The effects of tibial rotation on posterior translation in knees in which the posterior cruciate ligament has been cut.

BACKGROUND: One of the most useful clinical tests for diagnosing an isolated injury of the posterior cruciate ligament is the posterior drawer maneuver performed with the knee in 90 degrees of flexion. Previously, it was thought that internally rotating the tibia during posterior drawer testing would decrease posterior laxity in a knee with an isolated posterior cruciate ligament injury. In this study, we evaluated the effects of internal and external tibial rotation on posterior laxity with the knee held in varying degrees of flexion after the posterior cruciate and meniscofemoral ligaments had been cut. MATERIALS AND METHODS: Twenty cadaveric knees were used. Each knee was mounted in a fixture with six degrees of freedom, and anterior and posterior forces of 150 N were applied. The testing was conducted with the knee in 90 degrees, 60 degrees, 30 degrees, and 0 degrees of flexion with the tibia in neutral, internal, and external rotation. All knees were tested with the posterior cruciate and meniscofemoral ligaments intact and transected. Repeated-measures analysis of variance was used for statistical analysis. RESULTS: At 30 degrees, 60 degrees, and 90 degrees of flexion, there was a significant increase in posterior laxity following transection of the posterior cruciate and meniscofemoral ligaments. At 60 degrees and 90 degrees of flexion, there was significantly less posterior laxity when the tibia was held in internal compared with external rotation. At 0 degrees and 30 degrees of flexion, there was no significant difference in posterior laxity when the tibia was held in internal compared with external rotation. CONCLUSIONS: After the posterior cruciate and meniscofemoral ligaments had been cut, posterior laxity was significantly decreased by both internal and external rotation of the tibia. Internal tibial rotation resulted in significantly less laxity than external tibial rotation did at 60 degrees and 90 degrees of knee flexion.

Articular cartilage contact pressure after tibial tuberosity transfer. A cadaveric study.

Medial transfer of the tibial tuberosity has been commonly used for treatment of recurrent dislocation of the patella and patellofemoral malalignment. In this study, six fresh human cadaveric knees were used. Static intrajoint loads were recorded using Fuji Prescale pressure-sensitive film for contact pressure and contact area determination in a closed kinetic chain knee testing protocol. Peak pressures, average contact pressures, and contact areas of the patellofemoral and tibiofemoral joints were calculated on native intact knee specimens and after tibial tuberosity transfer. All native intact knee specimens had a normal Q angle. Medialization of the tibial tuberosity significantly increased the patellofemoral contact pressure. Medial displacement of the tibial tuberosity also significantly increased the average contact pressure of the medial tibiofemoral compartment and changed the balance of tibiofemoral joint loading. The results of our study suggest that caution should be used when transferring a patellar tendon in the face of a preexisting normal Q angle as this will result in abnormally high peak pressure within the tibiofemoral joint. Overmedialization of the tibial tuberosity should be avoided in the varus knee, the knee after medial meniscectomy, and the knee with preexisting degenerative arthritis of the medial compartment.

A comparison of preoperative imaging techniques for predicting patellar tendon graft length before cruciate ligament reconstruction.

Autogenous bone-patellar tendon-bone is commonly used as graft material for cruciate ligament reconstructions. If this type of graft is too long, graft fixation other than an interference screw may be required. If it is too short, selection of another type of graft may be necessary. If the length of the patellar tendon portion of the graft could be accurately predicted, preoperative planning could determine the adequacy of this graft and choice of fixation for the planned procedure. Using lateral radiographs with the knee flexed 30 degrees, standard magnetic resonance imaging, and magnetic resonance imaging with supplemental three-dimensional reconstructions, we measured the length of the patellar tendon in cadavers and then compared these measurements with the actual patellar tendon length measurements. Linear regression analysis resulted in r2 values of 0.80, 0.43, and 0.65 for lateral radiographs, standard magnetic resonance imaging, and magnetic resonance imaging with three-dimensional reconstructions, respectively. We concluded that lateral radiographs at 30 degrees of knee flexion are the most accurate predictors of patellar tendon length.

A biomechanical comparison of posterior cruciate ligament reconstruction techniques.

Most posterior cruciate ligament reconstruction techniques use both tibial and femoral bone tunnels for graft placement. Because of the acute angle the graft must make to gain entrance into the tibial tunnel, abnormal stresses are placed on the graft that could lead to graft failure. An alternative technique for posterior cruciate ligament reconstruction involves placement of the bone plug from the graft anatomically on the back of the tibia (inlay), preventing formation of an acute angle at the tibial attachment site. We used six pairs of human cadaver knees to compare the biomechanical properties of these two techniques. One knee from each pair underwent tunnel reconstruction while the other knee underwent inlay reconstruction. There was significantly less anterior-posterior laxity in the inlay group when compared with the tunnel group from 30 degrees to 90 degrees of knee flexion and after repetitive loading at 90 degrees of knee flexion. Evaluation of the grafts revealed evidence of mechanical degradation in the tunnel group but not in the inlay group. The inlay technique resulted in less posterior translation with less graft degradation than did the tunnel technique for posterior cruciate ligament reconstruction.

Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out.

BACKGROUND CONTEXT: Biomechanical studies show that bone-mineral density, pedicle morphology, and screw thread area affect pedicle screw pullout failure. The current literature is based on studies of cylindrical pedicle screw designs. Conical screws have been introduced that may provide better "fit and fill" of the dorsal pedicle as well as improved resistance to screw bending failure. However, there is concern about loss of fixation if conical screws must be backed out after insertion. PURPOSE: To determine that conical screws have comparable initial stiffness and fixation strength compared with standard, cylindrical screws, and to assess whether conical screw fixation deteriorates when screws are backed out from full insertion. STUDY DESIGN/SETTING: This biomechanical analysis compared pullout strength of cylindrical and conical pedicle screw designs, using porcine lumbar vertebrae in a paired testing format. METHODS: Porcine lumbar vertebrae were instrumented with conical and cylindrical pedicle screws with the same thread pitch, area and contour, and an equivalent diameter at the pedicle isthmus, 1.2 cm distal to the hub. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads, work and stiffness were recorded at 0.02-second intervals. Conical versus cylindrical screws were tested using three paired control configurations: fully inserted, backed out 180 degrees and backed out 360 degrees. Fully inserted values were compared with each set of back-out values to determine relative loss of fixation strength. Screw pullout data were analyzed using a Student's t test. RESULTS: Pullout loads in these porcine specimens were comparable to data from healthy human vertebrae. Conical screws provided a 17% increase in the pullout strength compared with cylindrical screws (P<.10) and a 50% increase in initial stiffness (P<.05) at full insertion. There was no loss in pullout strength, stiffness or work to failure when conical or cylindrical screws were backed out 180 or 360 degrees from full insertion. CONCLUSIONS: Conical screws offer improved initial fixation strength compared with cylindrical screws of the same size and thread design. Our results suggest that appropriately designed conical screws can be backed out 180 to 360 degrees for intraoperative adjustment without loss of pullout strength, stiffness or work to failure. Intraoperative adjustments of these specific conical screws less than 360 degrees should not affect initial fixation strength. These results may not hold true for screws with a smaller thread area or larger minor diameter.

Cranial defect repair using e-PTFE: part I. Evaluation of bone stiffness.

Autologous bone grafts are the preferred material for craniofacial reconstruction, but such procedures lead to increased operative time and bleeding, donor site morbidity, and graft resorption. The efficacy of expanded-polytetrafluoroethylene (e-PTFE) sheets to increase bone regeneration and remodeling in cranial defects using a rabbit model was evaluated by mechanical testing. New Zealand white rabbits were divided into 3 groups and sacrificed 6 months after surgery. In the Split Table group, (n = 16), a bilateral bone defect was created on the outer table of the parietal bones. In the Full Table group, (n = 16), a bilateral defect was created through both the inner and outer table of the cranium. The control group, (n = 10) was subjected to a sham operation. Indentation testing was performed to determine the stiffness of newly formed bone in and around the defect. Near the center of the defect, Split Table defects repaired with e-PTFE resulted in significantly stiffer bone than regenerated control bone. The Full Table defects repaired with e-PTFE also resulted in bone significantly stiffer than control regenerated bone around the central region of the defect. The data supports the hypothesis that e-PTFE improves the repair of cranial defects in a rabbit model. It is surmised that the porosity of the e-PTFE provides a stable scaffold for migration of tissue regenerating cells, which may be preferentially localized near the cranial suture lines. This porosity may also provide a barrier to fibrous tissue regenerating cells.

Biomechanical evaluation of dual-energy X-ray absorptiometry for predicting fracture loads of the infant femur for injury investigation: an in vitro porcine model.

OBJECTIVE: The purpose of this study was to determine the ability of bone mineral density (BMD) measured by dual-energy x-ray absorptiometry (DXA) and geometry measured by biplanar x-ray to predict fracture mechanics in vitro in an immature femur model. DESIGN: Prospective analysis of radiographic and biomechanical data was performed. SETTING: In vitro experimentation. INTERVENTIONS: Bone geometry and DXA data were obtained before mechanical testing. Twenty-two porcine femora from males and females (age 3 to 12 months; body weight 3.6 to 7.0 kilograms) were fractured. Mechanical tests were performed on the diaphysis of the femora in two loading configurations: (a) three-point bending to simulate loads that result in transverse fractures; and (b) torsion to simulate twisting injuries that result in spiral fractures. MAIN OUTCOME MEASURES: Correlation of radiographic data with the experimentally determined bone strength. RESULTS: Three-point bending consistently resulted in transverse fractures. Femoral diaphysis BMD (mean, 0.304 grams per square centimeter; SD, 0.028 grams per square centimeter) strongly correlated (r2 = 0.938) to fracture load in bending. Load at failure ranged from 530 to 1,024 N (mean, 726 N; SD, 138 N), consistent with the findings of Miltner. Empirically derived strength parameters coupling BMD with geometry accurately predicted bending loads (r2 = 0.84, p < 0.001) and energy to failure (r2 = 0.88, p < 0.05). Torsional loading failed to generate spiral fractures consistently, resulting in either end plate or diaphyseal fractures. Load at failure for torsion ranged from 1,383 to 3,559 Newton-millimeters (mean, 2,703 Newton-millimeters; SD, 826 Newton-millimeters). Because of these inconsistent fracture results, empirical strength parameters for torsion could not be derived. CONCLUSION: BMD coupled with geometry is a strong predictor of bending fracture loads in the immature femoral diaphysis. A similar relationship could not be shown for torsion because of inconsistent failure results. This study represents an initial attempt at developing a methodology for predicting the strength of young bones from radiographic measures. Further research is required to establish this methodology and to show the necessary correlation with immature human bone.

Characterization and application of thin film pressure sensors.

Data regarding intra-joint loads during range of motion is essential to understanding normal joint mechanics and pathology. The investigators configured and characterized the response of a 440 N range, 0.076 mm thick commercial thin film sensor to monitor joint loads through a range of motion. Following preconditioning, static and dynamic tests were performed to evaluate the sensor response under varied environmental conditions. Both tests utilized a fixture to align the sensor and applied load. Under conditions that included dry, wet, folded and kinked configurations, a static load of 100 N was applied and the sensor output monitored up to thirty minutes from the time of loading. A load of 400 N at 50 N/s was applied to the sensor to determine dynamic characteristics and calibration curves under the conditions described for static tests in addition to curved, hard and soft contact surfaces and sensor overload. Static kinked data was significantly different from the dry, wet and folded conditions. Dynamic data showed that inter-package variability was not significant but that differences between sensor packages and sensor configuration conditions were significant. To investigate applicability of these sensors to the field of orthopaedics, a cadaveric knee was instrumented with sensors to examine the role of the meniscus in load transmission and distribution across the knee. The sensors were placed bilaterally below each meniscus on the anterior, posterior and center of the tibial plateau. Sensor data were obtained at these locations during manually flexed range of motion for the intact, re-attached and lateral menisectomized conditions of the knee specimen.

Biomechanical testing of the lumbosacral spine.

OBJECTIVE: To assess various forms of anterior and posterior sacral fixation and to study the influence of anterior lumbosacral fixation and posterior pedicle fixation at L5 in conjunction with lumbosacral fixation. SUMMARY OF BACKGROUND DATA: Moments at the lumbosacral junction are high in the long constructs requiring lumbosacral fixation. The purpose of this study was to assess bending moments in flexion-extension and lateral bending and rotational forces at the lumbosacral junction involving a variety of long constructs to the lumbosacral junction. The incidence of pseudarthrosis in such constructs in the adult spine literature ranges from 7% to 40%. METHODS: An alignment jig was designed to display three-dimensional motion in the three orthogonal planes. Nine constructs of five specimens each were tested. These consisted of fixation at T12-L5-S1 (construct 1), T12-L5-S1 with anterior L5-S1 fixation and grafting (construct 2), T12-L5-S1, S2 with and without L5-S1 fixation and grafting anterior (constructs 3 and 4, respectfully), T12-S1, S2 with and without L5-S1 anterior grafting and fixation (constructs 5 and 6, respectfully), T12 Jackson intrasacral fixation with or without L5-S1 grafting anteriorly at the anterior fixation (constructs 7 and 8, respectfully), and T12-L5-S1, S2 fixation with anterior grafting only (construct 9). RESULTS: The use of anterior fixation statistically increased stiffness in extension. There was a trend toward increasing stiffness in constructs with anterior fixation (two anterior anterior-oblique L5-S1 screws) and in other loading modes as well. Failure to use L5 screw fixation significantly decreased torsional rigidity in long constructs without anterior fixation. CONCLUSIONS: In long constructs, particularly in scoliosis surgery requiring lumbosacral fixation, the addition of anterior fixation at L5-S1 is recommended. The addition of L5 fixation in addition to sacral fixation significantly decreases rotational stresses and is recommended as well.

Biomechanical comparison of antegrade and retrograde nailing of humeral shaft fracture.

A pair-controlled study was conducted to compare biomechanical properties of antegrade and retrograde nailing of humeral fractures. First, six paired fresh anatomic specimen humeri were used to compare the properties of humeri fractured at the middle to distal diaphyses junction that were nailed from the retrograde approach with the Humeral Locked nail with those of contralateral intact humeri. An 18 additional pairs were divided into three equal groups by distal, proximal, or mid-diaphysis location of a standardized 5-mm bone defect to simulate unstable fractures. The retrograde and antegrade nailings were performed in each pair in a random manner. Nail and bone constructs were tested for bending stiffness by nondestructive three-point bending and for torsional stiffness by destructive torsional tests. Compared with intact humeri, fractured humeri fixed with nails had 28.6% posteroanterior and 31.4% mediolateral bending stiffness, 22.5% torsional stiffness, and 43.3% failure torque. For distal fractures, retrograde nailing showed significantly more initial stability and higher bending and torsional stiffness; for proximal fractures, antegrade nailing showed similar properties. For middle to distal diaphyses junction fractures, retrograde and antegrade nailing were indistinguishable. The defect created as an entry portal for retrograde nailing reduced the bone strength only 11.1%. These results suggest that retrograde nailing, which is less detrimental to shoulder function than is antegrade nailing, is an acceptable alternative treatment for humeral shaft fractures. In addition, nailing from the short to the long bone segments can improve mechanical properties of the fixation construct because of better nail and bone interface purchase.

A comparison of methods of repairing the symphysis pubis in bladder exstrophy by tensile testing.

OBJECTIVE: To compare the efficacy of several fixation techniques in the reconstruction of diastasis of the symphysis pubis in bladder exstrophy. MATERIALS AND METHODS: The symphysis of 32 pelves removed from piglets about 1 month old were disrupted and repaired using one of eight methods. After repair, each pelvis was tested biomechanically for load-to-failure, stiffness and energy-to-failure. The various repair techniques were compared with one another and to a group of six pelves tested intact. RESULTS: Four of the methods tested, including a #2 nylon suture placed through bone in a horizontal mattress arrangement, several loops of #2 nylon suture tied around the pubes, Mersilene tape tied around the pubes, and Mitek G-II suture anchors placed into the superior pubic rami, showed the highest stiffness and load-to-failure. All methods were very weak compared with the intact symphysis; the best load-to-failure (#2 nylon horizontal mattress suture) was less than half of that for intact bone, and the best stiffness (mersilene tape) was less than one-third that for the intact symphysis. CONCLUSION: The repairs varied greatly in the variables tested and those which are most promising merit further investigation to assess methods of improving their performance.

Locking strength of Morse tapers used for modular segmental bone defect replacement prostheses.

Mechanical testing has been performed to characterize the locking strength of Morse taper locks used for reconstruction of large bone defects. Taper joint pairs were locked with a series of compressive loads increasing from 500 to 3500 N. Following each load application the taper locks were distracted with either an axial load or a torsional load. Additional tapers were loaded with 2 million cycles of axial compression or 2 million cycles of cantilever bending combined with axial compression, followed by axial distraction. The torsional and axial distraction loads increased linearly with the compressive load. Compared to a single compressive load application, cyclic axial loading had little influence on the joint strength, while a combination of axial loading and bending increased the joint strength. Based on these results, in vivo loading should increase the locking strength of Morse taper locks used for bone defect reconstruction.

Isolation and characterization of metallic wear debris from a dynamic intervertebral disc prosthesis.

A dynamic intervertebral disc prosthesis (DIDP) has been developed. It consists of a CoCrMo body and uses Ti6Al4V springs to replicate the mechanical function of the lumbar joint. Wear studies have been performed previously on the DIDP using two specialized simulators to test the wear properties of the moving parts of the disc prosthesis. A pin-in-slot simulator generates wear that would occur in the hinge-pin assembly of the prosthesis. A spring-in-pocket simulator approximates the conditions under which the springs would wear against the body of the prosthesis. The spring-pocket interface is responsible for the production of approximately 90% of the total wear occurring in the prosthesis, and is therefore the main focus of this study. Bovine serum with a preservative has been used as a lubricant in both simulators. The spring-in-pocket simulator compares the effects of two different manufacturing techniques of CoCrMo (HIPing and forging) on their wear characteristics against Ti6Al4V springs. Debris from the spring-in-pocket simulator has been isolated from the serum lubricant and characterized using scanning electron microscopy techniques. The morphology of the Ti6Al4V fragments is rough and irregularly shaped. The size of these fragments ranges from < 1 microns to > 30 microns. The forged CoCrMo alloy debris has an irregular polyhedral shape, with sizes in the same range as the spring fragments. The morphology of the HIPed CoCrMo debris is spherical with a size range < 5 microns to > 30 microns. Length and width measurements of micron-size particles were made with the particle measurements feature of the scanning electron microscope. Micron-size particles were found in all stations. This article provides a unique way to isolate and analyze debris from serum lubricants used in simulators.

Biomechanical analysis of screw load sharing in pedicle fixation of the lumbar spine.

Segmental fixation of the spine by means of rods or plates and pedicle screws posteriorly usually results in a stable and rigid construct. The magnitude of the stresses on the instrumentation particularly at the bone-screw and rod (or plate) interfaces will depend on the load distribution between the bone and the implant as well as the number of sites of bone purchase of the implant. If a fusion is to be obtained in the case of a degenerative spine, the role of the instrumentation is to prevent translation of the motion segments, thereby allowing the compressive loads to be transmitted through the vertebral bodies and the degenerated discs. In the case of a fracture, the instrumentation is made to withstand the bulk of the loads since the structural integrity of a motion segment may have been lost. This study was undertaken to evaluate the effect of different constructs on the stresses in Cotrel-Duboussett (C-D) pedicle (tulip) screws close to their junction with the rod bridging the motion segments to be immobilized. In order to minimize the effect of anatomical and material property variation between spines, adult porcine spines were used, thus providing a reproducible experimental model. Fresh 3-year-old porcine spines were potted in holders after the soft tissues were removed (with the exception of the ligamentous structures).(ABSTRACT TRUNCATED AT 250 WORDS)