dual rod systems with and without interbody structural support.

STUDY DESIGN: Nondestructive biomechanical testing was performed on bovine lumbar spines instrumented with multilevel scoliosis type anterior spine constructs. OBJECTIVE: To determine the biomechanical effects from the number of anterior rods (1 vs 2) and the effects of interbody structural support on construct stiffness after anterior multisegmental instrumentation. SUMMARY OF BACKGROUND DATA: Corrective surgery using anterior instrumentation for thoracolumbar and lumbar scoliosis has been performed with single rod and, more recently, with dual rod constructs. The biomechanical effect of one- or two-rod anterior instrumentation systems on construct stiffness and the addition or absence of interbody structural support have not been defined adequately in the literature. METHODS: Eight bovine lumbar spines each underwent instrumentation using four different constructs: one rod without interbody support; one rod with titanium mesh interbody support at the L2-L3, L3-L4, and L4-L5 disc spaces; two rods alone; and two rods with interbody support. Nondestructive cyclic testing in flexion-extension (+/-5 Nm), lateral bending (+/-5 Nm), and torsion (+/-2 Nm) were performed. The construct stiffness (Nm/ degrees ) of the four implant configurations was compared. RESULTS: With the addition of a second rod, the construct was significantly stiffer than a single rod construct in flexion (P = 0.006), extension (P = 0.02), and torsion (P = 0.01), but not in lateral bending. The addition of interbody structural support to the rod systems resulted in significantly stiffer constructs than those without cages in flexion (P = 0.03), but not in the other loading conditions (extension, lateral bending, torsion). CONCLUSIONS: Dual rod constructs were stiffer in torsion and flexion-extension loading than single rod constructs. Neither the number of rods nor the use of structural mesh interbody support had any effect on lateral bending stiffness. However, in a single rod system, the addition of interbody support increased stiffness in flexion. The use of structural support in dual rod constructs may be helpful in "setting" the desired lordosis, but adds little to construct stiffness.

Biomechanical comparison of stainless steel and titanium nails for fixation of simulated femoral fractures.

Flexible intramedullary nails are commonly used to treat femoral fractures in children. This study evaluated the biomechanical differences between stainless steel and titanium nails when securing transverse and comminuted fractures in a synthetic femur model. Retrograde flexible stainless steel and titanium nails placed in a divergent "C" pattern were mechanically tested, and axial rotation and compression stiffness were analyzed with a two-way ANOVA. Rotational stability was significantly greater for titanium nails than stainless steel nails for both fracture patterns. Axial compression stiffness was significantly greater for titanium nails than stainless steel nails for both fracture patterns. There was no statistical difference between materials for axial "failure" load that produced 5 mm of shortening. Titanium intramedullary nails were more stable than stainless steel nails in torsion and axial compression. Both materials stabilized simulated fractures at levels beyond physiologic non-weight-bearing loads without permanent deformation.