Raman tensor analysis of ultra-high molecular weight polyethylene and its application to study retrieved hip joint components

The angular dependences of the polarized Raman intensity of A(g), B(1g), B(2g), and B(3g) modes have been preliminary investigated on a model fiber sample of ultra-high molecular weight polyethylene (UHMWPE) in order to retrieve the Raman tensor elements, i.e. the intrinsic parameters governing the vibrational behavior of the orthorhombic structure of polyethylene. Based on this Raman analysis, a method is proposed for determining unknown crystallographic orientation patterns in UHMWPE biomedical components concurrently with the orientation distribution functions for orthorhombic lamellae. An application of the method is shown, in which we quantitatively examined the molecular orientation patterns developed on the surface of four in vivo exposed UHMWPE acetabular cups vs. an unused cup. Interesting findings were: (i) a clear bimodal distribution of orientation angles was observed on worn surfaces; and (ii) a definite and systematic increase in both molecular orientation and crystallinity in main wear zones vs. non-wear zones was found in all retrieved acetabular cups. The present crystallographic analysis is an extension of our previous Raman studies of UHMWPE acetabular cups related to assessments of oxidation and residual strain and suggests a viable path to track back wear-history information from the surface of UHMWPE, thus unfolding the in vivo kinematics of the bearing surfaces in hip joints on the microscopic scale.

No change detected by DEXA in bone mineral density after periacetabular osteotomy

The purpose of this study was to assess acetabular bone mineral density after periacetabular osteotomy and to examine whether bone mineral density correlates with postoperative migration of the osteotomised acetabular fragment. Twenty-five female and three male patients scheduled for periacetabular osteotomy were consecutively included. The patients were scanned by dual energy X-ray absorptiometry (DEXA) at 1 week, 1 year, and 2 1/2 years after surgery. Radiostereometric analyses (RSA) were done at 1, 4, 8, and 24 weeks after surgery. Two and a half years after periacetabular osteotomy, no significant changes in bone mineral density or any biological effect on bone remodelling due a changed loading pattern in the acetabulum could be detected. There was no significant correlation between bone mineral density and migration of the acetabulum. Dual energy X-ray absorptiometry is not an appropriate method to demonstrate the changes in bone mineral density after periacetabular osteotomy or to predict postoperative acetabular migration.

The functional anatomy of hip abductors

The gluteal region was dissected in 18 adult cadavers. The attachments, directions, and orientations of the fibres of the tensor fasciae latae, gluteus medius, and gluteus minimus muscles were noted. The gluteus medius was found to be formed of three distinct parts, while the gluteus minimus was formed of two parts only; each part of these muscles had its separate innervations from the superior gluteal nerve. The tensor fasciae latae muscle arose from the anterior part of the outer lip of the iliac crest and was attached to the iliotibial tract slightly below and in front of the greater trochanter. The direction of the fibres of the anterior and middle parts of the gluteus medius and the anterior part of the gluteus minimus suggested that they have vertical pull and initiate abduction which is then completed by the tensor fasciae latae. The function of the posterior parts of the gluteus medius and minimus, being parallel to the neck of the femur, would be stabilization of the femoral head in the acetabulum during the different stages of the gait cycle. By resolving the line of action of the tensor fasciae latae muscle, it was found to help the muscle to fix the hip and femur together during the stance phase and to counteract the weight of the body during standing position.

Total hip arthroplasty with the Secur-Fit and Secur-Fit plus femoral stem design a brief follow-up report at 5 to 10 years

This report represents a 5- to 10-year follow-up of our initial 2- to 5-year data with this proximally hydroxyapatite-coated stem design (Secur-Fit, Stryker Orthopaedics, Mahwah, NJ). This is a retrospective review of a single surgeon's consecutive cases. A total of 105 cases were available for complete review. The average follow-up was 6.7 years (60-123 months). All stems achieved bony ingrowth. Harris hip score and Oxford hip score averaged to 91 and 17, respectively, with no differences seen between the 132 degrees and the 127 degrees stem designs. Leg length measurements were considered equal side-to-side differences of 7 mm or less in 87% of cases: 82% of standard- and 92% of high-offset cases (P < .05). Four cases of osteolysis were identified and occurred only in patients with an elevated rim liner, implicating neck-liner impingement. The stem design performed exceedingly well over the course of this study. Having a high-offset option aids the surgeon in proper leg length management.

Testing the stability of the polyethylene acetabulum cemented on a frozen bone graft substrate on a model of an artificial hip joint

The stability of the polyethylene acetabulum cemented on a substrate made of frozen bone grafts was investigated. The force was applied to the edge of the acetabulum and the magnitude of the force and resulting displacement were recorded. These tests were preceded by stress testing, during which the acetabulum was subjected to loading forces of 1 or 3 kN through 100,000 cycles. Additionally the influence of the thickness of grafts layer on the overall stability of an implant was also determined. The experiments proved that such factors as initial compacting of bone grafts, magnitude of the loading force and thickness of grafts layer greatly affect the stability of the artificial acetabulum.

[Musculoskeletal load analysis. A biomechanical explanation for clinical results--and more?]

Mechanical loading of the lower extremities due to muscle and joint contact forces plays an important role in orthopaedic and trauma surgery. Detailed, patient specific information on the in vivo forces and their distribution is, however, currently not readily available to the surgeon in clinical routine. The goal of this study was to elucidate the relationship between the position of the cup and the musculoskeletal loading conditions at the hip using validated analyses, and further, to evaluate the predictions of the biomechanical conditions against the results of a clinical study. The results indicate that restoring the anatomical hip centre to its anatomical mediolateral position could help to reduce joint loads and add to the longevity of the reconstruction. The routine use of validated analyses of musculoskeletal loading conditions, such as in the presented example using standardised pre-operative planning and sound intra-operative decision support systems, could contribute to securing a high standard in patient treatment.

Regional load bearing of the canine acetabulum

OBJECTIVE

To determine the load bearing areas of the canine acetabulum.

MATERIALS AND METHODS

A kinematic study of four healthy dogs was used to determine the orientation of the femur to the pelvis at mid-stance. Femora and pelves from 10 canine cadavers were loaded with the physiological canine hip reaction force and angle being replicated. Impression material placed within the acetabulum was extruded from areas of load bearing. Digital images before and after loading were used to assess if six different regions of the acetabulum were fully, partially or non-load bearing.

RESULTS

All areas of the acetabulum were partially or fully load bearing. The cranial and caudal thirds of the acetabulum were 7.9 and 13.1 times more likely to be fully load bearing than the central third, respectively. There was a significant difference in load bearing between the axial, middle and abaxial thirds of the acetabulum in all tests, with the middle and abaxial thirds 72.4 and 351 times more likely to be fully load bearing than the axial third, respectively.

CONCLUSION

The cranial and caudal thirds and the middle and abaxial thirds of the canine acetabulum are fully load bearing.

CLINICAL RELEVANCE

The caudal third of the canine acetabulum is loaded and therefore recommendations that fractures in this area be managed conservatively need to be reconsidered.

Vitamin D and oestrogen receptor polymorphisms in developmental dysplasia of the hip and primary protrusio acetabuli--a preliminary study

We investigated the association of developmental dysplasia of the hip (DDH) and primary protrusion acetabuli (PPA) with Vitamin D receptor polymorphisms Taq I and Fok I and oestrogen receptor polymorphisms Pvu II and Xba I. 45 patients with DDH and 20 patients with PPA were included in the study. Healthy controls (n = 101) aged 18–60 years were recruited from the same geographical area. The control subjects had a normal acetabular morphology based on a recent pelvic radiograph performed for an unrelated cause. DNA was obtained from all the subjects from peripheral blood. Genotype frequencies were compared in the three groups. The relationship between the genotype and morphology of the hip joint, severity of the disease, age at onset of disease and gender were examined. The oestrogen receptor Xba I wild-type genotype (XX, compared with Xx and xx combined) was more common in the DDH group (55.8%) than controls (37.9%), though this just failed to achieve statistical significance (p = 0.053, odds ratio = 2.1, 95% CI = 0.9–4.6). In the DDH group, homozygosity for the mutant Taq I Vitamin D receptor t allele was associated with higher acetabular index (Mann-Whitney U-test, p = 0.03). Pvu II pp oestrogen receptor genotype was associated with low centre edge angle (p = 0.07). This study suggests a possible correlation between gene polymorphism in the oestrogen and vitamin D receptors and susceptibility to, and severity of DDH. The Taq I vitamin D receptor polymorphisms may be associated with abnormal acetabular morphology leading to DDH while the Xba I oestrogen receptor XX genotype may be associated with increased risk of developing DDH. No such correlations were found in the group with PPA.

Acetabular Loading in Active Abduction

Operative fixation of fragments in acetabular fracture treatment is not strong enough to allow weight bearing before the bone is healed. In some patients, even passive or active nonweight-bearing exercises could lead to dislocation of fragments and posttraumatic osteoarthritis. Therefore, early rehabilitation should avoid loading the acetabulum in the regions of fracture lines. The aim of the paper is to estimate acetabular loading in nonweight-bearing upright, supine, and side-lying leg abduction. Three-dimensional mathematical models of the hip joint reaction force and the contact hip stress were used to simulate active exercises in different body positions. The absolute values of the hip joint reaction force and the peak contact hip stress are the highest in unsupported supine abduction (1.3 MPa) and in side-lying abduction (1.2 MPa), lower in upright abduction (0.5 MPa), and the lowest in supported supine abduction (0.2 MPa). All body positions the hip joint reaction force and the peak contact hip stress are the highest in the posterior-superior quadrant of acetabulum, followed by anterior-superior quadrant, posterior-inferior quadrant, and finally anterior-inferior quadrant. Spatial distribution of the average acetabular loading shows that early rehabilitation should be planned according to location of the fracture lines.

Parametric analysis of the stress distribution on the articular cartilage and subchondral bone

Few studies have stressed on the sensitivity of stress distribution in different mechanical properties of the articular cartilage and subchondral bone. The purpose of this study was to establish parametric variations of mechanical factors individually and examine how these biomechanical effects influenced the cartilage and subchondral bone plate stress fields in the hip joint. A finite element model including acetabulum and proximal femur was established to study the stress change associated with the thinning of cartilage, the increasing of subchondral bone modulus and the thickening of subchondral bone plate individually. The stress distributions in bone/cartilage interface were evaluated. Sensitivity of the stress magnitudes to the parametric changes was also analyzed. The results indicated that cartilage thinning has more significant effect than subchondral bone modulus increasing or thickening on the shear stress levels in subchondral bone/cartilage interface. Subchondral bone plate modulus increasing has mild effect on the shear stress in subchondral bone/cartilage interface. Cartilage thinning acts as a major influence on the development of the articular cartilage damage.

No effect of bone morphogenetic protein-7 (OP-1) on the incorporation of impacted bone grafts in a realistic acetabular model

Bone morphogenetic proteins (BMPs) accelerate bone repair in experimental and clinical conditions. Impacted Morsellized Cancellous Bone grafts (MCB) are successfully used to reconstruct bone defects after failed hip implants. The main question in this study was if BMP-7 (OP-1) mixed with MCB could accelerate the incorporation of MCB and prevents the formation of a soft tissue interface after remodeling of the MCB. A large loaded defect in the acetabulum of goats was reconstructed with a wire mesh and with MCB or MCB mixed with OP-1. After 6 weeks, no differences were found in the revascularization process, in the number of osteoclasts resorbing the MCB, and in the thickness and appearance of the fibrous interface between MCB with or without OP-1. After 6 weeks, enchondral bone had formed in the bone graft layer and on the periosteal anterior and superior rim in the OP-1 group only. More periosteal bone and more bone in the holes of the mesh had been formed in most OP-1 goats. Most MCB was replaced by new lamellar bone after 15 weeks in both groups. We speculate that during or directly after impaction most of the OP-1 is released from the carrier inducing an early effect outside the reconstructive layer at the periosteal side of the acetabulum. Probably most OP-1 has left the reconstruction by the time new vessels and progenitors reached the bone graft. These results do not support the use of OP-1 in impaction bone grafting in patients.

Bone remodeling around the Cambridge cup: a DEXA study of 50 hips over 2 years

BACKGROUND

In a prospective 2-year study we have used dual-energy X-ray absorptiometry to measure periprosthetic bone mineral density (BMD) following implantation of a novel, "physiological", acetabular component designed using composite materials.

METHOD

The acetabular components were implanted in hydroxyapatite (HA) and HA-removed options. They were implanted in conjunction with a cemented femoral component in 50 female patients who presented with displaced, subcapital, fractures of the neck of the femur. Regions of interest (ROI) were defined according to De Lee and Charnley. BMD during follow-up was compared with immediate postoperative values for the affected limb.

RESULTS

The mean precision error (CV%) was 1.01%, 2.26% and 1.12%, for ROI I, II and III respectively. The mean change in BMD, for both cups, was analyzed. There was no significant difference between the BMD changes induced with the HA- and non-HA-coated cups.

INTERPRETATION

After an initial fall in BMD in all 3 ROI at 6 months, ROI I and ROI II showed return to baseline BMD by 2 years. ROI III showed no significant decrease in BMD beyond 6 months, but did not return to baseline levels. Statistical analysis revealed no significant decrease in BMD in ROI I and ROI II at 2 years, compared with immediate postoperative values. The changes in BMD reflect a pattern of maximally reduced stress in the non-weight-bearing zone (ROI III), with preservation of bone density in weight bearing zones ROI I and ROI II. These results support the design principles of the Cambridge cup.

Compliant layer acetabular cups: friction testing of a range of materials and designs for a new generation of prosthesis that mimics the natural joint

Total joint replacements (TJRs) have a limited lifetime, but the introduction of components that exhibit good lubricating properties with low friction and low wear could extend the life of TJRs. A novel acetabular cup design using polyurethane (PU) as a compliant layer (to mimic the natural joint) has been developed. This study describes a series of friction tests that have been used to select the most appropriate material, optimize the design parameters, and fine-tune the manufacturing processes of these joints. To determine accurately the mode of lubrication under which these joints operate, a synthetic lubricant was used in all these tests. Friction tests were carried out to assess the lubrication of four PU bearing materials. Corethane 80A was the preferred material and was subjected to subsequent testing. Friction tests conducted on acetabular cups, manufactured using Corethane 80A articulating against standard, commercially available femoral heads, demonstrated friction factors approaching those for full-fluid-film lubrication with only approximately 1 per cent asperity contact. As the joint produces these low friction factors within less than half a walking cycle after prolonged periods of loading, start-up friction was not considered to be a critical factor. Cups performed well across the full range of femoral head sizes, but a number of samples manufactured with reduced radial clearances performed with higher than expected friction. This was caused by the femoral head being gripped around the equator by the low clearance cup. To avoid this, the cup design was modified by increasing the flare at the rim. In addition to this the radial clearance was increased. As the material is incompressible, a radial clearance of 0.08 mm was too small for a cup diameter of 32 mm. A clearance of between 0.10 and 0.25 mm produced a performance approaching full-fluid-film lubrication. This series of tests acted as a step towards the optimization of the design of these joints, which has now led to an in vivo ovine model.

Cementless acetabular component inserted without screws - the effect of immediate weight-bearing

Seventy-four total hip arthroplasties using a cementless acetabular component without holes for supplemental screw fixation were evaluated during a follow-up period of 2-6 years. Immediate full weight-bearing walking was allowed after the operation. A comparison of anteroposterior (AP) radiographs of the pelvis taken in the early postoperative period and when the patient returned for the final follow-up revealed that there was no migration of the acetabular component. Based on the early postoperative radiographs and using the zones described by Martell et al. [J Bone Joint Surg (1993) 75-A:554-571], gaps between the acetabular component and the bone occurred at zone A2 in three hips (4.1%), at zone B1 in 12 hips (16.2%), and at zone B2 in seven hips (9.5%). The final follow-up radiographs, however, revealed only one hip to have a gap of 1.1 mm at zone B2; the majority of the gaps had disappeared. The results suggest that immediate weight-bearing walking did not result in the migration of the cementless acetabular component.

Hedrocel trabecular metal monoblock acetabular cups: mid-term results

The most important factors for long-term survival of cementless prostheses are the initial fixation, the osteoconducting properties of the metal shell and the bony response. Porous tantalum, a new biomaterial with a geometric structure similar to trabecular bone, was reported in animal studies to allow for bone ingrowth even when 3-mm gaps exist between the implant and the bone. This new material may improve the durability and stability of hip arthroplasties. We analysed the behaviour of the underlying acetabular bone, based on radiographs taken 46 months or longer after implantation of monoblock cementless acetabular cups made of porous tantalum. Clinical evaluation was done by means of the Harris Hip Score. The acetabular ARA-score, ranging from poor to excellent, was excellent in 80 % of the cases, 46 months or longer after implantation. The clinical condition of the patients as assessed with the Harris Hip Score was excellent for 65 % of the patients.

Der acetabuläre Pressfit bei äquatorialer Beschichtung der zementfreien Hüftpfanne – eine finite-Elemente-Analyse Pressfit of equatorially roughened cementless acetabular components – a finite elements analysis

AIM

Does the pressfit anchorage of cementless acetabular cups depend on the roughness of the pole? To answer this question the primary pressfit of two cementless acetabular cups which differ only with regard to the roughness of their poles were compared by means of finite elements analysis.

MATERIALS AND METHODS

It was assumed that the material properties of bone are homogeneous, isotropic and linearly elastic. Material-specific values of cancellous bone with three different bone densities were used. Assumption of isotropy represents an approximation.

RESULTS

Comparison of the two prosthesis designs revealed that both designs/shapes cause similar patterns of bone deformation and tension.

CONCLUSIONS

It can therefore be concluded that with regard to pressfit anchorage the prosthesis with milled polar surface is according to FEA mechanically equivalent to the prosthesis with non-milled polar surface.

The frontal pelvic plane provides a valid reference system for implantation of the acetabular cup: spatial orientation of the pelvis in different positions

BACKGROUND

The frontal pelvic plane has traditionally served as the reference plane for implantation of the acetabular cup during total hip arthroplasty, with referencing performed with the patient supine on the operating table. During daily activities in an upright position, the frontal pelvic plane changes from a horizontal to a vertical orientation. If this change in orientation is accompanied by a substantial change in pelvic inclination angle, it would mean that the use of the frontal pelvic plane as a reference plane for implantation of the acetabular cup would not be valid for proper alignment of the cup. To evaluate this possibility, we measured the change of inclination of the pelvis from the supine to the standing position.

SUBJECTS AND METHODS

We evaluated 120 patients, first positioned in a standing position and then supine on a table. Three pelvic landmarks were digitized percutaneously, and the spatial coordinates were calculated with regard to pelvic orientation in the horizontal and the vertical plane.

RESULTS

We found a mean inclination of 6.7 degrees in the standing position and 5.6 degrees in the supine position. Patients who were more than 60 years of age who did not have coxarthrosis had a greater inclination angle (8.7 degrees ) while standing. Pelvic orientation was stable with regard to the supine and standing positions. These results were independent of sex, level of arthrosis, or status after implantation of a total hip replacement.

INTERPRETATION

The frontal pelvic plane is a valid reference plane for implantation of the acetabular cup.

Tensile properties of the human acetabular labrum-the first report

The tensile properties of the human acetabular labrum were investigated using a uniaxial tension testing apparatus. The superior quadrant of the acetabular labrum was harvested from patients who underwent hip surgery. The obtained labra were sliced and shaped into rectangles for measuring uniaxial tension. We hypothesized that several characteristics such as gender, age, degeneration due to primary ailment, and the severity of the acetabular dysplasia would influence the tensile properties of the labrum. Antero-posterior radiographs of the pelvis have been used to evaluate the severity of the acetabular dysplasia and to diagnose the developmental dysplasia of the hip joint (DDH) clinically. Thus, we investigated the correlation between each of two representative radiological measurements-the CE angle and the Sharp angle-and tensile properties to assess the influence of the severity of the acetabular dysplasia. The tensile stress-strain curves for the labrum assumed a sigmoid shape. The mean tensile stress at failure for all specimens was 8.8+/-4.1 MPa. The mean strain at failure for all specimens was 48.5+/-10.4%. The mean tensile modulus was 66.4+/-42.2 MPa. Mean tensile stress at failure for specimens from males was greater than that of specimens from females. A significant difference was found in the maximum stress at failure among the three diagnosis-based groups: the other group withstood the highest stress before failure, whereas the osteoarthritis (OA) group withstood the lowest. No significant correlations with age or radiological characteristics were found for tensile stress, strain at failure, or tensile modulus. Our results suggest that labra obtained from male patients have stronger tensile stress than those from female patients, and that degenerative changes may influence the properties of the acetabular labrum.

Pelvic tilt makes acetabular cup navigation inaccurate

BACKGROUND

Modern navigation techniques allow precise positioning of the acetabular cup relative to the anterior pelvic plane. Variations in pelvic tilt will affect the resulting spatial orientation of the cup.

METHODS

We measured pelvic tilt in 30 volunteers with an inclinometer combined with an ultrasonographic position measurement system. A mathematical algorithm was developed to calculate the resulting cup position measured on standard radiographs, depending on pelvic tilt.

RESULTS

Average pelvic tilt at rest was -4 degrees in the lying position and -8 degrees in the standing position, and ranged from -27 degrees to +3 degrees. Pelvic reclination of 1 degree will lead to functional anteversion of the cup of approximately 0.7 degree.

INTERPRETATION

Pelvic tilt makes navigation systems referring to the anterior plane inaccurate.

[Hip joint radiographic technique: radiographic technique for false profile view (a study of optimum foot position)]

PURPOSE

To establish a more effective false profile (FP) radiography standard via theoretical consideration of patients' non-inspection-side foot position.

PROBLEM

Existing FP radiography admits to two difficulties: 1) A 65-degree pelvic rotation results in differing inter-femoral head distances in individual patients, and 2) the desired angle of rotation of the pelvis is difficult to maintain throughout the examination period.

METHOD

The following investigations were performed in this study: 1) inter-femoral head distance using computed tomography (CT) images; 2) optimum outside rotation angle of the non-inspection side for standing comfort; and 3) optimum foot position of the non-inspection side.

RESULTS

1) CT examination revealed an average inter-femoral head distance of 28.7 mm in males and 40.3 mm in females, with a maximum distance of 57.5 mm and minimum distance of 44.1 mm. 2) Investigation established the optimum outside angle of rotation to be 60 degrees. 3) Investigation revealed that the optimum foot position requires that the patient's heel be retracted 3 cm along the axis created by a pelvic rotation of 65 degrees. CT was used to verify the accuracy of the FP view method. CT comparison revealed that X-rays are able to obtain a view quite similar in quality to that of CT.

CONCLUSION

It is of utmost importance in FP radiography to establish the optimum foot position for a pelvic rotation value of 65 degrees.

The effect of geometry and abduction angle on the stresses in cemented UHMWPE acetabular cups--finite element simulations and experimental tests

BACKGROUND

Contact pressure of UHMWPE acetabular cup has been shown to correlate with wear in total hip replacement (THR). The aim of the present study was to test the hypotheses that the cup geometry, abduction angle, thickness and clearance can modify the stresses in cemented polyethylene cups.

METHODS

Acetabular cups with different geometries (Link: IP and Lubinus eccentric) were tested cyclically in a simulator at 45 degrees and 60 degrees abduction angles. Finite element (FE) meshes were generated and two additional designs were reconstructed to test the effects of the cup clearance and thickness. Contact pressures at cup-head and cup-cement interfaces were calculated as a function of loading force at 45 degrees, 60 degrees and 80 degrees abduction angles.

RESULTS

At the cup-head interface, IP experienced lower contact pressures than the Lubinus eccentric at low loading forces. However, at higher loading forces, much higher contact pressures were produced on the surface of IP cup. An increase in the abduction angle increased contact pressure in the IP model, but this did not occur to any major extent with the Lubinus eccentric model. At the cup-cement interface, IP experienced lower contact pressures. Increased clearance between cup and head increased contact pressure both at cup-head and cup-cement interfaces, whereas a decreased thickness of polyethylene layer increased contact pressure only at the cup-cement interface. FE results were consistent with experimental tests and acetabular cup deformations.

CONCLUSION

FE analyses showed that geometrical design, thickness and abduction angle of the acetabular cup, as well as the clearance between the cup and head do change significantly the mechanical stresses experienced by a cemented UHMWPE acetabular cup. These factors should be taken into account in future development of THR prostheses. FE technique is a useful tool with which to address these issues.

Contact pressures in the flexed hip joint during lateral trochanteric loading

Acetabular fractures are an especially problematic outcome of motor vehicle side impacts. While fracture type has been correlated with impact direction and femoral orientation, actual contact pressures in the hip joint have not been quantified for lateral loading conditions. In the present study, we used pressure sensitive film to measure contact areas and pressures in seven hip joints from four cadavers under quasi-static lateral loading through the greater trochanter. The aim was to quantify the interactions of the femoral head with the acetabulum associated with variations in femoral orientation. Three angles of hip flexion (80 degrees , 90 degrees , 100 degrees ) and hip abduction (-10 degrees , 0 degrees , 10 degrees ) were tested, producing nine test orientations for each joint. We observed that contact areas, pressures, and forces varied significantly with femoral orientation for the adducted hip. The principal locations of load transmission were in the anterior and posterior regions of the acetabulum. For the abducted femur, contact pressures were concentrated anteriorly, and with increased adduction, anterior contact pressures diminished while posterior and superior pressures increased. The movement of pressure sites was consistent with mechanisms of acetabular fractures described by Letournel and Judet and provides new data for validation of finite element models of the pelvis in side impact.

Regional load bearing of the feline acetabulum

OBJECTIVE

To test if the caudal acetabulum of the cat is unloaded at mid-stance as has been asserted.

MATERIALS AND METHODS

A kinematic study of five healthy cats was performed to ascertain the mid-stance angle of the hip joint, and the orientation of the pelvis and femur. Femora and pelves from 10 feline cadavers were loaded at physiological load at the mid-stance angle. Impression material placed within the acetabulum was extruded from areas of load bearing. Digital images before and after loading were used to assess whether three areas of the acetabulum, cranial, central and caudal, were fully, partially or non-load bearing.

RESULTS

There was a significant difference in load bearing in the three regions (p < 0.001, Kruskal-Wallis test), with a significant difference in load bearing between the cranial and central thirds (p < 0.001) and the cranial and caudal thirds (p < 0.001) but no difference between the central and caudal thirds of the acetabulum (Mann-Whitney-U test).

CONCLUSIONS

The load bearing areas of the feline acetabula are the caudal and central thirds.

CLINICAL RELEVANCE

The caudal acetabulum of the cat is loaded, therefore the recommendations for simple fractures being treated conservatively needs to be reconsidered.

The effect of pelvic osteotomy plate type on axial rotation of the acetabular segment in the triple pelvic osteotomy

OBJECTIVE

The purpose of this study is to evaluate the changes in structural anatomy of cadaver pelves, with specific emphasis on axial rotation, resulting from the application of TPO plates differing by type, angle and orientation.

SAMPLE POPULATION

Nine cadaver pelves procedure: The degree of axial rotation was determined for three different pelvic osteotomy plate types, four different plate angles and two different orientations of the same plate.

RESULTS

The observed degree of axial rotation was significantly different than the degree of axial rotation predicted by the plate angle for each group except the Rooks pelvic osteotomy plate.

CONCLUSIONS/CLINICAL RELEVANCE

In the triple pelvic osteotomy the final degree of axial rotation may be significantly different than the predetermined plate angle.