Stability of proximal femoral grafts in canine hip arthroplasty

In a canine model, the fixation stability of a prosthesis and proximal bone graft composite were measured relative to the distal femur. One group had the prosthesis graft composite cemented into the distal femur. The second group had the prosthesis graft composite press fit into the distal femur for biologic ingrowth. Displacements of the proximal femoral grafts relative to the host bone in each group were measured after ex vivo (acute with graft) implantation and 4 months after implantation. A third group with no osteotomy (acute intact) simulated perfect graft to host bone union. Relative displacements representing 6 degrees freedom (translation and rotation) were calculated from the displacement values measured by 9 eddy current transducers. Measurements of displacement were used to test the hypothesis that distal press fit fixation equals distal cement fixation at 4 months after implantation. In all cases the measured translations and rotations of the graft to implant construct were small and of a magnitude that should encourage bone ingrowth (< 0.05 mm and < 0.1 degree, respectively). The stability of the press fit group at 4 months was not significantly different from the cemented group in axial and transverse displacement during axial and transverse loading, respectively. There was no difference in stabilities at 4 months between distal press fit and cemented fixation in hip replacements requiring a proximal femoral graft.     

Subsidence of an uncemented canine femoral stem

Factors contributing to subsidence were analyzed by radiographic evaluation and mechanical testing of 36 canine cadaver femora during and after insertion of an uncemented porous-coated femoral stem and by radiographic evaluation of 35 canine total hip arthroplasties. Mean percentage of canal fill in immediate postoperative radiographs, and percentage of canal fill at midimplant and distal implant locations, were accurate predictors of subsidence. Force required to implant the femoral stem was strongly correlated with force required for implant subsidence. Femoral morphology and percentage of canal fill at the middle and distal sites were accurate predictors of subsidence. Implants in femora with a stovepipe morphology (canal flare index less than or equal to 1.8) were six times more likely to subside than implants in femora that had a normal appearance (canal flare index 1.8 to 2.5), and 72 times more likely to subside than implants in champagne-fluted femora (canal flare index greater than or equal to 2.5). Femora with more than 85% mean, middle, or distal canal fill were less likely to subside.     

Fixation of humeral prostheses and axial micromotion

Surgeons often avoid cementing a proximal humeral prosthesis. Occasionally bony augmentation is needed. This study was undertaken to compare proximal cementation in combination with distal press with total cementation or press fit alone. In phase 1 axial micromotion with axial loading was measured in 15 pairs of humeri: 5 fully cemented versus proximally cemented, 5 fully cemented versus press fit, and 5 proximally cemented versus press fit. X-ray films of the specimens were obtained to assess canal fill. In phase 2 axial micromotion was measured in 5 pairs of high mineral density and 5 pairs of low mineral density to compare proximal cementation with press fit. The 3 M modular prosthesis was used in both phases. No difference was found in phase 1 among the 3 fixation techniques. A strong reverse statistical correlation (P = .007) (r = .55) was seen between axial micromotion and fill of the canal with the prosthesis. In the second phase no statistically significant difference was found between the techniques of fixation or between the 2 bone densities. Fill of the canal at the distal end of the prosthesis was the only variable found that affected axial micromotion, but it accounted for only approximately 30% of the variance. Bone quality and augmentation of the proximal bone with cement did not affect axial micromotion in this experiment but warrant further study.     

Cubital tunnel syndrome pathophysiology

Cubital tunnel syndrome is the second most common peripheral compression neuropathy. The unique anatomic relationships of the ulnar nerve at the elbow place it at risk for injury. Normally with elbow range of motion, the ulnar nerve is subjected to compression, traction, and frictional forces. As the elbow is flexed the arcuate ligament elongates producing a decrease in canal volume of 55%. Intraneural and extraneural pressures increase and have been shown to exceed 200 mm Hg with elbow flexion and flexor carpi ulnaris contraction. Because the ulnar nerve courses behind the elbow axis of rotation, elbow flexion produces excursion of the nerve proximal and distal to the medial epicondyle. The ulnar nerve also elongates 4.7 to 8 mm with elbow flexion. Cubital tunnel syndrome may develop because of various factors including repetitive elbow motion, prolonged elbow flexion, or direct compression. An understanding of the anatomy and pathophysiology associated with cubital tunnel syndrome will aid in patient evaluation and determination of the appropriate treatment.     

The transepicondylar axis approximates the optimal flexion axis of the knee

The traditional understanding of knee kinematics holds that no single fixed axis of rotation exists in the knee. In contrast, a recent hypothesis suggests that knee kinematics are better described simply as two simultaneous rotations occurring about fixed axes. Knee flexion and extension occurs about an optimal flexion axis fixed in the femur, whereas tibial internal and external rotations occur about a longitudinal rotation axis fixed in the tibia. No other translations or rotations exist. This hypothesis has been tested. Tibiofemoral kinematics were measured for 15 cadaveric knees undergoing a realistic loadbearing activity (simulated squatting). An optimization technique was used to identify the locations of the optimal flexion and longitudinal rotation axes such that simultaneous rotations about them could best represent the measured kinematics. The optimal flexion axis was compared with the transepicondylar axis defined by bony landmarks. The longitudinal rotation axis was found to pass through the medial joint compartment. The optimal flexion axis passed through the centers of the posterior femoral condyles. No significant difference was found between the optimal flexion and transepicondylar axes. To an average accuracy of better than 3.4 mm in translation, and 2.9 degrees in orientation, knee kinematics were represented successfully by simple rotations about the optimal flexion and longitudinal rotation axes. The optimal flexion axis is fixed in the femur and can be considered the true flexion axis of the knee. The transepicondylar axis axis, which is identified easily by palpation, closely approximates the optimal flexion axis.     

Maintenance of proximal cortical bone with use of a less stiff femoral component in hemiarthroplasty of the hip without cement. An investigation in a canine model at six months and two years

A canine model of hemiarthroplasty of the hip was used to determine if the use of a less stiff femoral stem can reduce the amount of bone loss induced by stress-shielding. Two types of stem were used: the stiffer stems were made of a titanium alloy, and the less stiff stems were composed of a cobalt-chromium-alloy core with an outer polymer layer. The stems were identical in shape, and both types were circumferentially coated along their entire length (except for the distal five millimeters) with commercially pure titanium fiber metal. Ten dogs with each type of stem were followed for six months, and twelve dogs with each type of stem were followed for two years. Loss of cortical bone from the proximal part of the femur was associated with both types of stem, but typically 50 per cent less bone was lost with the less stiff implants. Most of the cortical loss occurred at the subperiosteal surface. The amount of medullary bone adjacent to the proximal and distal aspects of both types of stem increased; the less stiff stems were associated with a greater increase in the proximal region, and the stiffer stems were associated with a greater increase in the distal region. Similarly, there were peaks in the amount of bone growth into the proximal and distal portions of both types of stem, with a greater peak in proximal bone growth into the less stiff stems and a greater peak in distal bone growth into the stiffer stems.     

Cement-mantle thickness affects cement strains in total hip replacement

Bone cement is commonly used to affix femoral implants to the bone during total hip reconstruction. Previous studies suggest that the expected life of a cemented femoral implant may depend on the thickness of the cement mantle surrounding the implant and the implant geometry. The purpose of this study was to determine whether different cement-mantle thicknesses and femoral stem sizes affected strain patterns in the bone cement around cemented femoral stems. Two different sizes of cobalt-chromium stems were cemented into composite femora with varying cement-mantle thickness. Strain gages were embedded in the cement mantle and the implanted stems were loaded axially and under conditions simulating walking and standing. An increase in stem size with the same cement-mantle thickness (approximately 2.2 mm) caused a 65% decrease in proximal medial cement strains. Increasing cement mantle thickness from 2.4 to 3.7 mm caused substantial strain reductions in the distal cement (40-49%). We conclude that increased cement-mantle thickness around femoral stems may increase the fatigue life of a bone-implant system by reducing peak strains within the cement.     

Cementless implant composition and femoral stress. A finite element analysis

Proximal atrophy and thigh pain are recognized problems with some cementless femoral stems in total hip arthroplasty. It is thought that reduced femoral stress from alterations in load transfer caused by an intramedullary stem contributes to proximal femoral atrophy. An increase in flexural rigidity and bone stress near the stem tip is thought to contribute to thigh pain. A three-dimensional finite element analysis study was performed to calculate stresses in the proximal femur and bone near the stem tip before and after implantation of a collared, proximally coated, cementless femoral prosthesis. The influence of prosthetic material was examined by changing implant composition from cobalt chrome to titanium alloy and leaving all other parameters constant. Femoral stress was increased twofold immediately below the collar with the titanium implant compared with the cobalt chrome. However, the proximal femoral stress in the titanium implanted model was still 1/10 that in the corresponding region of the unimplanted femur model. At the stem tip, as much as a 30% reduction in femoral stress was seen with the titanium stem compared with the cobalt chrome. These findings suggest biomechanical evidence of an advantage for titanium as an implant material compared with cobalt chrome for cementless femoral stems.     

Functional adaptation and ingrowth of bone vary as a function of hip implant stiffness

The purpose of the present study was to test the hypothesis that cortical bone loss, trabecular bone density and the amount of bone ingrowth vary as a function of stem stiffness in a canine cementless hip replacement model. The study was motivated by the problem of cortical bone atrophy in the proximal femur following cementless total hip replacement. Two stem stiffnesses were used and both designs were identical in external geometry and porous coating placement. The high stiffness stem caused approximately 26% cortical bone stress-shielding and the low stiffness stem caused approximately 7.5% stress-shielding, as assessed by beam theory. Each group included nine adult, male canines who received unilateral arthroplasties for a period of six months. The animals with the low stiffness stems tended to lose less proximal cortical bone than the animals with high stiffness stems (4% +/- 9 as opposed to 11% +/- 14), but the difference was not statistically significant (p = 0.251). However, the patterns of bone ingrowth into the implant and change in medullary bone density adjacent to the implant were fundamentally different as a function of stem stiffness (p < 0.01). Most importantly, while the high stiffness group had peaks in these variables at the distal end of the stem, the low stiffness group had peak values proximally. These different patterns of functional adaptation are consistent with the idea that reduced stem stiffness enhances proximal load transfer.     

Further studies into proximal interphalangeal joint dimensions for the design of a surface replacement prosthesis: medullary cavities and transverse plane shapes

The proximal and middle phalanges from 83 proximal interphalangeal joints (PIPJs) were set in clear plastic and sectioned in the transverse plane leaving the heads whole. The sections were cleaned, shadowgraphed and measured. The medullary canals were marked on sagittal and frontal plane shadowgraphs of the intact bones and analysed. The information was then used in the design of a surface replacement prosthesis for the PIPJs. The main dorsal surface of the proximal phalanx (PP) was found to be angled to the longitudinal baseline of the bone by a mean of 5.19 degrees. This angle increased just proximal to the phalangeal head to a mean of 11.84 degrees. The mean ratio between these angles was 2.71. The phalangeal shaft bone was thicker laterally than dorsally and palmarly, and thicker dorsally than palmarly for the proximal and middle phalanges throughout the length of the bone. The shape and size of the transverse cross-section of the medullary canal changed throughout the length of the shaft. The centreline of the PP medullary canal coincided with the midline of the bone in the frontal plane and was approximately a straight line along the length of the canal. In the sagittal plane the centreline was slightly palmar to the midline and the angle between it and the longitudinal baseline of the bone changed along the length of the canal. In the region of the shaft just proximal to the PP head (where the stem of a surface replacement prosthesis would fit) the mean angle was 10.63 degrees. The centreline was offset dorsally from the centre of rotation of the PIPJ by a mean of 0.83 mm, 0.83 mm, 0.80 mm and 0.57 mm for the index, middle, ring and little fingers respectively, with an overall mean of 0.76 mm. The mean PP head heights (transverse plane) were 9.17 mm, 9.33 mm, 8.73 mm and 7.40 mm and the mean PP widths (transverse plane) were 12.86 mm, 13.25 mm, 12.75 mm and 10.54 mm for the index, middle, ring and little fingers respectively. The mean angle between the lateral sides of the condyles to the transverse baseline was 78.35 degrees and the mean distance from the centreline of the PP head (transverse plane) to the bases of the two condyles was 4.69 mm. The mean maximum depth of the PP head intercondylar sulcus in the frontal plane was 0.72 mm and in the transverse plane, the mean maximum depth of the intercondylar sulcus on the anterior face was 0.82 mm.     

Monitoring of occupational exposure to 1-butanol by diffusive sampling and urinalysis

This study was designed to define the relationship between different combinations of prosthetic head diameters, neck lengths, and acetabular containment angles, and range of motion before component impingement. Three cadaveric pelves with attached lower limbs were mounted in their correct anatomic position. Acetabular and modular femoral stems were inserted into each of the six hips. For each combination of femoral head diameter, neck length, and acetabular liner overhang, the range of motion to impingement was measured in flexion, extension, abduction, adduction, external rotation, internal rotation, and internal rotation with 90 degrees hip flexion. These experiments suggest that the maximum range of motion before impingement can be attained by increasing the prosthetic head diameter and avoiding longer neck lengths with skirts. Acetabular liners with greater overhang decrease motion in all planes except flexion when the overhang is positioned posteriorly.     

Kinematics of cervical spine discectomy with and without bone grafting: quantitative evaluation of late fusion in a sheep model

OBJECTIVE: This study was conducted to evaluate the kinematic response of late fusion results for cervical spine discectomies with and without bone grafting. MATERIALS AND METHODS: Fifteen Barbados Black Belly sheep underwent sham operations (Group A, n = 5), C2-C3 discectomies only (Group B, n = 5), and C2-C3 discectomies with autologous iliac bone grafting (Group C, n = 5). Ten months after surgery, the animals were killed. Fresh ligamentous spines (C1-C5) were subjected to the relevantly applied loads through a loading frame attached to the C1. Each vertebra (from C2 to C4) was attached with a set of three infrared light-emitting diodes to record the spatial location relating to each load application using a Selspot II system (Selcom Selective Electronics, Inc., Valdese, NC). The load-deformation data of the C2-C3 and C3-C4 motion segments were recorded and analyzed for the three groups. RESULTS: At the C2-C3 motion segment, the results indicated that Group B displayed larger motion ranges of rotation and lateral bending loads than did the other two groups. Significantly larger motion ranges of rotation loads were found in Group B than in Group C (P<0.05, for both comparisons). In contrast, Group C had the smallest motion ranges of flexion, lateral bending, and rotation loads. At the C3-C4 motion segment, both groups that had undergone discectomies had a significantly larger motion range of flexion load compared with Group A (P<0.05, for both comparisons). A significant increase in the motion range of right axial rotation was found in Group B (P<0.05), but not in Group C, compared with Group A. Group B exhibited larger motion ranges responding to all six tested loads than did Group C. CONCLUSION: The results indicate that anterior fusion after C2-C3 cervical discectomies, regardless of the presence or absence of bone grafting, decreases the motion range of flexion load at the C2-C3 motion segment, and contrary data were seen at the C3-C4 motion segment. For axial rotation loads, discectomies without bone grafting resulted in increased motion ranges of both C2-C3 and C3-C4 motion segments whereas discectomies with bone grafting did not. The data may have clinical relevance regarding the role of bone grafting in cases of cervical spine disease.     

Neuronal death induced by SIN-1 in the presence of superoxide dismutase: protection by cyclic GMP

OBJECTIVES: To test whether femoral ostectomy level, subtrochanteric bone mass removal, and stemsize selection significantly affect stem positioning in canine total hip replacement, and to determine ability of the femoral stem component to restore geometry of the normal femoral head and neck. SAMPLE POPULATION: Femurs from 8 adult mixed-breed canine cadavers. PROCEDURE: Femurs were systematically prepared, using 8 combinations of 3 surgical preparation techniques that included level of ostectomy (cervical isthmus vs lesser trochanter), subtrochanteric bone block removal, and femoral stem size (recommended, undersized). Computer-aided analysis of specimen photographs was used to evaluate femoral head offset and position and variability of femoral stem positioning for each of the preparation combinations. RESULTS: Original femoral head offset and position were reconstructed to within a mean of 0.052 and 0.031 cm, respectively, using an undersized femoral stem after ostectomy at the level of the lesser trochanter. Implantation of an undersized femoral stem after subtrochanteric bone block removal improved ability to centralize the distal tip of the implant and reduce the angle between the femoral diaphyseal and implant axes. Ostectomy at the level of the cervical isthmus tended to force femoral implants into a varus position, and ostectomy at the level of the lesser trochanter tended to force implants into a valgus position. CONCLUSIONS: Geometry of normal canine femurs was most closely reconstructed by implantation of an undersized femoral component after ostectomy at the level of the lesser trochanter. Implantation of an undersized femoral component after subtrochanteric bone block removal resulted in the best alignment and centralization of the stem.     

Relative effects of wound healing and mechanical stimulus on early bone response to porous-coated implants

We hypothesized that early bone adaptation to well fixed porous-coated implants is influenced more by wound healing than by mechanical loading. To test this hypothesis, two groups of dogs with identical, hydraulically controlled porous-coated implants interference fit within distal femoral trabecular bone were used. One group had no load: the other had 35 N of load applied to the implants. At 5 weeks after surgery, the resulting adaptation of bone around the implants was quantified on a cellular basis by cytochemical analysis of type-I procollagen synthesis and on a structural basis using three-dimensional micro-computed tomography imaging. The percentage of trabecular surfaces covered by osteoblasts expressing type-I procollagen was significantly increased in bone surrounding the implant in both groups compared with contralateral control bone tissue. There was no difference between the groups with no load or 35 N of load. In addition, measures of trabecular bone structure did not differ significantly between the load and no-load groups. Taken together, these results suggest that wound healing plays a much greater role in the early response of bone to well fixed porous-coated implants than does mechanical stimulus.     

Hip motion and moments during gait relate directly to proximal femoral bone mineral density in patients with hip osteoarthritis

The present study examined the loads at the hip joint during gait and the bone mineral density of the proximal femur in 25 patients with end-stage hip osteoarthritis. Dual energy X-ray absorptiometry was used to determine the bone mineral density of the greater trochanter, femoral neck and Ward's triangle of the osteoarthritic group. The bone mineral density was normalized for the patient's age, gender, weight and ethnic origin (Z score). Gait analysis was used to determine the external hip joint moments and motion during walking for the osteoarthritic group and a control group of 21 normal subjects. The gait parameters of the osteoarthritic group which were significantly diminished compared to the normal group (p < 0.001) accounted for as much as 42% (p < 0.001) of the variation in the normalized bone mineral density. Specifically, the dynamic sagittal plane hip motion during gait (maximum flexion minus maximum extension) and peak external rotation and adduction moments were significantly correlated with greater trochanter (R = 0.429-0.648, p = 0.032-0.0001) and Ward's triangle (R = 0.418-0.532, p = 0.038-0.006) normalized bone mineral density while the adduction moment was also significantly correlated with the femoral neck normalized bone mineral density (R = 0.5394, p = 0.005). The normalized bone mineral density of the femoral neck and Ward's triangle was elevated while that of the greater trochanter was decreased as compared to normal reference values. The significant correlation between the hip joint moments during gait and femoral bone mineral density indicate that hip joint loads need to be included when explaining local variation in bone mineral density in hip osteoarthritis.     

Failure mechanisms of anatomic porous replacement I cementless total hip replacement

The APR-I Hip System was designed for metaphyseal bone attachment by proximal patch porous coating and the acetabular component was a hemisphere fixed with screws. The results of 100 consecutive primary total hip arthroplasties performed with the APR-I were studied in patients still alive after an average of 6.7 years. The selection of hips for implantation of this hip system was limited to those patients in whom a satisfactory intraoperative fit could be obtained. This resulted in a distribution of patients such that 75% were younger than age 65 years and only 15% had osteoporotic bone. The revision rate was 16% with a mechanical failure rate of 11%. Seventy percent of hips had progressive loss of fixation. loss of femoral component fixation was correlated with younger patient age, higher patient activity level, metaphyseal fill of less than 90%, and increased polyethylene wear and osteolysis. Eighty-nine percent of hips maintained femoral neck contact with the porous coated collar. The acetabular component was well fixed in 97% of hips. The failure rate of the APR-I stem is unacceptably high and this stem is no longer used by the authors.     

Intracellular distribution of phospholipase C gamma 1 in cell lines with different levels of transformation

PURPOSE OF THE STUDY: To assess after 83 months of follow-up, the results of 19 femoral revisions carried out according to an original method combining a cemented stem and bone reconstruction by means of impacted-morcelized bone allograft protected by a titanium mesh. MATERIALS: Twenty hips (18 patients mean aged 58 at surgery) were included between 1986 and 1991. Five hips had a least one previous prosthetic revision, one hip was revised because of septic loosening. No patient was lost for follow-up, but two had died during the follow-up period: one patient died one month after surgery was excluded, one other died 7 years after the index procedure and was included with his last hip rating. Loss of femoral bone stock was severe according to the SOFCOT four stage rating system: 2 femurs were grade II, 14 grade III, and 3 grade IV. Femoral stem migration was assessed with landmarks recommended by Walker. All the measurements were performed with a digitizer (OrthoGraphics). METHODS: All the procedures were carried out through a posterolateral approach, augmented by 4 trochanteric osteotomies and 5 distal femoral windows. After prosthesis and cement removal, a bone plug was placed into the medullary canal. Then, cancelous bone morcelized allografts were impacted in the femoral defects through the medullary canal. A titanium mesh cylinder was placed into the femur to separate the graft from the cement introduced later to obtain fixation of the revision stem. The stem was extended about 5 centimeters over the distal edge of the grafts in order to bridge the femoral defects. The mesh was extended only in front of the grafts and was used to protect them from excessive cement penetration. RESULTS: Functional improvement was noticeable since the Merle d'Aubigné Hip score improved from 9.8 to 16.3 at follow-up. The pain score improved from 2.1 to 5.5 and walking score from 2.3 to 5. Adverse effects occurred during the first cases and were related to cement removal: 3 greater trochanter fractures, 5 distal femoral perforations and 2 non displaced femoral shaft fractures. The septic revision had recurrence of infection associated with radiolucent lines > 2 millimeters and the only one graft resorption. One trochanteric non-union was observed but no prosthetic dislocation. Only one femoral stem migration (4.4 millimeters) was detected without any other radiographic features of loosening after 9 years of follow-up. This stem was considered as loosed, but was not revised because of few clinical symptoms. Only 2 radiolucent lines less than 2 millimeters at the bone cement interface in Gruen's zones 3 to 5. Likewise, no radiographic feature of stress-shielding was observed. On follow-up X-rays, 3 hips had corticalisation of the grafts, and 12 hips demonstrated normal cancelous trabeculations in the grafts. CONCLUSION: Satisfactory functional and radiographic results were obtained with this method after 5 to 10 years of follow-up instead of severe preoperative femoral bone stock impairement. Likewise, we observed only one recurrence of loosening diagnosed with the help of digitized X-ray examination. Only one significant (> 3.5 mm) femoral stem migration was detected. Radiographic features of femoral reconstruction were observed but without histologic proof of graft integration. This method uses a longer stem than the "Exeter", but avoids a high rate of femoral stem migration and appears compatible with femoral bone reconstruction.     

The effect of femoral tunnel position and graft tensioning technique on posterior laxity of the posterior cruciate ligament-reconstructed knee

We report the effects of femoral tunnel position and graft tensioning technique on posterior laxity of the posterior cruciate ligament-reconstructed knee. An isometric femoral tunnel site was located using a specially designed alignment jig. Additional femoral tunnel positions were located 5 mm proximal and distal to the isometric femoral tunnel. With the graft in the proximal femoral tunnel, graft tension decreased as the knee flexed; with the graft in the distal femoral tunnel, graft tension increased as the knee flexed. When the graft was placed in the isometric femoral tunnel, a nearly isometric graft tension was maintained between 0 degrees and 90 degrees of knee flexion. One technique tested was tensioning the graft at 90 degrees of knee flexion while applying an anterior drawer force of 156 N to the tibia. This technique restored statistically normal posterior stability to the posterior cruciate ligament-deficient knee between 0 degrees and 90 degrees for the distal femoral tunnel position, between 0 degrees and 75 degrees for the isometric tunnel position, and between 0 degrees and 45 degrees for the proximal tunnel position. When the graft was tensioned with the knee in full extension and without the application of an anterior drawer force, posterior translation of the reconstructed knee was significantly different from that of the intact knee between 15 degrees and 90 degrees for all femoral tunnel positions.     

Role of ligaments and facets in lumbar spinal stability

STUDY DESIGN: The issue of segmental stability using finite element analysis was studied. Effect of ligament and facet (total and partial) removal and their geometry on segment response were studied from the viewpoint of stability. OBJECTIVES: To predict factors that may be linked to the cause of rotational instabilities, spondylolisthesis, retrospondylolisthesis, and stenosis. SUMMARY OF BACKGROUND DATA: The study provides a comprehensive study on the role of facets and ligaments and their geometry in preserving segmental stability. No previous biomechanical study has explored these issues in detail. METHODS: Three-dimensional nonlinear finite element analysis was performed on L3-L4 motion segments, with and without posterior elements (ligaments and facets), subjected to sagittal moments. Effects of ligament and facet (partial and total) removal and their orientations on segment response are examined from the viewpoint of stability. RESULTS: Ligaments play an important role in resisting flexion rotation and posterior shear whereas facets are mainly responsible for preventing large extension rotation and anterior displacement. Facet loads and stresses are high under large extension and anterior shear loading. Unlike total facetectomy, selective removal of facets does not compromise segmental stability. Facet loads are dependent on spatial orientation. CONCLUSIONS: Rotational instability in flexion or posterior displacement (retrospondylolisthesis) is unlikely without prior damage of ligaments, whereas instability in extension rotation or forward displacement (spondylolisthesis) is unlikely before facet degeneration or removal. The facet stress and displacement distribution predicts that facet osteoarthritis or hypertrophy leading to spinal stenosis is most likely under flexion-anterior shear loading. Selective facetectomy may restore spinal canal size without compromising the stability of the segment. A facet that is more sagittally oriented may be linked to the cause of spondylolisthesis, whereas a less transversely oriented facet joint may be linked to rotational instabilities in extension.     

A mathematical expression to describe the ventilatory response to hypoxia and hypercapnia

Analytical techniques using multiple-exposure roentgenograms were employed to investigate surgical repositioning of either the femoral or the tibial attachment of the medial collateral ligament. The motion of the femoral attachment of the ligament with respect to the tibial attachment was used to compute the changes in length of the borders of the ligament for normal knees and for knees with repositioned attachments. The results support the conclusion that when advancement of the medial collateral ligament is utilized in the treatment of medial instability, optimization is accomplished by distal and anterior advancement with the knee in 30 degrees of flexion. Femoral displacement (proximal realignment) or tibial displacement at knee-flexion angles greater than 45 degrees is not recommended.