Effects of posterior distraction forces on anterior column intradiscal pressure in the dual growing rod technique.

BACKGROUND: Little evidence is available addressing biomechanical properties of posterior distraction forces and their effects on anterior spinal column in the growing rod technique. The question is often asked if posterior distraction forces may be kyphogenic. The goal of this study is to determine whether posterior distraction forces transmitted anteriorly through different foundation constructs (i.e., screws vs. hooks) affect intradiscal pressure. METHODS: Six skeletally immature porcine spines were harvested leaving soft tissues and rib heads intact. Pedicle screws served as the lower foundation on a L3-L4 motion segment while pedicle screws and laminar hooks were randomly used at T3-T4 levels. Proximal constructs (hook vs. screw) were switched after initial distraction testing. The dual rod distractor was instrumented with strain gauges and calibrated using a custom force transducer. During distraction, intradiscal pressures immediately inferior to the superior foundation and the level equidistant between foundations were measured using needle pressure transducers. Maximum distraction force and maximum anterior disc pressure change were compared between hook and pedicle screw anchors using one-way ANOVA (p < 0.05). RESULTS: Upper foundations with pedicle screws had significantly greater distraction forces (416 ± 101 N) than those with upper level hooks (349 ± 100 N). There were no significant differences in disc pressures between levels or between upper foundation constructs. Disc pressures adjacent to the upper foundation demonstrated greater reduction (disc expansion) than the level equidistant within the construct. Pedicle screw constructs demonstrated greater endplate separation (distraction) compared to hook constructs. CONCLUSIONS: Posterior distraction forces result in anterior disc separation (distraction) and are distributed across multiple levels rather than delivered to the disc immediately adjacent to a foundation. Constructs with upper foundation hooks had lower distraction forces possibly due to hook motion during distraction. The load distribution at multiple levels may assist with curve control and may affect vertebral growth. The distraction forces may not be kyphogenic as is commonly believed.

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 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.

Biomechanical evaluation of transfacet screw fixation for stabilization of multilevel cervical corpectomies.

STUDY DESIGN: Cadaveric biomechanical investigation. OBJECTIVES: To test the feasibility of transfacet screws as a minimally invasive posterior fixation device for the cervical spine by comparing the biomechanical stability of transfacet screws to lateral mass screws and rods in a multilevel cervical corpectomy model. SUMMARY OF BACKGROUND DATA: Minimally invasive surgery (MIS) of the spine has gained increasing acceptance and popularity. However, a minimally invasive means of instrumenting the posterior cervical spine has yet to be discovered. Posterior transfacet screws have been described as a means of posterior fixation. In addition, they have the potential of being placed percutaneously through stab incisions. However, validation of transfacet screws in an unstable cervical model in which posterior instrumentation may be necessary has not been carried out till date. METHODS: Sixteen cadaveric cervical spines were randomized to transfacet or lateral mass instrumentation groups. The spines were tested in the following conditions: (a) intact, (b) after multilevel corpectomies with strut graft placement with stand-alone posterior fixation, and (c) with an additional anterior plate over the strut graft. Corpectomy site loading was measured with a custom-designed strut graft. Data were collected for spinal stiffness, range of motion, and strut graft loading, and was analyzed using 2-way analysis of variance (P<0.05). RESULTS: Stand-alone transfacet screw fixation was found to provide inferior spinal stability and resulted in increased spinal motion and graft loading compared with the other constructs (P<0.05 for all). CONCLUSIONS: It is unclear what kind of mechanical stiffness is necessary to stabilize the cervical spine and obtain solid fusion. However, decreased stability and increased graft loading suggest that transfacet screws may not be the ideal method of posterior fixation to supplement multilevel anterior cervical corpectomies and fusions despite their potential as a minimally invasive method for posterior cervical instrumentation.

Comparison of mechanical stability in double-row rotator cuff repairs between a knotless transtendon construct versus the addition of medial knots.

PURPOSE: Our purpose was to investigate the importance of medial-row knot tying to mechanical stability in a double-row rotator cuff repair by comparing a knotless construct with transtendon anchor passage versus a similar construct implementing medial knots. METHODS: A standard defect was created in the infraspinatus tendons of 14 bovine humeri. All defects were repaired with 2 medial and 2 lateral anchors (SutureCross System; KFx Medical, Carlsbad, CA). The medial anchors were either placed by transtendon passage in a knotless construct or placed directly into bone with needle passage of suture to create bursal-sided knots medially. Constructs were subjected to a cyclic loading protocol and then loaded to failure. RESULTS: The medially knotted constructs had a statistically higher stiffness at both the initial and final cycles (P < .001 and P < .001, respectively) and a lower displacement during cyclic loading (P < .02). There were strong trends toward decreased gauge displacement (P = .12) and decreased cycles to 3 mm of displacement (P = .07) in the medially knotted group. Maximal yield strength was greater in the medially knotted group (350 +/- 270 N v 650 +/- 530 N), although this was not found to be statistically significant (P = .5). CONCLUSIONS: Our data suggest that creation of medial knots increases construct stiffness and stability in arthroscopic double-row cuff repair. This is likely because of increased load transfer to the lateral anchor and suture-tendon interface in the knotless construct. CLINICAL RELEVANCE: Medial knots create increased mechanical stability that theoretically may improve rotator cuff healing. This mechanical advantage must be weighed against surgical efficiency, with consideration given to factors such as tissue quality.

An evaluation of fracture stabilization comparing kyphoplasty and titanium mesh repair techniques for vertebral compression fractures: is bone cement necessary?

STUDY DESIGN: In vitro biomechanical investigation using human cadaveric vertebral bodies. OBJECTIVE: To evaluate differences in biomechanical stability of vertebral compression fractures (VCFs) repaired using an expandable titanium mesh implant, with and without cement, as compared with standard balloon kyphoplasty. SUMMARY OF BACKGROUND DATA: Vertebral augmentation, either in the form of vertebroplasty or kyphoplasty, is the treatment of choice for some VCFs. Polymethylmethacrylate, a common bone cement used in this procedure, has been shown to possibly cause injury to neural and vascular structures due to extravasation, embolization, and may be too rigid for an osteoporotic spine. Therefore, suitable alternatives for the treatment of VCFs have been sought. METHODS: Individual vertebral bodies from 5 human cadaveric spines (from T4 to L5) were stripped of all soft tissues, and compressed at 25% of the intact height using methods previously described. Vertebral bodies were then randomly assigned to the following repair techniques: (1) conventional kyphoplasty, (2) titanium implant with cement, (3) titanium implant without cement. All vertebral bodies were then recompressed at 25% of the repaired height. Yield load, ultimate load, and stiffness were recorded and compared in these groups before and after treatment. RESULTS: There were no differences in biomechanical data between intact groups, and between repaired groups. In all 3 treatment groups, yield load and ultimate load of repaired vertebrae were similar to that of intact vertebrae. However, the stiffness following repair was found to be statistically less than the stiffness of the intact vertebral body (P < 0.05 for all comparisons). CONCLUSION: Based on the biomechanical data, the titanium mesh implant with or without cement was similar to polymethylmethacrylate fixation by kyphoplasty in the treatment of VCFs. Avoiding the adverse effects caused by using cement may be the main advantage of the titanium mesh implant and warrants further study.

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.

Reconstruction plates for stabilization of mid-shaft clavicle fractures: differences between nonlocked and locked plates in two different positions.

Reconstruction plates permit contouring to the irregular anatomic shape of the clavicle. This study evaluated the biomechanical stability of locking and nonlocking clavicle reconstruction plates for treating midshaft, transverse fractures, comparing anterior-inferior to superior plate position. Twenty-four synthetic clavicles with mid-shaft fractures were repaired with either a locking or nonlocking clavicle reconstruction plate in either the anterior-inferior or superior plate position (n = 6/group). Repaired constructs were tested in axial compression, axial torsion, and cantilever bending failure. In compression, anterior-inferior plates were significantly stiffer than superior plates and locked plates stiffer than nonlocked. In torsion, anterior-inferior plates were stiffer, with a significant interaction term that favored anterior-inferior locked and superior nonlocked plates. In cantilever bending, superior plates had a significantly higher bending failure load and stiffness. Anterior-inferior plates failed at a significantly lower load ( approximately 40 N or approximately 4 kg), which could potentially occur in the postoperative period.

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.

Unilateral and bilateral sacropelvic fixation result in similar construct biomechanics.

STUDY DESIGN: In vitro biomechanical investigation of lumbosacropelvic spinal instrumentation. OBJECTIVE: Determine whether unilateral iliac fixation, with or without an L6 to S1 interbody graft, provides equivalent biomechanical stability compared with bilateral iliac fixation. SUMMARY OF BACKGROUND DATA: Recent clinical evidence has shown improved clinical outcomes of unilateral iliac fixation compared with bilateral instrumentation that contradicts biomechanical data supporting bilateral instrumentation, although no specific investigation has compared unilateral versus bilateral instrumentation. METHODS: Sixteen porcine spines were instrumented with bilateral segmental pedicle screws from L1 to S1 and 5.5-mm titanium rods. Spines were randomized to either have an intact L6 to S1 disc space (n = 8/group) or a full discectomy and intervertebral cage at L6 to S1 (n = 8/group). Four reflective noncolinear markers were attached to both L6 and S1. Spines were tested with bilateral ilium, unilateral ilium, and sacrum-only fixation in flexion, extension, lateral bending, and axial torsion between +/-7.0 Nm. L6 to S1 range of motion (degrees) and mechanical stiffness (Nmm/degrees) were compared between groups with a 2-way analysis of variance (P < 0.05). RESULTS: No significant differences were found in construct stiffness or L6 to S1 motion between unilateral or bilateral fixation for any test direction and both demonstrated significantly less L6 to S1 motion compared with sacrum-only fixation for all tests (all P values <0.005). Bilateral fixation was significantly stiffer than sacrum-only fixation in flexion and extension (P < 0.0001). The interbody cage significantly decreased construct stiffness in extension, lateral bending, and axial torsion (P < 0.002), and significantly increased L6 to S1 motion in torsion compared with an intact disc (P < 0.03). CONCLUSION: There were no biomechanical differences between bilateral and unilateral iliac screw fixation. Intervertebral cage with full discectomy was significantly less stiff than intact. This study provides biomechanical data to correlate with improved clinical outcomes using unilateral iliac screw fixation, and evidence contraindicating full discectomy with intervertebral cage placement.

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.

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.

Residual motion on flexion-extension radiographs after simulated lumbar arthrodesis in human cadavers.

Flexion-extension radiographs are commonly used to assess lumbar fusion. Recommended criteria for solid fusion have varied from 1 to 5 degrees of angular motion between vertebrae. Notwithstanding this wide variation, the validity of these criteria have never been biomechanically tested. As a preliminary and initial step, it was the authors' purpose to quantify measurable angular motion after simulating solid lumbar fusion in human cadaver spines. Seven cadaveric spines (L1 to L4) were tested in a radiolucent jig fixed to a servohydraulic testing apparatus. Flexion and extension moments of 10 Nm were applied. Fusion was simulated using metallic implants spanning the L2-L3 motion segment. These included transverse process plates, a spinous process plate, pedicle screw construct, or an anterior vertebral body plate to simulate an intertransverse, interspinous process, facet, and interbody fusions, respectively. Angular movements were measured on lateral radiographs and statistically compared using a repeated measures analysis of variance. Simulated intertransverse fusion resulted in 13+/-4 degrees of motion; interspinous fusion, 9+/-4 degrees; posterior facet fusion, 5+/-3 degrees; and interbody fusion with plate, 3+/-2 degrees. Compared with the intact, only posterior facet fusion and interbody fusion with plate had statistically significantly less motion (P=0.006 and 0.0001, respectively). The amount of radiographically detectable flexion-extension motion with simulated fusions varies widely and seems to be influenced by fusion type. This study documents a range of measurable motion on flexion-extension radiographs after several types of simulated lumbar fusion. However, as the degrees of motion seemed to be high, future studies should use a fusion simulation other than metallic implants that more closely resembles bony arthrodesis.

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.

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 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.

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.