Locking plates improve torsional resistance in the stabilization of three-part proximal humeral fractures

This study quantified the torsional resistance provided by locking plates and angled blade plates used to stabilize proximal humeral fractures. Three-part proximal humeral fractures were created in 6 pairs of cadaveric humeri. One specimen of each pair was reconstructed with a proximal humeral locking plate, whereas the other specimen was reconstructed with an angled blade plate. An external rotation torque, varying from 0 to 5 N-m, was applied to the humeral head until the head rotated 30 degrees or 10,000 loading cycles were applied. The mean initial torsional stiffness was significantly larger for the locking plates (0.99 N-m/degree) than for the blade plates (0.59 N-m/degree). For each pair, the maximum rotation was larger for the blade plate than for the locking plate. For this in vitro model of a reconstructed 3-part proximal humeral fracture, the locking plate provided better torsional fatigue resistance and stiffness than the blade plate.     

Bending and torsional stiffness in cadaver humeri fixed with a self-locking expandable or interlocking nail system: a mechanical study

OBJECTIVES: This study was designed to gain data about a new expandable, noninterlocked intramedullary nail's capacity to stabilize unstable transverse humeral shaft fractures without the need for interlocking, thus making nail implantation simpler and to prove our goal hypothesis: that in a midshaft osteotomy of the humeral shaft the expandable humeral nail will show the same bending and torsional stiffness as an interlocked humeral nail, when implanted correctly according to the manufacturer's instructions. DESIGN: Pair randomization. SETTING: Mechanical laboratory testing. PARTICIPANTS: Eight pairs of freshly harvested cadaveric humeri. INTERVENTIONS: Fracture model was a midshaft transverse osteotomy, gapped to 3 mm. Each humerus pair received an expandable humeral nail (Fixion) or an interlocked humerus nail (Synthes) through a retrograde approach. The humeri were fixed in polymethylmethacrylate cylinders and tested in a servo-pneumatic material-testing machine. MAIN OUTCOME MEASUREMENTS: Torsional stiffness and bending stiffness of the nail-bone-construction. RESULTS: Expandable nails (interlocked nails) showed a lateral bending stiffness of 0.73 +/- 0.14 (0.63 +/- 0.1) KN/mm (P = 0.026) and a frontal bending stiffness of 0.67 +/- 0.18 (0.58 +/- 0.09) KN/mm (P = 0.084). Torsional stiffness values were 0.13 +/- 0.19 (0.43 +/- 0.09 Nm/degrees) (P = 0.012). Lower torsional stiffness in the expandable nail group was observed in humeri with a funnel shaped proximal intramedullary canal. CONCLUSIONS: The nail systems showed similar characteristics for frontal bending (P = 0.084), but not for lateral bending (P = 0.026). For lateral bending, the Fixion nail showed significantly more stiffness than the UHN nail (P = 0.026). There was significantly lower torsional stiffness with expandable nails compared with interlocked nails. Clinical correlation would suggest that in rotationally unstable fractures (A2 and A3 diaphyseal fractures), interlocked nails would provide increased stability over expandable nails.     

Biomechanical analysis of distal femur fracture fixation: fixed-angle screw-plate construct versus condylar blade plate

OBJECTIVE: The objective of this study is to establish the relative strength of fixation of a locking distal femoral plate compared with the condylar blade plate. METHODS: Eight matched pairs of fresh-frozen cadaveric femurs were selected and evaluated for bone density. A gap osteotomy model was used to simulate an OTA/AO A3 comminuted distal femur fracture. One femur of each pair was fixed with the blade plate; the other, with a locking plate. After 100 N preload and 10,000 cycles between 100 N and 1000 N, total displacement of each specimen was assessed. After completion of cyclic loading, maximum load to failure was tested. RESULTS: Significantly greater subsidence (total axial displacement) occurred with the blade plate (1.70 +/- 0.45 mm; range, 1.21-2.48 mm) than with the locking plate fixation (1.04 +/- 0.33 mm; range, 0.67-1.60 mm) after cyclic loading (P = 0.03). In load-to-failure testing, force absorbed by the locking plate before failure (9085 +/- 1585 N; range, 7269-11,850 N) was significantly greater than the load tolerated by the blade plate construct (5591 +/- 945 N; range, 3546-6684 N; P = 0.001). Variability in bone mineral density did not affect the findings (fixed angle distal femoral plate r = 0.1563; condylar blade plate r = 0.0796). CONCLUSIONS: The locking screw-plate construct proved stronger than the blade plate in both cyclic loading and ultimate strength in biomechanical testing of a simulated A3 distal femur fracture. Although differences were small, the biomechanical performance of the locking plate construct over the blade plate may lend credence to use of the locking plate versus the blade plate in the fixation of comminuted distal femur fractures.     

Physiological axial compressive preloads increase motion segment stiffness, linearity and hysteresis in all six degrees of freedom for small displacements about the neutral posture.

The stiffness of motion segments, together with muscle actions, stabilizes the spinal column. The objective of this study was to compare the experimentally measured load-displacement behavior of porcine lumbar motion segments in vitro with physiological axial compressive preloads of 0, 200 and 400 N equilibrated in a physiological fluid environment, for small displacements about the neutral posture. These preloads are hypothesized to increase stiffness, hysteresis and linearity of the load-displacement behavior.At each preload, displacements in each of six degrees of freedom (+/-0.3 mm AP and lateral translations, +/-0.2 mm axial translation, +/-1 degrees lateral bending and +/-0.8 degrees flexion/extension and torsional rotations) were imposed. The resulting forces and moments were recorded. Tests were repeated after removal of posterior elements. Using least squares, the forces at the vertebral body center were related to the displacements by a symmetric 6 x 6 stiffness matrix. Six diagonal and two off-diagonal load-displacement relationships were examined for differences in stiffness, linearity and hysteresis in each testing condition.Mean values of the diagonal terms of the stiffness matrix for intact porcine motion segments increased significantly by an average factor of 2.2 and 2.9 with 200 and 400 N axial compression respectively (p<0.001). Increases for isolated disc specimens averaged 4.6 and 6.9 times with 200 and 400 N preload (p<0.001). Changes in hysteresis correlated with the changes in stiffness. The load-displacement relationships were progressively more linear with increasing preload (R(2)=0.82, 0.97 and 0.98 at 0, 200 and 400 N axial compression respectively). Motion segment and disc load-displacement behaviors were stiffer, more linear and had greater hysteresis with axial compressive preloads.     

Stability of radial head and neck fractures: a biomechanical study of six fixation constructs with consideration of three locking plates

PURPOSE: Open reduction and internal fixation of radial neck fractures can lead to secondary loss of reduction and nonunion due to insufficient stability. Nevertheless, there are only a few biomechanical studies about the stability achieved by different osteosynthesis constructs. METHODS: Forty-eight formalin-fixed, human proximal radii were divided into 6 groups according to their bone density (measured by dual-energy x-ray absorptiometry). A 2.7-mm gap osteotomy was performed to simulate an unstable radial neck fracture, which was fixed with 3 nonlocking implants: a 2.4-mm T plate, a 2.4-mm blade plate, and 2.0-mm crossed screws, and 3 locking plates: a 2.0-mm LCP T plate, a 2.0-mm 6x2 grid plate, and a 2.0-mm radial head plate. Implants were tested under axial (N/mm) and torsional (Ncm/ degrees ) loads with a servohydraulic materials testing machine. RESULTS: The radial head plate was significantly stiffer than all other implants under axial as well as under torsional loads, with values of 36 N/mm and 13 Ncm/ degrees . The second-stiffest implant was the blade plate, with values of 20 N/mm and 6 Ncm/ degrees . The weakest implants were the 2.0-mm LCP, with values of 6 N/mm and 2 Ncm/ degrees , and the 2.0-mm crossed screws, with values of 18 N/mm and 2 Ncm/ degrees . The 2.4-mm T plate, with values of 14 N/mm and 4 Ncm/ degrees , and the 2.0-mm grid plate, with values of 8 N/mm and 4 Ncm/ degrees came to lie in the midfield. CONCLUSIONS: The 2.0-mm angle-stable plates-depending on their design-allow fixation with comparable or even higher stability than the bulky 2.4-mm nonlocking implants and 2.0-mm crossed screws.     

Cervical spine locking plate: in vitro biomechanical testing

Summary The AO cervical spine locking plate (CSLP) for anterior subaxial fixation was recently received increasing clinical acclaim, yet to date the in vitro mechanical properties of this implant have not been reported. To determine the in vitro biomechanical properties of this device, five fresh human cadaver cervical spines were subjected to nondestructive testing in flexion and torsion in three stages: stage 1: intact spine; stage 2: destabilized spine; stage 3: destabilized spine with CSLP. Stage 3 specimens were also subjected to large angular displacement testing to assess the integrity of the fixation. In flexion, mean spinous process displacement was 1.21 mm for stage 1, 3.19 mm for stage 2, and 1.37 mm, for stage 3. Mean torsional stiffness was 2.86 Nm/degree in stage 1, 1.82 Nm/degree in stage 2, and 2.20 Nm/degree in stage 3. Large angular displacement testing in stage 3 resulted in screw loosening from the bone in two specimens; no screw plate loosening occurred. In our severely destabilized in vitro model, the CSLP restored flexion stability but not rotational stability. This suggests that supplemented bracing or fixation may be required to restore torsional stability.     

The significance of postoperative stability for osseous repair of a multiple fragment fracture. Animal experiment studies

The significance of postoperative mechanical stability for bony repair of a comminuted fracture was investigated in an animal experimental study comparing four commonly applied operative methods of stabilizing fractures: (1) flate osteosynthesis combined with lag screw fixation after reduction of the fragments; (2) bridging plate osteosynthesis; (3) external fixation; (4) static interlocking intramedullary nailing. As the fracture model, a triple-wedge osteotomy of the right sheep tibia was used. The results of in vitro testing of stiffness (N/mm) of each of the four osteosyntheses was as follows: anatomical plate: 746 N/mm; bridging plate 434 N/mm; external fixation 625 N/mm; nailing 416 N/mm. Eight weeks after the operation, the tibiae were explanted and the contralateral tibiae of six sheep were taken as a control group. The three-point bending test revealed no significant difference in bending deviation: anatomical plate 47.58 +/- 22.57 microns; bridging plate 33.93 +/- 7.67 microns; external fixation 33.83 +/- 8.02 microns; nailing 33.0 +/- 17.23 microns. However, it was noted that there was a slightly higher tendency towards stiffness of the bones after bridging plate osteosynthesis, external fixation and interlocking intramedullary nailing and that the amount of stiffness resembled that in non-operated control animals (25.56 +/- 6.66 microns). On the other hand, anatomical plate osteosynthesis showed less stiffness. To assess the tensile strength at the osteotomy area, bone samples were prepared and tested for failure on a material testing machine. The tensile strength of the bone samples showed a distinct difference in all experimental groups according to their anatomical location.(ABSTRACT TRUNCATED AT 250 WORDS)     

A biomechanical comparison between two volar locking plate systems for distal radius fractures

The biomechanical properties of two distal radius volar locking plate systems were investigated. A cadaveric model simulating distal radius fractures with dorsal comminution was selected. The biomechanical properties of fracture fixation using the Synthes volar locking T-plate and the Hand Innovation DVR plate were characterized. The average stiffness was 137.1 +/- 19.9 N/mm for the DVR group and 119.1 +/- 9.9 N/mm for the Synthes group (P = .49), and the average changes of the opening angle describing plastic deformation were 10.1 degrees +/- 4.0 degrees and 8.6 degrees +/- 13.2 degrees, respectively (P = .23). This study demonstrates that both the Synthes T-plate and Hand Innovations DVR plate fixation systems show comparable biomechanical characteristics.     

Construct stiffness of different fixation methods for supracondylar femoral fractures above total knee prostheses

Supracondylar fracture of the femur after total knee arthroplasty has an estimated frequency of 0.6%-2.5% among total knee recipients and presents an extremely difficult problem when encountered. The goal of this study is to determine the most stable method of fixation of these supracondylar fractures among currently available devices. Synthetic composite femurs with properties similar to human bone were used, and identical, unstable supracondylar fractures were created in each. Osteotomized specimens were placed into four groups of five. Each group was then tested with one of four devices: the Green-Seligson-Henry (GSH) intramedullary nail, AO 95 degrees blade plate, dynamic condylar screw and sideplate, and condylar buttress plate. After stabilization with the different types of fixation, the constructs were tested individually for bending stiffness in four modes: flexion, extension, varus, and valgus bending. The stiffest fixation was determined in each of the four bending planes. Resistance to all tested directions was greatest for the condylar screw and sideplate construct. Resistance to flexion (stiffness = 30.96 N/mm), extension (stiffness = 36.36 N/mm), varus (stiffness = 35.46 N/mm), and valgus forces (stiffness = 32.26 N/mm) was highest in the group fixed with the dynamic condylar screw. This may be due to the purchase gained by the large lag screw into the distal femur, or it may be the result of the total rigidity of the implant. Although the femoral samples used in this study do not duplicate the typical osteopenic bone encountered at the site of a total knee arthroplasty, they do allow direct comparison of the fixation devices by removing the variability associated with cadaveric bone samples.     

Load displacement behavior of the human lumbo-sacral joint

The three-dimensional load displacement behavior of nine fresh adult L5-S1 spine motion segments was studied. Static test forces up to 160 N in anterior, posterior, and lateral shear, test forces up to 320 N in compression, and test moments up to 15.7 Nm in flexion, extension, lateral bending, and torsion were used. The six displacements of the center of the inferior L5 endplate were measured 15 and 60 s after the load was applied. Specimens were then retested after posterior element excision. The results show that at the maximum test force, intact specimen mean (SD) displacements ranged from 1.65 mm (0.63 mm) in lateral shear to 2.21 mm (0.87 mm) in posterior shear. Posterior element excision resulted in an average 1.66-fold increase in shear translations. At the maximum moment, rotations ranged from 3.38 degrees (1.03 degrees) in torsion to 7.19 degrees (1.77 degrees) in flexion. Posterior element excision resulted in an average 2.09-fold increase in bending rotations and a 2.74-fold increase in the average torsional rotation. In general, these L5-S1 joints were stiffer than more cranial lumbar segments in flexion, extension, and lateral bending and were less stiff in torsion tests.     

In situ forces in the human posterior cruciate ligament in response to muscle loads: a cadaveric study.

The objectives of this study were to determine the effects of hamstrings and quadriceps muscle loads on knee kinematics and in situ forces in the posterior cruciate ligament of the knee and to evaluate how the effects of these muscle loads change with knee flexion. Nine human cadaveric knees were studied with a robotic manipulator/universal force-moment sensor testing system. The knees were subjected to an isolated hamstrings load (40 N to both the biceps and the semimembranosus), a combined hamstrings and quadriceps load (the hamstrings load and a 200-N quadriceps load), and an isolated quadriceps load of 200 N. Each load was applied with the knee at full extension and at 30, 60, 90, and 120 degrees of flexion. Without muscle loads, in situ forces in the posterior cruciate ligament were small, ranging from 6+/-5 N at 30 degrees of flexion to 15+/-3 N at 90 degrees. Under an isolated hamstrings load, the in situ force in the posterior cruciate ligament increased significantly throughout all angles of knee flexion, from 13+/-6 N at full extension to 86+/-19 N at 90 degrees. A posterior tibial translation ranging from 1.3+/-0.6 to 2.5+/-0.5 mm was also observed from full extension to 30 degrees of flexion under the hamstrings load. With a combined hamstrings and quadriceps load, tibial translation was 2.2+/-0.7 mm posteriorly at 120 degrees of flexion ut was as high as 4.6+/-1.7 mm anteriorly at 30 degrees. The in situ force in the posterior cruciate ligament decreased significantly under this loading condition compared with under an isolated hamstrings load, ranging from 6+/-7 to 58+/-13 N from 30 to 120 degrees of flexion. With an isolated quadriceps load of 200 N, the in situ forces in the posterior cruciate ligament ranged from 4+/-3 N at 60 degrees of flexion to 34+/-12 N at 120 degrees. Our findings support the notion that, compared with an isolated hamstrings load, combined hamstrings and quadriceps loads significantly reduce the in situ force in the posterior cruciate ligament. These data are in direct contrast to those for the anterior cruciate ligament. Furthermore, we have demonstrated that the effects of muscle loads depend significantly on the angle of knee flexion.     

桡骨远端骨折锁定钢板刚度的拓扑优化研究

目的对桡骨远端骨折锁定钢板结构进行优化设计,以应对骨折内固定个性化刚度需求。方法运用三维建模和计算机辅助设计软件完成桡骨远端骨折模型和常规钢板的模型构建,基于初始有限元分析结果,以轴向刚度下调33.33%且保留扭转刚度的90.00%以上作为优化目标,对常规钢板进行拓扑优化和重设计;通过有限元分析计算,对比常规钢板和优化钢板在轴向压缩和扭转工况下的内固定刚度和产生的骨折区应变。结果所获得的优化钢板轴向刚度为636.5 N/mm,下调幅度为19.7%,基本接近既定的目标刚度,优化后的扭转刚度为634.12 Nmm/°,下调幅度为8.8%,并未超出既定的目标限值;而骨折区应变变化方面,轴向应变相比切向应变呈现出更为显著的增加趋势,与刚度调控效果基本一致。结论通过拓扑优化的方法从钢板结构层面进行重设计,可实现骨折愈合的个性化内固定刚度调控。     

Proximal humerus fractures: a comparative biomechanical analysis of intra and extramedullary implants

Introduction The biomechanical stability of a newly developed humerus nail (Sirus™) for the treatment of fractures of the proximal humerus was analyzed in comparison to established systems. In total, three randomized groups were formed (n = 4 pairs) from 12 matched pairs of human cadaver humeri. Materials and methods All intact bones were mechanically characterized by five subsequent load cycles under bending and torsional loading. The bending moment at the osteotomy was 7.5 N m the torsional moment was 8.3 N m over the hole specimen length. Loading was consistently initiated at the distal epiphysis and the deformation at the distal epiphysis was continuously recorded. Prior to implant reinforcement, a defect of 5 mm was created to simulate an unstable subcapital humerus fracture. For paired comparison, one humerus of each pair was stabilized with the Sirus proximal humerus nail while the counterpart was stabilized by a reference implant. In detail, the following groups were created: Sirus versus Proximal humerus nail (PHN) with spiral blade (group I); Sirus versus PHILOS plate (group II); Sirus versus 4.5 mm AO T-plate (group III). Results The Sirus nail demonstrated significantly higher stiffness values compared to the reference implants for both bending and torsional loading. The following distal epiphyseal displacements were recorded for a bending moment of 7.5 N m at the osteotomy: Sirus I: 8.8 mm, II: 8.4 mm, III: 7.7 mm (range 6.9–10.9), PHN 21.1 mm (range 15.7–25.2) (P = 0.005), PHILOS plate 27.5 mm (range 21.6–35.8) (P < 0.001), 4.5 AO T-plate 26.3 mm (range 24.3–33.9) (P = 0.01). The rotations corresponding to 8.3 N m torsional moment were: Sirus I: 9.1°, II: 9.3°, III: 10.6° (range 7.5–12.2), PHN 13.5° (range 10.3–15.6) (P = 0.158), PHILOS plate 15.6° (range 13.7–20.8) (P = 0.007), 4.5 AO T-Platte 14.1° (range 11.5–19.7) (P = 0.158). Conclusion The intramedullary load carriers were biomechanically superior when compared to the plating systems in the fracture model presented here. Supplementary, the Sirus Nail showed higher stiffness values than the PHN. However, the latter are gaining in importance due to the possibility of minimal invasive implantation. Whether this will be associated with functional advantages requires further clinical investigation.     

颈椎可调控式融合固定器山羊模型的生物力学对照研究

目的 利用山羊颈椎模型分析比较颈椎可调控式融合固定器(AC-AFF)与其他内固定方式的生物力学差异.方法 将18只山羊随机分为3组,椎体次全切除后分别植入AC-AFF、钛网+钢板或髂骨块+钢板,人工饲养6个月后羊颈椎模型经处理再进行生物力学测试,施加载荷为0～150N,加载速率为1.4mm/min,测试的运动工况包括颈椎前屈、后伸、侧屈及旋转,测量指标包括颈椎的变形与位移、强度和刚度及极限力学性能.结果 三种重建方式中,AC-AFF组在相同载荷作用下应变最小,较钛网+钢板组、髂骨块+钢板组分别小2%～4%(P＞0.05)及10%～16%(P＜0.05);在轴向压缩、前屈、后伸或侧屈状态下植骨融合处的应力集中最小;平均水平位移AC-AFF组为0.44 mm,钛网+钢板组为0.51 mm,髂骨块+钢板组为0.70 mm.轴向位移AC-AFF较钛网+钢板小3%～4%(P＞0.05),较髂骨块+钢板小16%～24%(P＜0.05);AC-AFF的水平剪切刚度、轴向刚度及弯曲刚度最高,扭矩及扭转刚度最大.极限破坏实验显示AC-AFF的极限载荷为1107 N,钛网+钢板组为998 N,髂骨块+钢板组为879 N.结论 与钛网+钢板及髂骨块+钢板比较,AC-AFF的生物力学稳定性更高.     

Screw pull-out force is dependent on screw orientation in an anterior cervical plate construct

Two common justifications for orienting cervical screws in an angled direction is to increase pull-out strength and to allow use of longer screws. This concept is widely taught and has guided implant design. Fixed versus variable angle systems may offer strength advantages. The purpose of our study is to test the influence of screw orientation and plate design on the maximum screw pull-out load. Variable and fixed angle 4.0 x 15 mm and 4.0 x 13 mm self-tapping screws were used to affix a Medtronic Atlantis cervical plate to polyurethane foam bone samples (density 0.160/cm). This synthetic product is a model of osteoporotic cancellous bone. The fixed angle screws can only be placed at 12 degrees convergent to the midline and 12 degrees in the cephalad/caudal ("12 degrees up and in") direction. Three groups were tested: (1) all fixed angle screws, (2) variable angle, all screws 12 degrees up and in, (3) variable angle, all screws 90 degrees to the plate. Plate constructs were pulled off with an Instron DynaMight 8841 servohydrolic machine measuring for maximum screw pull-out force. There was no difference between group 1, fixed angle (288.4 +/- 37.7 N) (mean +/- SD) and 2, variable angle group (297.7 +/- 41.31 N P< or =0.73). There was a significant increase in maximum pull-out force to failure for the construct with all screws at 90 degrees (415.2+/-17.4 N) compared with all screws 12 degrees "up and in" (297.4 +/- 41.3 N, P< or =0.0016). Group 3 done with 13 mm screws, showed a trend toward better pull-out strength, compared to group 2 w/15 mm screws (345.2 +/- 20.5 vs. 297.4 +/- 41.3, P< or =0.06). In this plate pull-out model, screw orientation influences maximum force to failure. When all 4 screws are 90 degrees to the plate the construct has the greatest ability to resist pullout. Fixed angle designs show no advantage over variable angle. These findings are contrary to current teaching.     

Blade plate compared with locking plate for tibiotalocalcaneal arthrodesis: a cadaver study.

BACKGROUND: We hypothesized that a locking plate would be stronger than a blade plate for tibiotalocalcaneal arthrodesis under dorsiflexion and torsional loading. MATERIALS AND METHODS: Nine pairs of matched cadaveric lower extremities were used. BMD was obtained for each specimen. Each received a retrograde augmentation screw and a stainless steel LC-angled blade plate (Synthes, Paoli, PA) or a stainless steel LCP proximal humerus locking plate (Synthes, Paoli, PA). Specimens were cyclically loaded in dorsiflexion to simulate 6 weeks of partial weightbearing and then monotonically loaded to failure. Specimens were removed from the load frame and remounted to simulate fusion. The specimen received an axial load of 720 N and was externally rotated proximal to the construct at 5 degrees/sec to fracture. Data were compared with a Student's t-test. Pearson correlation analysis was used to determine whether bone mineral density was significantly related to measured parameters. Significance was set at p < or = 0.05. RESULTS: The locking plate group had higher initial stiffness, higher dorsiflexion and torsional load to failure, and lower construct deformation than the blade plate group. Bone mineral density was positively correlated with dorsiflexion failure load and torsional failure load in the locking plate construct. CONCLUSION: Fixation with the locking plate was superior to that with the blade plate. CLINICAL RELEVANCE: Use of a locking plate may be an effective fixation technique in tibiotalocalcaneal arthrodesis, especially in complex hindfoot reconstructions with bone loss or deformity.     

Biomechanics of external fixation of distal tibial extra-articular fractures: is spanning the ankle with a foot plate desirable?

OBJECTIVES: To compare the mechanical stability of external fixation with and without spanning of the ankle joint with a foot plate in an in vitro model of extra-articular distal tibia fractures. DESIGN: A laboratory investigation was performed to evaluate the mechanical behavior of external fixation of extra-articular distal tibia fractures using a fixator with and without a foot plate. Ten fresh-frozen lower extremities (5 pairs) with a simulated OTA 43-A3.3 fracture were stabilized with an Ilizarov hybrid fixator with and without a foot plate. SETTING: All mechanical testing was performed with a servohydraulic test frame (MTS Bionix 858, Minneapolis, MN). MAIN OUTCOME MEASUREMENT: Deformation characteristics as a function of load were compared for an Ilizarov fixator with and without a foot plate under identical conditions of forefoot loading from 0 to 100 N. Relative interfragmentary motions (vertical and horizontal translations and rotation) were measured. RESULTS: There was significantly more vertical translation (2.57 +/- 0.97 mm vs. -0.83 +/- 0.64 mm) and angular displacement (4.49 +/- 0.45 degrees vs. -1.15 +/- 0.61 degrees ) of the distal fragment in the arrangement without a foot plate compared with the construct with a foot plate. The anterior translation of the distal fragment was similar with (1.12 +/- 0.98 mm) and without a foot plate (1.19 +/- 1.23 mm). CONCLUSIONS: This study supports the mechanical importance of spanning of the ankle with a foot plate in most cases of external fixation for unstable extra-articular and periarticular distal tibia fractures. Further studies are needed to validate these results before widespread changes in clinical treatment can be recommended.     

Structural properties of the subscapularis tendon.

The subscapularis muscle is an important mover and stabilizer of the glenohumeral joint. The purpose of this study was to measure regional variations in the structural properties of the subscapularis tendon in two joint positions. Subscapularis tendons from cadaveric shoulders were divided into four sections superiorly to inferiorly and tested to failure at 0 or 60 degrees of glenohumeral abduction. Arm position had a significant influence on stiffness in the inferior and superior portions (p < 0.05). The inferior region showed a higher stiffness in the hanging-arm position (0 degrees) than at 60 degrees of abduction (27.4+/-17.7 compared with 9.5+/-5.9 N/mm). Meanwhile, stiffness of the superior portion was higher at 60 degrees of abduction than in the hanging-arm position (208.7+/-60.9 compared with 147.2+/-32.3 N/mm). In the hanging-arm position (0 degrees) and at 60 degrees of abduction, the superior and midsuperior portions failed at significantly higher loads (superior: 623.2+/-198.6 and 478.2+/-206.6 N at 0 and 60 degrees of abduction, respectively; midsuperior: 706.2+/-164.6 and 598.4+/-268.4 N, respectively) than did the inferior portion (75.1+/-54.2 and 30.3+/-13.0 N, respectively). Likewise, stiffness of the superior and midsuperior portions was significantly higher than that of the inferior region in both positions. Higher stiffness and ultimate load in the superior tendon region may explain the infrequent extension of rotator cuff tears into the subscapularis tendon. Conversely, the significantly lower ultimate load and stiffness in the inferior tendon region could facilitate anterior dislocation of the humeral head when this portion stabilizes the joint in a dislocated position. Therefore, repair of torn inferior portions of the subscapularis tendon should be considered in surgery for glenohumeral instability.     

Measurement of anterior instability of the knee. A new apparatus for clinical testing

We have developed an apparatus to measure the anteroposterior stability of the knee to forces of up to 250 N, applied at 20 degrees of flexion. We measured anterior laxity at 200 N, anterior stiffness at 50 N and total laxity at +/- 200 N. A study of cadaveric knees revealed that the soft tissues surrounding the bones had a significant influence on the force-displacement curve, and emphasised that differences between injured and normal pairs of knees are much more important than the absolute values of the parameters. In 61 normal volunteers we found no significant left to right differences in anterior laxity at 200 N and anterior stiffness at 50 N. In 92 patients with unilateral anterior cruciate deficiency there were significant differences (p less than 0.0005) in anterior laxity, anterior stiffness and total laxity, the injured-normal differences averaging 6.7 mm, 1.3 N/mm, and 8.1 mm respectively.     

A biomechanical comparison of plate configuration in distal humerus fractures

OBJECTIVES: Our aim was to test the hypothesis that two plates placed parallel to each other are stronger and stiffer than plates placed perpendicular to each other for fixation of a distal humerus fracture model. METHODS: We created an artificial distal humeral fracture model by osteotomizing two groups of identical epoxy resin humera. Screw and plate constructs were built to mimic osteosynthesis. In the first group, 3.5-mm reconstruction plates were placed parallel to each other along each of the medial and lateral supracondylar ridges. In the second group, 3.5-mm reconstruction plates were placed perpendicular to each other with a medial supracondylar ridge plate and a posterolateral plate. Stiffness and strength data of the two constructs were obtained by testing to failure with sagittal plane bending forces. RESULTS: The parallel plate group (n = 7) had a mean stiffness of 214.9 +/- 43.3 N/mm and a mean strength of 304.4 +/- 63.5 N. The perpendicular plate group (n = 8) had a mean stiffness of 138.3 +/- 44.6 N/mm and a mean strength of 214.9 +/- 43.3 N. These differences were significant (Student's t test, P < 0.05). CONCLUSIONS: As theoretically expected, a parallel plate configuration is significantly stronger and stiffer than a perpendicular plate configuration when subjected to sagittal bending forces in a distal humerus fracture model.