Biomechanical Analysis of Pedicle Screw Fixation for Thoracolumbar Burst Fractures.

Treatment of unstable thoracolumbar burst fractures remains controversial. Long-segment pedicle screw constructs may be stiffer and impart greater forces on adjacent segments compared with short-segment constructs, which may affect clinical performance and long-term out come. The purpose of this study was to biomechanically evaluate long-segment posterior pedicle screw fixation (LSPF) vs short-segment posterior pedicle screw fixation (SSPF) for unstable burst fractures. Six unembalmed human thoracolumbar spine specimens (T10-L4) were used. Following intact testing, a simulated L1 burst fracture was created and sequentially stabilized using 5.5-mm titanium polyaxial pedicle screws and rods for 4 different constructs: SSPF (1 level above and below), SSPF+L1 (pedicle screw at fractured level), LSPF (2 levels above and below), and LSPF+L1 (pedicle screw at fractured level). Each fixation construct was tested in flexion-extension, lateral bending, and axial rotation; range of motion was also recorded. Two-way repeated-measures analysis of variance was performed to identify differences between treatment groups and functional noninstrumented spine. Short-segment posterior pedicle screw fixation did not achieve stability seen in an intact spine (P<.01), whereas LSPF constructs were significantly stiffer than SSPF constructs and demonstrated more stiffness than an intact spine (P<.01). Pedicle screws at the fracture level did not improve either SSPF or LSPF construct stability (P>.1). Long-segment posterior pedicle screw fixation constructs were not associated with increased adjacent segment motion. Al though the sample size of 6 specimens was small, this study may help guide clinical decisions regarding burst fracture stabilization. [Orthopedics. 2016; 39(3):e514-e518.].

Contribution of the medial malleolus to tibiotalar joint contact characteristics.

BACKGROUND: Isolated medial malleolus fractures are typically treated operatively to minimize the potential for articular incongruity, instability, nonunion, and posttraumatic arthritis. The literature, however, has not clearly demonstrated inferior outcomes with conservative treatment of these injuries. This study measured the effects of medial malleolus fracture and its resultant instability on tibiotalar joint contact characteristics. We hypothesized that restoration of anatomical alignment and stability through fixation would significantly improve contact characteristics. METHODS: A Tekscan pressure sensor was inserted and centered over the talar dome in 8 cadaveric foot and ankle specimens. Each specimen was loaded at 700 N in multiple coronal and sagittal plane orientations. After testing fractured samples, the medial malleolus was anatomically fixed before repeat testing. Contact area and pressure were analyzed using a 2-way repeated-measure ANOVA. RESULTS: In treated fractures, contact areas were higher, and mean contact pressures were lower for all positions. These differences were statistically significant in the majority of orientations and approached statistical significance in pure plantarflexion and pure inversion. Decreases in contact area varied from 15.1% to 42.1%, with the most dramatic reductions in positions of hindfoot eversion. CONCLUSIONS: These data emphasize the importance of the medial malleolus in maintaining normal tibiotalar contact area and pressure. The average decrease in contact area after simulated medial malleolar fractures was 27.8% (>40% in positions of hindfoot eversion). Such differences become clinically relevant in cases of medial malleolar nonunion or malunion. Therefore, we recommend anatomical reduction and fixation of medial malleolus fractures with any displacement. LEVEL OF EVIDENCE: Therapeutic Level V-Cadaveric Study.

Cervical total disc replacement exhibits similar stiffness to intact cervical functional spinal units tested on a dynamic pendulum testing system.

BACKGROUND CONTEXT: The pendulum testing system is capable of applying physiologic compressive loads without constraining the motion of functional spinal units (FSUs). The number of cycles to equilibrium observed under pendulum testing is a measure of the energy absorbed by the FSU. OBJECTIVE: To examine the dynamic bending stiffness and energy absorption of the cervical spine, with and without implanted cervical total disc replacement (TDR) under simulated physiologic motion. STUDY DESIGN: A biomechanical cadaver investigation. METHODS: Nine unembalmed, frozen human cervical FSUs from levels C3-C4 and C5-C6 were tested on the pendulum system with axial compressive loads of 25, 50, and 100 N before and after TDR implantation. Testing in flexion, extension, and lateral bending began by rotating the pendulum to 5°, resulting in unconstrained oscillatory motion. The number of rotations to equilibrium was recorded and the bending stiffness (Newton-meter/°) was calculated and compared for each testing mode. RESULTS: In flexion/extension, with increasing compressive loading from 25 to 100 N, the average number of cycles to equilibrium for the intact FSUs increased from 6.6 to 19.1, compared with 4.1 to 12.7 after TDR implantation (p<.05 for loads of 50 and 100 N). In flexion, with increasing compressive loading from 25 to 100 N, the bending stiffness of the intact FSUs increased from 0.27 to 0.59 Nm/°, compared with 0.21 to 0.57 Nm/° after TDR implantation. No significant differences were found in stiffness between the intact FSU and the TDR in flexion/extension and lateral bending at any load (p<.05). CONCLUSIONS: Cervical FSUs with implanted TDR were found to have similar stiffness, but had greater energy absorption than intact FSUs during cyclic loading with an unconstrained pendulum system. These results provide further insight into the biomechanical behavior of cervical TDR under approximated physiologic loading conditions.

Effect of insertion of a single interference screw on the mechanical properties of porcine anterior cruciate ligament reconstruction grafts.

The objective of this study was to evaluate the mechanical properties of soft-tissue grafts following a single interference screw insertion of 4 different commercially available bioabsorbable interference screws. Twenty-four bovine proximal tibiae (12 matched pairs) were prepared and sagittally split to make 48 bone samples for testing. Tibiae were prepared for a 9 mm porcine tendon graft and were instrumented with 1 of 4 commercially available 10 x 35 mm composite screws, each with a different thread design. The samples were tensile loaded to failure at 200 mm/min and values for yield load, maximum load, and stiffness were recorded to quantify any differences on the function of the grafts. No graft showed macroscopic evidence of laceration following screw insertion and there were no statistically significant differences for yield load (P = .41), maximum load (P = .35), or stiffness (P = .68) among the different screw types. There is no significant difference in the mechanical properties of an anterior cruciate ligament graft following insertion of the 4 bioabsorbable screws tested in this study, in terms of yield load, stiffness, or failure load.

Dynamic biomechanical examination of the lumbar spine with implanted total spinal segment replacement (TSSR) utilizing a pendulum testing system.

BACKGROUND: Biomechanical investigations of spinal motion preserving implants help in the understanding of their in vivo behavior. In this study, we hypothesized that the lumbar spine with implanted total spinal segment replacement (TSSR) would exhibit decreased dynamic stiffness and more rapid energy absorption compared to native functional spinal units under simulated physiologic motion when tested with the pendulum system. METHODS: Five unembalmed, frozen human lumbar functional spinal units were tested on the pendulum system with axial compressive loads of 181 N, 282 N, 385 N, and 488 N before and after Flexuspine total spinal segment replacement implantation. Testing in flexion, extension, and lateral bending began by rotating the pendulum to 5°; resulting in unconstrained oscillatory motion. The number of rotations to equilibrium was recorded and bending stiffness (N-m/°) was calculated and compared for each testing mode. RESULTS: The total spinal segment replacement reached equilibrium with significantly fewer cycles to equilibrium compared to the intact functional spinal unit at all loads in flexion (p<0.011), and at loads of 385 N and 488 N in lateral bending (p<0.020). Mean bending stiffness in flexion, extension, and lateral bending increased with increasing load for both the intact functional spinal unit and total spinal segment replacement constructs (p<0.001), with no significant differences in stiffness between the intact functional spinal unit and total spinal segment replacement in any of the test modes (p>0.18). CONCLUSIONS: Lumbar functional spinal units with implanted total spinal segment replacement were found to have similar dynamic bending stiffness, but absorbed energy at a more rapid rate than intact functional spinal units during cyclic loading with an unconstrained pendulum system. Although the effects on clinical performance of motion preserving devices is not fully known, these results provide further insight into the biomechanical behavior of this device under approximated physiologic loading conditions.

Biomechanical evaluation of mini-fragment hardware for supination external rotation fractures of the distal fibula.

BACKGROUND: Supination external rotation distal fibula fractures are common, requiring fixation when associated with talar displacement. Subcutaneous distal fibula hardware may become painful, necessitating operative removal. We hypothesize that mini-fragment and small-fragment constructs will demonstrate similar biomechanical stability. METHODS: A biomechanical comparison was performed in synthetic osteoporotic sawbones. The first arm compared two 2.4-mm lag screws with one 3.5-mm lag screw for fixation of a simulated supination external rotation distal fibula fracture. The second arm compared a 2.4-mm plate-screw construct with a 3.5-mm lag screw and one-third tubular neutralization plate. During torsional testing, torque and displacement were recorded, and stiffness and peak torque were determined. RESULTS: Differences in mean stiffness and mean load at failure were not statistically significant with lag screw-only fixation. The 3.5-mm plate-screw construct outperformed the 2.4-mm plate-screw construct, but neither mean stiffness nor mean load at failure were statistically significantly different. Dynamic testing also demonstrated similar results. CONCLUSION: Our data suggest that isolated 2.4-mm screws function similarly to one 3.5-mm screw. Although the 3.5-mm plate-screw construct was stiffer, mean load at failure was equivalent for the 2 constructs. These data provide biomechanical evidence to support further investigation in the use of mini-fragment hardware for distal fibula fracture fixation. LEVELS OF EVIDENCE: Therapeutic, Level V.

A three-dimensional comparison of intramedullary nail constructs for osteopenic supracondylar femur fractures.

OBJECTIVES: This study developed a new 6 degree-of-freedom, unconstrained biomechanical model that replicated the in vivo loading environment of femoral fractures. The objective of this study was to determine whether various distal fixation strategies alter failure mechanisms and/or offer mechanical advantages when performing retrograde intramedullary nail (IMN) stabilization of supracondylar femur fractures in osteoporotic bone. METHODS: Forty fresh-frozen human femora were allocated into 2 groups of matched pairs: "locked" (fixed angle locking construct with both distal locking screws rigidly attached to the IMN) versus "unlocked" (conventional locking technique with 2 distal locking screws targeted through the distal locking screw holes of the IMN) and "locked" versus "washer" (fixed angle locking with the most distal screw exchanged for a bolt with condyle washers) distal fixation of a retrograde IM nails. A comminuted fracture (OTA 33-A3) was simulated with a wedge osteotomy. Bone density measurements were completed on all specimens before instrumentation. Instrumented femurs were loaded axially to failure, whereas 6 degree-of-freedom translations and angulations were measured using Roentgen stereophotogrammetric analysis. RESULTS: Mean (± SD) load born by "locked" specimens (1609 ± 667 N) at clinical failure was 38.1% greater (P = 0.09) than the corresponding mean load born by "unlocked" specimens (1165 ± 772 N). Clinical failure for the "washer" group (1738 ± 772 N) was 29.9% greater (P = 0.07) than the corresponding mean of the "locked" counterparts (1338 ± 822 N). Failure load was most clearly related to bone density in the "unlocked" fixation group. CONCLUSIONS: Predicting failure load based on bone density using a least squares estimate suggests that the washer construct provides superior fixation to other treatment techniques. The failure mechanism for a comminuted, supracondylar fracture cannot be analyzed accurately with a 1-dimensional measurement. The most common failure mechanism in this model was medial translation and varus angulation.

Locking buttons increase fatigue life of locking plates in a segmental bone defect model.

BACKGROUND: Durability of plate fixation is important in delayed union. Although locking plates result in stronger constructs, it is not known if locking affects the fatigue life of a plate. Two locking screws on either side of the nonunion could decrease working length and increase strain in the plate. However, the reinforcing effect of the locking head on the plate may compensate, so that it is unclear whether locking reduces fatigue life. QUESTIONS/PURPOSES: We determined whether locking screws, compression screws, and locking buttons reduce or increase the fatigue life of a plate. METHODS: We tested fatigue life of four constructs using an eight-hole locking plate in a segmental defect model: (1) all locking screws (Locked; n = 5); (2) all compression screws (Unlocked; n = 5); (3) six compression screws with two locking buttons in the central holes (Button; n = 6); and (4) six compression screws with two open central holes (Open; n = 6). RESULTS: The Button group had the longest fatigue life (1.3 million cycles). There was no difference between the Locked and Unlocked groups. All of the constructs failed by fracture of the plates through a screw hole adjacent to the defect. CONCLUSIONS: Locking screws did not improve fatigue life, however a locking button increased the fatigue life of a locking plate in a segmental bone defect model. CLINICAL RELEVANCE: Locking buttons in holes adjacent to a defect may improve durability, which is important when delayed union is a possibility.

Dynamic biomechanical examination of the lumbar spine with implanted total disc replacement using a pendulum testing system.

STUDY DESIGN: Biomechanical cadaver investigation. OBJECTIVE: To examine dynamic bending stiffness and energy absorption of the lumbar spine with and without implanted total disc replacement (TDR) under simulated physiological motion. SUMMARY OF BACKGROUND DATA: The pendulum testing system is capable of applying physiological compressive loads without constraining motion of functional spinal units (FSUs). The number of cycles to equilibrium observed under pendulum testing is a measure of the energy absorbed by the FSU. METHODS: Five unembalmed, frozen human lumbar FSUs were tested on the pendulum system with axial compressive loads of 181 N, 282 N, 385 N, and 488 N before and after Synthes ProDisc-L TDR implantation. Testing in flexion, extension, and lateral bending began by rotating the pendulum to 5º resulting in unconstrained oscillatory motion. The number of rotations to equilibrium was recorded and bending stiffness (N·m/º) was calculated and compared for each testing mode. RESULTS: In flexion/extension, the TDR constructs reached equilibrium with significantly (P < 0.05) fewer cycles than the intact FSU with compressive loads of 282 N, 385 N, and 488 N. Mean dynamic bending stiffness in flexion, extension, and lateral bending increased significantly with increasing load for both the intact FSU and TDR constructs (P < 0.001). In flexion, with increasing compressive loading from 181 N to 488 N, the bending stiffness of the intact FSUs increased from 4.0 N·m/º to 5.5 N·m/º, compared with 2.1 N·m/º to 3.6 N·m/º after TDR implantation. At each compressive load, the intact FSU was significantly stiffer than the TDR (P < 0.05). CONCLUSION: Lumbar FSUs with implanted TDR were found to be less stiff, but absorbed more energy during cyclic loading with an unconstrained pendulum system. Although the effects on clinical performance of motion-preserving devices are not fully known, these results provide further insight into the biomechanical behavior of these devices under approximated physiological loading conditions.

Examination of cervical spine kinematics in complex, multiplanar motions after anterior cervical discectomy and fusion and total disc replacement.

BACKGROUND: The biomechanical behavior of total disc replacement (TDR) and anterior cervical discectomy and fusion (ACDF) incomplex multiplanar motion is incompletely understood. The purpose of this study was to determine whether ACDF or TDR significantly affects in vitro kinematics through a range of complex, multiplanar motions. METHODS: Seven human cervical spines from C4-7 were used for this study. Intact cervical motion segments with and without implanted TDR and ACDF were tested by use of unconstrained pure bending moment testing fixtures in 7 mechanical modes: axial rotation (AR); flexion/extension (FE); lateral bending (LB); combined FE and LB; combined FE and AR; combined LB and AR; and combined FE, LB, and AR. Statistical testing was performed to determine whether differences existed in range of motion (ROM) and stiffness among spinal segments and treatment groups for each mechanical test mode. RESULTS: ACDF specimens showed increased stiffness compared with the intact and TDR specimens (P < .001); stiffness was not found to be different between TDR and intact specimens. ACDF specimens showed decreased ROM in all directions compared with TDR and intact specimens at the treated level. For the coupled motion test, including AR, LB, and FE, the cranial adjacent level (C4/C5) for the intact specimens (2.7°) showed significantly less motion compared with both the TDR (6.1°, P = .009) and ACDF (6.8°, P = .002) treatment groups about the LB axis. Testing of the C4/C5 and C6/C7 levels in all other test modes yielded no significant differences in ROM comparisons, although a trend toward increasing ROM in adjacent levels in ACDF specimens compared with intact and TDR specimens was observed. CONCLUSIONS: This study compared multiplanar motion under load-displacement testing of subaxial cervical motion segments with and without implanted TDR and ACDF. We found a trend toward increased motion in adjacent levels in ACDF specimens compared with TDR specimens. Biomechanical multiplanar motion testing will be useful in the ongoing development and evaluation of spinal motion-preserving implants.

The mechanical axes of the wrist are oriented obliquely to the anatomical axes.

BACKGROUND: the complex motions of the wrist are described in terms of four anatomical directions that are accomplished through the multiple articulations of the carpus. With minimal tendinous insertions, the carpus is primarily a passive structure. This emphasizes the importance of its mechanical properties, which few studies have examined to date. The purpose of the present study was to determine the mechanical properties of the wrist in twenty-four different directions of wrist motion. METHODS: the moment-rotation mechanical behavior of six fresh-frozen cadaver wrists was determined in four directions: flexion, extension, ulnar deviation, and radial deviation. Twenty other directions that were a combination of these anatomical directions were also studied. A custom-designed jig was interfaced with a standard materials testing system to apply unconstrained moments. Moments of ± 2 Nm were applied, and the moment-rotation data were recorded and analyzed to determine the neutral zone, range of motion, and stiffness values as well as the orientation of the envelope of these values. RESULTS: the envelope of wrist range-of-motion values was ellipsoidal in shape and was oriented obliquely (p < 0.001) to the direction of pure flexion-extension by a mean (and standard deviation) of 26.6° ± 4.4°. The largest wrist range of motion was a mean of 111.5° ± 10.2°, in the direction of ulnar flexion, 30° from pure flexion. The largest stiffness (mean, 0.4 Nm/deg) was in the direction of radial flexion, while the smallest stiffness (mean, 0.15 Nm/deg) was in the direction of ulnar flexion. CONCLUSIONS: the mechanical axes of the wrist are oriented obliquely to the anatomical axes. The primary mechanical direction is one of radial extension and ulnar flexion, a direction along a path of the dart thrower's wrist motion. CLINICAL RELEVANCE: understanding the mechanical function of the wrist can aid clinical treatment decisions, arthroplasty, and implant designs. The findings of this study provide new evidence that the mechanical axes of the wrist are not collinear with the anatomical axes.

The mechanical effect of commercially pure titanium and polyetheretherketone rods on spinal implants at the operative and adjacent levels.

STUDY DESIGN: Single-level cadaveric lumbar constructs were instrumented with either polyetheretherketone (PEEK) or commercially pure (CP) titanium (Ti) rods and biomechanically evaluated. Strain from gauged bone screws and interbody (IB) spacers, kinematic motion, and caudal disc pressure measurements were recorded during testing. OBJECTIVE: The objective of this study was to determine the biomechanical differences in CP Ti rods and PEEK rods in conjunction with PEEK interbody spacers. SUMMARY OF BACKGROUND DATA: Very little biomechanical data exist substantiating the performance of PEEK as a spinal rod material. This study is unique, because it combines strain, motion, and pressure measurement techniques to evaluate cadaveric constructs. METHODS: Twelve human cadaveric lumbar spine segments (T12-L3 and L4-S1) were tested in compression, flexion-extension, bilateral lateral bending, and bilateral axial torsion. Bending, axial, and shear strains were recorded from a gauged bone screw; axial and shear strains were also recorded from a gauged PEEK interbody spacer. Planar motion data and subadjacent disc pressure measurements were also collected. RESULTS: Highest screw strains were in bending; the lowest screw strains derived from the shear and axial gauges. Spacer strain was high to medium in some cases, especially in compression and flexion. PEEK constructs attained higher interbody strains than Ti constructs. Conversely, Ti construct screw strains were higher in most tests. Planar motion showed no differences at any level in almost every test. There was a trend toward decreased caudal intradiscal pressure for Ti constructs in compression. CONCLUSION: Rigid CP Ti rods resulted in increased screw strain (bone-screw interface forces) and less interbody spacer compression (higher stress shielding). Furthermore, there was a trend toward decreased intradiscal pressure with Ti rods at the caudal segment. These trends suggest that segments instrumented with PEEK more closely mimicked intact physiologic loading in the subadjacent level, which may reduce the likelihood of adjacent level disease.

Collagen scaffold supplementation does not improve the functional properties of the repaired anterior cruciate ligament.

Primary suture anterior cruciate ligament (ACL) repair was abandoned in favor of reconstruction due to a high rate of clinical failures. However, the insertion of a collagen scaffold loaded with platelets into the wound at the time of suture repair ("enhanced primary repair") has been shown to improve functional healing in animal models. Our objectives were to determine if using a collagen scaffold alone (without platelets) would be sufficient to increase the structural properties of the repaired ACL and decrease postoperative knee laxity compared to suture repair without the scaffold. Eight Yucatan minipigs underwent bilateral ACL transection and suture repair. In one knee, the repair was augmented with a collagen scaffold (SCAFFOLD group) while the other had suture alone (SUTURE group). After 13 weeks of healing, knee joint laxity and the structural properties of the ACL were measured. The addition of the collagen scaffold to suture repair of a transected ACL did not significantly improve the mean anteroposterior knee laxity [SCAFFOLD vs. SUTURE: 6.1 +/- 1.4 vs. 4.4 +/- 2.0 mm (p = 0.07), 8.1 +/- 2.0 vs. 7.6 +/- 2.0 mm (p = 0.66), and 6.2 +/- 1.2 vs. 6.1 +/- 1.8 mm (p = 0.85) at 30 degrees, 60 degrees, and 90 degrees flexion, respectively]. Likewise, there were no significant differences in the structural properties [SCAFFOLD vs. SUTURE: 367 +/- 185.9 vs. 322 +/- 122.0N (p = 0.66) and 90.7 +/- 29.5 vs. 85.0 +/- 30.3N/mm (p = 0.74) for the yield load and linear stiffness, respectively]. The use of a collagen scaffold alone to enhance suture repair of the ACL was ineffective in this animal model. Future work will be directed at stimulating biological activity in the scaffold.

The effect of coordinate system choice and segment reference on RSA-based knee translation measures.

Roentgen stereophotogrammetric analysis (RSA) can be utilized to accurately describe joint kinematics, but even when measuring small displacements within radiographically discernible structures, standardized reference frames are imperative for useful comparison across patients and across studies. In the current paper, accurately controlled laboratory models demonstrated the considerable influence that a mere 1.9-cm offset of the origin of the coordinate system from the rotation axes could exert on translation measures when rotations were occurring. In addition, the use of two different coordinate systems to gauge translation on a radiographic anterior-posterior (A-P) knee laxity exam resulted in a significant correlation (R(2)=0.562) between the two systems; however, differences of up 9.28 mm were found between corresponding measurements. This implies that clinical conclusions can potentially be upheld or refuted, based on the same data set, subject to coordinate system definition. Although the data analyzed presently involved the knee joint, similar issues surround the RSA motion analysis of other joints as well.