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.

Characterization of hamstring reflexes during anterior cruciate ligament disruption: in vivo results from a goat model.

The existence of an anterior cruciate ligament-hamstring reflex arc, the extent to which these reflexes can protect the knee, and the extent to which they are affected by rupture of the anterior cruciate ligament remain controversial. We evaluated the temporal components of the anterior cruciate ligament-hamstring synergy by simulating an injury to the ligament in a goat model. Reflexive hamstring activation in anesthetized goats was evaluated when the anterior cruciate ligament was loaded with static subfailure, dynamic subfailure, and dynamic failure loads. Reflexive hamstring activation was not found in response to static subfailure loading but was observed in response to dynamic subfailure and failure loading. The latency of the reflex evoked by dynamic failure loading was shorter than that evoked by dynamic subfailure loading. The findings suggest that the extent to which the hamstring reflexes can protect the knee may be bounded by the ability of these muscles to generate force rapidly and the amplitude and time-course of the loads applied to the knee joint. The present data present a framework for further investigation of the contribution of anterior cruciate ligament-hamstring reflexes to the stability of the knee joint under high loads and loading rates.

Influence of stability and mechanical properties of a spinal fixation device on production of wear debris particles in vivo.

A prospective and quantitative animal study was performed to evaluate the production of wear particles from a spinal fixation device, and to test the hypothesis that the concentration of wear debris particles adjacent to spinal fixation hardware is correlated with the stiffness of the spinal fusion construct and local bone formation at the fusion site. An established canine segmental spinal fusion model with three interfacet fusions was used in this study. Several bone substitute materials were grafted to the area of the interfacet fusion. Internal fixation was performed on both sides of the spinous processes at each site using a stainless steel plate system in 19 dogs. After 12 weeks, spinal segments were excised, then 3-dimensional computerized tomography was used to measure bone volume and bone area of the individual fusion sites. The stiffness of each segment was tested using a servohydraulic materials testing machine. Biopsies were obtained from the soft tissues immediately around the plate system, and wear particles were collected and characterized using an electrical resistance particle analyzer, light and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX). Biopsies from para-spinal tissue from adjacent, unoperated spinal levels served as negative controls. Histologically, 24 of 57 specimens (42.1%) showed only fibrous tissue with no recognizable macrophages, inflammation, or debris. Fourteen of 57 specimens (24.6%), however, contained many particles that were composed of Fe, Cr, and Ni, corresponding to elements found in the fixation hardware. Another 19 specimens showed only occasional particles. The mean concentration of particles from the tissue around the plate system was 2.8 x 10(9) per gram dry tissue weight, compared to 0.5 x 10(9) particles per gram for controls (p < 0.05). Statistical analyses showed significant inverse correlation between the log particle number and stiffness (r = -0.41, p < 0.01), bone volume (r = -0.28, p < 0.05), and bone area (r = -0.34, p < 0. 05) of the corresponding segments. The concentration of particles in the tissue showed a significant inverse correlation with stiffness, bone volume, and bone area of the fusion constructs.

Cell, matrix changes and alpha-smooth muscle actin expression in repair of the canine meniscus.

Processes in the repair of a crevice in the knee joint meniscus were investigated in 10 dogs. Two 2-mm cylindrical plugs from each medial meniscus were removed, rendered acellular by freezing and thawing, and then reinserted into the meniscus. Dogs were euthanized at intervals of 3-52 weeks after surgery. The crevice between the plug and meniscus at 3 weeks after surgery was filled with a tissue containing alpha-smooth muscle actin-positive cells. One year after surgery, the plug had remodeled and was populated with spindle-shaped and fibrochondrocyte-like cells. The plug had an appearance intermediate between that of hyaline and fibrocartilage at this time, with a seamless integration in sites between the remodeled plug and the surrounding meniscus. alpha-smooth muscle actin-positive cells were concentrated at the interface of the remodeled plug and adjacent meniscus and at the surface of the plug. Therefore, remodeling of both the plug and meniscal tissue and the participation of alpha-smooth muscle actin-positive cells appear essential for integration of the plug into the adjacent meniscal tissue. Cells in the superficial zone of the meniscus seem to be active in the repair process. A change in both the phenotype of the cells and the quality of the matrix toward a more hyaline state appears to be an integral part of the remodeling process in the meniscus.

Experimental wound healing with electrical stimulation.

The effect of alternating current (AC) and direct current (DC) stimulation was studied on experimental pressure ulcer healing in a new monoplegic pig model. The study was conducted in 30 healthy young Hanford minipigs. The rate of wound healing, histology, vascularization, collagen formation, microbiology, perfusion, and the mechanical strength of the healed wounds were studied. Normal pigskin was compared to denervated control and denervated AC and DC stimulated healed skin. Hind limb denervation was by right unilateral extradural rhizotomies from the L2 to S1 nerve roots. Reproducible uniformly controlled Stage III or higher tissue ulcers were created. When compared to the control wounds, both the AC and DC stimulated wounds showed reduced healing time and increased perfusion in the early phases of healing. DC stimulation reduced the wound area more rapidly than AC, but AC stimulation reduced the wound volume more rapidly than DC. The electrical stimulation did not reduce the strength of the healing wounds below those of the nonstimulated controls. The applied current appears to orient new collagen formation even in the absence of neural influences.

Spin doctors: new innovations for centrifugal apheresis.

The preparation of plasma from blood has a long history dating back to the early 1900s when the concept of blood washing replaced the traditional blood letting. Over the next 57 years landmark discoveries such as centrifugal and membrane filtration systems led to different and rapid plasma, solute, and cell separation. These were not singular events but rather events influenced by the converging chemical, physiological, and engineering advances that have characterized the latter half of the 20th century. These events have led to entire new fields of biomedical research. The biotechnology for on-line plasma separation and plasma treatment has opened a new era, expanding the application of extracorporeal technology to modern therapeutic medicine. The association of biochemical or cellular abnormalities with various disease states provides the rationale for therapeutic plasma exchange (the removal of large amounts of patient's plasma, alone or with replacement with crystalloid) and therapeutic cytopheresis (removal of cellular elements). The purpose of this review is to provide a historical picture of the innovative ideas of the spin doctors and their devices, which predate the centrifugal blood and cell separators commonplace to any hospital or blood bank worldwide. The emphasis is to define the historical events and their impacts on the development of centrifugal devices and apheresis technologies.

Effects of age, density, and geometry on the bending strength of human metatarsals.

In this basic study, we investigated the relative roles of donor age, bone density, and bone geometry in determining structural strength of human metatarsals tested in a four-point bending configuration. Density measurements were made noninvasively using dual energy X-ray absorptiometry, and geometric measurements were made by digitally imaging cross-sections of specimens. Correlations between area bone mineral density and metatarsal strength were strong (r2 = 0.83, 0.81 for second and third metatarsals, respectively) and were not improved by including cross-sectional area or minimum moment of inertia in multiple regression analyses. Increased donor age was associated with decreased bending strength (r2 = 0.51 and 0.45, respectively), which was expected because increased age correlated significantly with decreased bone density (r2 = 0.69 and 0.80, respectively). These results indicate that the strength of human metatarsals generally decreases with age and that this decrease is likely attributable to decreased bone density. Moreover, the results indicate that noninvasive dual energy x-ray absorptiometry measurements of metatarsal density are useful for assessing metatarsal strength and that additional measurements of bone geometry are not required.

Patella-tibial transfixation for posterior cruciate ligament repair and reconstruction: a biomechanical analysis.

The posterior cruciate ligament (PCL) restricts posterior translation of the tibia on the femur. Because flexion of the knee increases tension on the PCL, the knee is usually immobilized in extension after PCL repair or reconstruction. Patella-tibial transfixation (olecranization), however, has been proposed to reduce the tension on the PCL without requiring immobilization of the knee. The objective of this study was: (1) to evaluate the distribution of strain in the anterolateral and posterior oblique fiber bundles of the PCLs in eight cadaveric knees before and after olecranization and (2) to measure the patellofemoral contact pressures at various degrees of knee flexion. Olecranization significantly (P < 0.05) reduced the strain on the anterolateral fiber bundles of the PCL at 15 degrees -45 degrees of flexion. No significant strain reduction was observed in the posterior oblique fiber bundles. Patellofemoral contact pressures measured from digitized Fuji sensitive film indicated significantly increased contact pressures (P < 0.05) following olecranization from 0 degrees -60 degrees of knee flexion. Increased parapatellar soft tissue tightness limited knee flexion to 90 degrees and patella lift-off occurred at 75 degrees. Although olecranization of the patella does reduce strain on the intact PCL within a selected range of motion, the beneficial effect of allowing early motion may be negated by the potentially harmful effects imposed upon the patellofemoral articular cartilage by increased contact pressures.

Biomechanical analysis of canine bone lengthened by the callotasis method.

The mechanical integrity of tibial bone treated by distraction osteogenesis (callotasis) using an Orthofix external fixator was evaluated in a group of 6 dogs. Diaphyseal corticotomies were performed on Day 0. Distraction was begun on Day 8, at a rate of 0.33 mm every 8 hours. On Day 34, the Orthofix frame was locked. On Day 56, the frames were removed and the dogs were euthanized. Radiographs were taken weekly to assess regenerate bone formation and extent of tibial bone lengthening, which averaged 12 mm (range, 5-20 mm). One dog showed incomplete bony union that was attributed to pin loosening. Torsional testing of lengthened and contralateral control tibiae resulted in spiral diaphyseal fractures. Five of 6 lengthened tibiae bones fractured within the diaphysis, but outside the regenerate area, suggesting that the strength of the lengthened segment exceeded that of the original diaphysis which was exposed to stress shielding by the external fixator. These observations have led to a method to test and evaluate lengthened bone at the time of frame removal and to monitor the effectiveness of this type of technique.

Influence of AC and DC electrical stimulation on wound healing in pigs: a biomechanical analysis.

To evaluate the effects of electrical stimulation on the mechanical properties of healing skin, 20 Hanford mini-pigs weighing 10-15 kg with trochanteric pressure ulcers were subjected to electrical stimulation. Examination of the biomechanical properties of the skin and changes in wound area and volume was done on previously wounded and healing pigskin subject to AC or DC electrical stimulation. The behavior of normal pigskin was compared to (1) denervated controls, (2) denervated AC-stimulated skin, and (3) denervated DC-stimulated skin. A denervated limb trochanteric pressure sore model developed in house permitted the use of a 6.5-mm percutaneous cancellous screw for wound formation and a 3-cm-diameter spring compression indentor to create reproducible and uniformly controlled grade 3 or higher tissue ulcers in the monoplegic hind limbs. Denervation was accomplished by right unilateral extradural rhizotomies from L2 to S1 nerve roots. Electrodes were placed 1 cm distal and proximal to the wound periphery, and wounds were stimulated 2 h/day, 5 days/week for 30 days. Dumbbell-shaped skin specimens with a length to width ratio of 3:1 were uniaxially loaded in tension until failure at an extension rate of 150 mm/min. The stiffness values for skin samples oriented parallel to the current flow were reduced by nearly half the values obtained for normal controls. Statistical differences (P < .05) were found for stress, Young modulus, and stiffness when compared to normal skin. Samples oriented in the perpendicular direction were comparable to normal skin (P = NS).(ABSTRACT TRUNCATED AT 250 WORDS)

A model for evaluating the strength of bones lengthened by distraction osteogenesis.

In an effort to evaluate the strength of bones lengthened by distraction osteogenesis, a modified Ilizarov-type external fixator was applied to the right tibiae of five adult mongrel dogs. Diaphyseal corticotomies were performed, and distraction (lengthening) was carried out at a rate of approximately 1 mm per day for 20 days. After completion of the lengthening, the animals were killed, the fixators were removed, and the right and left tibiae were harvested and stripped of soft tissue. The ends of all harvested bones were fixated in polymethylmethacrylate to allow testing on a materials testing system (MTS) machine. Testing under torsional loads was performed on the MTS machine (50 degrees maximum angle of rotation at ten seconds) until failure. The right tibiae were lengthened an average of 12 mm (range, 7-15 mm). The slopes of the linear portion of the torque versus rotation curves were determined. A comparison of slopes revealed a mean change in stiffness between test and control bones of 20.32% (range, 0.64%-40.5%). All tibiae exhibiting mature regenerate bone demonstrated stiffness greater than the matched controls (range, 0.64-24.09% stiffer). Four of five dogs had problems with proximal and distal ring impingement. This study presents a reproducible model for evaluating the strength of bones lengthened by distraction osteogenesis. The lengthened bones that exhibited bony or mature regeneration were as strong as the control bones.

Effects of design geometry of intravascular endoprostheses on stenosis rate in normal rabbits.

To investigate the effects of gaps between the individual wire coils of a shape memory Nitinol alloy intravascular endoprosthesis, 20 stents with and without gaps were implanted transluminally into the infrarenal abdominal aortas of 10 normal rabbits after balloon angioplasty. Digital subtraction angiography (DSA) was done at 4, 12, 16, 20, and 24 weeks after stent implantation to examine the stenosis rate and major side branch patency. Stenosis rate within stents with gaps were significantly lower than those without gaps: 8.1 +/- 5.0% versus 15.0 +/- 6.8% at 12 weeks, 13.6 +/- 6.0% versus 26.0 +/- 9.4% at 24 weeks by DSA, p greater than 0.005 and p less than 0.01 respectively. The maximum and mean neointimal thickness measured histologically at the time of animal death correlated significantly to the narrowest diameter obtained from the DSA studies, (r = 0.84 and r = 0.79, respectively, p less than 0.01). Greater hyperplasia of the neointima was evident in the stented arterial segments with stents without gaps compared with those with gaps (83 +/- 22 versus 187 +/- 46 microns mean thickness, p less than 0.001). The patency rate of the side branches in the stented arterial segment was significantly (p less than 0.05) higher in stents having gaps. These results suggest that the placement of gaps between wire pitches reduced the neointimal thickness within stents and prevented the obstruction of arterial side branches.

Vascular stenting in normal and atherosclerotic rabbits. Studies of the intravascular endoprosthesis of titanium-nickel-alloy.

Percutaneous transluminal balloon angioplasty would be more effective if the rate of recurrent stenosis were reduced. To evaluate the prevention of restenosis after percutaneous transluminal angioplasty, intravascular endoprosthetic stents of titanium-nickel-alloy were implanted transluminally in seven normal and 21 atherosclerotic rabbits. In normal rabbits, a 3.5-mm diameter stent was implanted in the aorta and a 2.5-mm diameter stent in the right iliac artery, which were followed with serial angiograms from 6 weeks (n = 7) to 8 months (n = 4). There was a mean stenosis of 13.1% in the 2.5-mm and 13.6% in the 3.5-mm stent. There was no significant narrowing compared with the adjacent control segments of artery; histopathology showed a thin, fibrous neointima with smooth muscle cells. Each atherosclerotic rabbit was balloon dilated at two separate stenotic sites; each site was 2.0 cm in length. The aortic site (with 28.8 +/- 13.8% mean stenosis [+/- SD]) was dilated with a 3.5-mm balloon, and the iliac site (with 36.5 +/- 14.2% stenosis) was dilated with a 2.5-mm balloon. In each site, an intravascular stent of corresponding diameter and 7-mm length was implanted in one half of the dilated segment, assigned randomly, and the other half served as the angioplasty control. Angiographically observed restenosis rates and the corresponding histopathology were similar in the atherosclerotic segments that had angioplasty alone versus the atherosclerotic segments that had angioplasty plus stenting. The mean neointimal thickness in the aortas and iliac arteries, respectively, measured 247 +/- 181 microns (+/- SD) and 218 +/- 77 microns after 6 weeks (n = 8) versus 321 +/- 168 and 308 +/- 189 microns after 20 weeks (n = 5, p = NS). At 20 weeks follow-up, there was 29.1 +/- 29.8% (median, 16.4%) stenosis in the aortic stent versus 38.9 +/- 24.1% (median, 34.0%) stenosis in the percutaneous transluminal angioplasty control segment of aorta (n = 5, p = NS) and 81.4 +/- 25.5% stenosis in the iliac artery stent versus 89.3 +/- 15.3% stenosis in the PTA control segment of the right iliac artery (n = 5, p = NS). Comparing stenotic arterial segments treated with angioplasty alone with angioplasty plus intravascular stenting in the atherosclerotic rabbits showed that there was no significant difference in either the histopathologic changes or the restenosis rates.

Intravascular endoprostheses. Effect of surface geometry on restenosis and side branch patency.

To investigate the effects of gaps between the wire pitches of a shape memory Nitinol alloy intravascular endoprosthesis (IVEP), 42 stents with and without gaps were implanted transluminally into the infrarenal abdominal aortae of 11 normal and 10 atherosclerotic rabbits. Digital subtraction angiography was done every 4 weeks to examine the restenosis rate and major side branch patency. Restenosis rates within IVEPs with gaps were significantly lower than in stents without gaps in both normal and atherosclerotic rabbits. The side branch patency rate was significantly higher with IVEPs having gaps. These results suggest that the placement of gaps between wire pitches reduced the restenosis rate within IVEPs, and prevented the obstruction of major side branches.