Biomechanics of large femoral heads: what they do and don't do

The stability and durability of total hip reconstruction is dependent on many factors that include the design and anatomic orientation of prosthetic components. An analysis of femoral component head size and acetabular component orientation shows an interdependency of these variables and joint stability. Increased femoral component head size can increase hip stability by increasing the prosthetic impingement-free range of hip motion and by increasing the inferior head displacement required before hip dislocation. Increasing the femoral head size from 22 mm to 40 mm increases the required displacement for dislocation by about 5 mm with the acetabular component at 45 degrees of abduction; however, increasing acetabular component abduction greatly diminishes this stability advantage of larger femoral heads. Vertical acetabular component orientation and femoral component head subluxation are each predicted to more than double the tensile stress with acetabular component polyethylene compared with components at 45 degrees of abduction. With a desirable acetabular component orientation, the use of larger femoral heads may result in improved joint stability and durable use of polyethylene. With high abduction acetabular component orientation, the use of larger femoral heads contributes little to joint stability and contributes to elevated stress within the polyethylene that may result in implant failure.

Friction of ceramic and metal hip hemi-endoprostheses against cadaveric acetabula

INTRODUCTION

Studies of hip arthroplasty have dealt mainly with total endoprosthesis, while tribology measurement values of hemi-endoprosthetic implants are rare. The small amount of experimental tribological data concerning materials of hemi-endoprosthetic implants in the form of pendulum trials, animal experiments, in vivo measurements on human hip joints and pin on disc studies report friction coefficients between 0.014 and 0.57; the friction coefficients measured in fresh human cadaver hip joints were determined between 0.001 and 0.08.

MATERIALS AND METHODS

The HEPFlEx-hip simulator was constructed to test the friction coefficients of unipolar femur head hemi-endoprostheses made of metal or ceramic against fresh cadaveric acetabula. Its plane of movement is uniaxial with a flexion-extension movement of +30/-18 degrees . The force is produced pneumatically dynamic with amounts of 2.5 kN. Newborn calf serum serves as a lubricant. We mounted 20 fresh porcine acetabula and 10 fresh human cadaver acetabula in the HEPFlEx-hip simulator and compared the two unipolar femur head hemi-endoprostheses (metal vs. ceramic).

RESULTS

The mean friction coefficients against porcine acetabula were micro=0.017-0.082 for ceramic and micro=0.020-0.101 for metal; against human cadaver acetabula micro=0.017-0.083 for ceramic and micro=0.019-0.118 for metal. The frictional coefficient deltas (metal-ceramic) values of all measurements were Deltamicro=0.004 for porcine acetabula and Deltamicro=0.001 for cadaver acetabula. Box-plots graphics document significantly lower frictional coefficients of the ceramics.

CONCLUSIONS

The lower frictional coefficients of ceramic compared to metal against fresh cadaveric acetabula may have a clinical impact on the process of the protrusion of the corresponding femoral head through the acetabulum.

Bioactive cement or ceramic/porous coating vs. conventional cement to obtain early stability of the acetabular cup. Randomised study of 96 hips followed with radiostereometry

Ninety patients (96 hips) scheduled for THA were stratified to fixation of the acetabular component in three main groups of about equal size. Fluoride cement, porous coated press-fit cup with ceramic coating or Palacos cum Gentamicin cement were used. All patients received Spectron EF stem. The migration of the cups and the femoral head penetration into the socket were measured with radiostereometric analysis. At 2 years the choice of fixation did not influence the migration or rotation of the cup. Patients with compromised bone quality showed increased three-dimensional (3D or total) migration. Proximal and 3D penetration rates were increased in cemented compared with the uncemented cups (p<0.001), which probably not could be related to the choice of fixation. Appearance of radiolucent lines was almost equal in the two cemented groups. Uncemented cups had less radiolucent lines at 2 years. Fluoride containing cement or uncemented fixation did not improve the early postoperative stability of the socket.

Effects of removal of the acetabular labrum in a sheep hip model

OBJECTIVE

The aim of this study was to learn whether partial resection of the acetabular labrum would lead to degenerative arthritis in an ovine model.

METHODS

A 2 cm segment of labrum was removed from one hip in 18 mature Swiss Alpine sheep and a sham procedure was performed on the opposite side. Animals were permitted ad lib activity until sacrifice at 6, 12 or 24 weeks. The hip joint was removed en bloc, and loaded with a force of one body weight, using a custom device designed to recreate a physiologic joint reaction force. While under load, the joint was plunge frozen, and then fixed by freeze substitution using aldehydes in methanol/acetone solvents. The entire joint was embedded in methacrylate and sectioned in a standardized frontal plane following the reaction force and including the femoral neck and the acetabular fossa. The sections were evaluated for concentricity and evidence of arthrosis. Six hip joints of three sheep with no surgical procedure were loaded with high or low loads and served as non-surgical controls.

RESULTS

Degenerative changes were present in all surgical hips, but the changes were symmetrical and mild. In 16 of 18 hips, the labrum regenerated to the extent that dense fibrous scar extending from the surgically denuded origin filled the defect.

CONCLUSION

Resection of the labrum does not cause rapid degeneration or altered stability of the sheep hip.

Structure and vascularization of the acetabular labrum with regard to the pathogenesis and healing of labral lesions

INTRODUCTION

The aim of this study is to examine the structure and vascularization of the acetabular labrum with regard to the pathogenesis and healing of labral tears.

MATERIALS AND METHODS

The labral tissue was characterized immunohistochemically and under light microscopy; the collagen fibril texture was demonstrated by scanning electron microscopy after temporary staggered maceration of the tissue; and the vascularization of the acetabular labrum was studied immunohistochemically using antibodies against laminin.

RESULTS

The peripheral aspect of the acetabular labrum consists of dense connective tissue. The internal layer consists of type II collagen-positive fibrocartilage. Scanning electron microscopy revealed three distinct layers in the acetabular labrum: (1) the articular surface was covered by a meshwork of thin fibrils; (2) beneath the superficial network, there is a layer of lamella-like collagen fibrils; (3) the majority of the collagen fibrils are oriented in a circular manner. Blood vessels enter the labrum from the adjacent joint capsule. The distribution of vessels within the labrum is not homogenous. Blood vessels can be detected only in the peripheral one-third of the labrum. The internal part is avascular.

CONCLUSION

The result of this study demonstrates that the structure of the acetabular labrum is highly significant for the direction of traumatic and dysplastic labral lesions. The biomechanical analysis of the structure suggests that the labrum is stressed by compressive load. Therefore, excision or removal of the labrum may alter physiological functions such as enhancing joint stability and load distribution. The vascular pattern identified should encourage surgeons to develop repair strategies of peripheral labral tears to maintain its functions in the hip.

7th Yahya Cohen Lecture. Significance of the lateral epiphysis of the acetabulum to hip joint stability

Normal development of the acetabulum is crucial to the growth and stability of the hip. Twenty-five New Zealand white rabbits (postweaning) weighing 1.5 to 2.2 kg were used. Both hips were exposed through an anterolateral approach. On the right hip, a fixed area of superolateral physeal cartilage was damaged with drills. Sham open reduction was performed for the left hip. Radiographic changes of the right hips were evident at 6 weeks. At 12 weeks, the right hip dislocated in a posterior and superior direction. The left hip remained normal. Histopathologic analysis correlated strongly with the radiographic findings. There were thinning of cartilage cells of the acetabulum, with disorganisation. The acetabular roof was poorly formed. The lateral acetabular epiphysis is vital to the development of the acetabular roof. Damage to this epiphysis may result in acetabular maldevelopment and subsequent hip instability. We tested the hypothesis that abnormality in this epiphysis can give rise to abnormal acetabular development.

Posterior-anterior glide of the femoral head in the acetabulum: a cadaver study

STUDY DESIGN

Descriptive study employing cadaver dissection and measurement of posterior-anterior (PA) glide of the femoral head in the acetabulum.

OBJECTIVE

To quantify PA glide of the femoral head in the acetabulum in a cadaveric sample.

BACKGROUND

Posterior-anterior glide of the femoral head within the acetabulum is a joint mobilization procedure described in orthopaedic physical therapy texts, yet there is no published evidence that the joint structures of the hip allow such movement. This study attempted to quantify PA glide of the femoral head in the hip joints of embalmed cadavers.

METHODS

Twelve hips, 3 male and 9 female, from 8 embalmed cadavers were employed in this study. Hips were dissected to the level of the joint capsule and a metal rod inserted through the femoral neck served as a mobilizing handle. A load cell was installed into this handle so that mobilizing forces could be monitored. A dial gauge, which recorded displacement of the femoral head, was mounted to the pelvis via bone pins and an external fixator.

RESULTS

Using mobilizing forces of 89, 178, 267, and 356 N, mean femoral head displacements of 0.57, 0.93, 1.20, and 1.52 mm were recorded. Within the 89-N trials, PA displacement ranged from a minimum of 0.04 mm to a maximum of 1.54 mm. Within the 356-N trials, PA displacement of the femoral head ranged from a minimum of 0.25 mm to a maximum of 2.90 mm.

CONCLUSION

In an embalmed cadaveric model, measurable PA glide of the femoral head within the acetabulum does exist and it is highly variable between individuals.

An in vitro investigation of the acetabular labral seal in hip joint mechanics

Labrum pathology may contribute to early joint degeneration through the alteration of load transfer between, and the stresses within, the cartilage layers of the hip. We hypothesize that the labrum seals the hip joint, creating a hydrostatic fluid pressure in the intra-articular space, and limiting the rate of cartilage layer consolidation. The overall cartilage creep consolidation of six human hip joints was measured during the application of a constant load of 0.75 times bodyweight, or a cyclic sinusoidal load of 0.75+/-0.25 times bodyweight, before and after total labrum resection. The fluid pressure within the acetabular was measured. Following labrum resection, the initial consolidation rate was 22% greater (p=0.02) and the final consolidation displacement was 21% greater (p=0.02). There was no significant difference in the final consolidation rate. Loading type (constant vs. cyclic) had no significant effect on the measured consolidation behaviour. Fluid pressurisation was observed in three of the six hips. The average pressures measured were: for constant loading, 541+/-61kPa in the intact joint and 216+/-165kPa following labrum resection, for cyclic loading, 550+/-56kPa in the intact joint and 195+/-145kPa following labrum resection. The trends observed in this experiment support the predictions of previous finite element analyses. Hydrostatic fluid pressurisation within the intra-articular space is greater with the labrum than without, which may enhance joint lubrication. Cartilage consolidation is quicker without the labrum than with, as the labrum adds an extra resistance to the flow path for interstitial fluid expression. However, both sealing mechanisms are dependent on the fit of the labrum against the femoral head.

Slide track analysis of eight contemporary hip simulator designs

In an earlier paper, the authors presented a new method of computation of slide tracks in the relative motion between femoral head and acetabular cup of total hip prostheses. For the first time, computed tracks were verified experimentally and with an alternative method of computation. Besides being an efficient way to illustrate hip kinematics, the shapes of the slide tracks are known to be of fundamental importance regarding the wear behaviour of prostheses. The verified method was now applied to eight contemporary hip simulator designs. The use of correct motion waveforms and an Euler sequence of rotations in each case was again found to be essential. Considerable differences were found between the simulators. For instance, the shapes of the tracks drawn by the resultant contact force included a circle, ellipse, irregular oval, leaf, twig, and straight line. Computation of tracks correctly for the most widely used hip simulator, known as biaxial, was made possible by the insight that the device is actually three-axial. Slide track patterns have now been computed for virtually all contemporary hip simulators, and both for the heads and for the cups. This comparative analysis forms a valuable basis for studies on the relationship between the type of multidirectional motion and wear. These studies can produce useful information for the design of joint simulators, and improve the understanding of wear phenomena in prosthetic joints.

Reduction in variability of acetabular cup abduction using computer assisted surgery: a prospective and randomized study

Optimal orientation of the acetabular component of a total hip prosthesis is an important factor in determining the early and long-term result of a total hip arthroplasty (THA). Conventional positioning of the cup component is usually done using a free-hand method, or with the help of a mechanical acetabular alignment guide. However, these methods have proven to be inaccurate, and a great variation in orientation of the cup is found postoperatively. In this study, we wished to determine if the variability of the abduction angle of acetabular cups could be reduced with the use of computer navigation. The abduction angles of the acetabular components of three groups of 50 THAs were assessed. In the first group, a free-hand method was used to position the cup component. This group was operated in the period before we started using computer navigation for hip surgery. In the second group, CT-based computer navigation was used to plan and help position the cup. The third group consisted of 50 THA cases in which a free-hand method was used to position the cup, although these procedures were performed in the period after we had begun using the Computer Assisted Surgery (CAS) system. The variability in cup abduction angle was assessed in all three groups and compared. There was a significant reduction in variability in the CAS group compared to the first group. There was also a reduction in variability in the CAS group compared to the third group, although this was not statistically significant. It is concluded that the use of computer navigation helped the surgeon to place the cup component with less variability of the abduction angle, and, more importantly, we found that no cups were placed in the more extreme positions (outliers).

Load transfer and fixation mode of press-fit acetabular sockets

Adequate initial fixation is a prerequisite for osseointegration and secondary stability of noncemented cups. Physiologic force transmission between the cup and acetabulum guarantees the best long-term fixation. To study load transfer within the natural hip joint and in the bone-implant interface of 2 different hemispherical noncemented press-fit cups, 10 hips were investigated in an experimental setup simulating single-leg stance. Load distribution and contact area were measured using prescale pressure-sensitive films and digital image analysis. Three dominant locations near the periphery of the acetabulum could be identified. Main load transfer occurs in the cranial region of the acetabulum, where it is buttressed by the iliac bone; the second location is at the posterior-inferior region at the ischial facet, and the third location is at the anterior region, where support is provided by the pubic bone. Peripheral rim contact was present in both cups but not completely circumferential. It showed marked loading at the same 3 locations similar to the natural hip joint. The ilioischial diagonal axis produced the highest press-fit. Peak local forces were found at the ischial and iliac facets. Local forces can be grouped into an iliac, an ischial, and a pubic group contributing 55%, 25%, and 20% to the total hip joint force. Pole contact was not present in the natural hip and with the biradial press-fit cup with flattened pole area but was observed with the pure hemispherical cup. Hence, stable fixation of an acetabular cup is achieved best by a 3-point-like bony support at the iliac, ischial, and pubic bone. The acetabular fovea does not provide functional support of the femoral head or endoprosthetic socket. In revision surgery, remaining peripheral bone stock at the iliac, ischial, and pubic locations allows stable implantation of primary cups.

The influence of the acetabular labrum seal, intact articular superficial zone and synovial fluid thixotropy on squeeze-film lubrication of a spherical synovial joint

A model of synovial fluid (SF) filtration by articular cartilage (AC) in a step-loaded spherical synovial joint at rest is presented. The effects of joint pathology (such as a depleted acetabular labrum, a depleted cartilage superficial zone consistent with early osteoarthritis and an inflammatory SF) on the squeezed synovial film are also investigated. Biphasic mixture models for AC (ideal fluid and elastic porous transversely isotropic two-layer matrix) and for SF (ideal and thixotropic fluids) are applied and the following results are obtained. If the acetabular labrum is able to seal the pressurised SF between the articular surfaces (as in the normal hip joint), the fluid in the synovial film and in the cartilage within the labral ring is homogeneously pressurised. The articular surfaces remain separated by a fluid film for minutes. If the labrum is destroyed or absent and the SF can escape across the contact edge, the fluid pressure is non-homogeneous and with a small jump at the articular surface at the very moment of load application. The ensuing synovial film filtration by porous cartilage is lower for the normal cartilage (with the intact superficial zone) than if this zone is already depleted or rubbed off as in the early stage of primary osteoarthritis. Compared with the inflammatory (Newtonian) SF, the normal (thixotropic) fluid applies favourably in the squeezed film near the contact centre only, yielding a thicker SF film there, but not affecting the minimum thickness in the fluid film profile at a fixed time. For all that, in the unsealed case for both the normal and pathological joint, the macromolecular concentration of the hyaluronic acid-protein complex in the synovial film quickly increases due to the filtration in the greater part of the contact. A stable synovial gel film, thick on the order of 10(-7)m, protecting the articular surfaces from the intimate contact, is formed within a couple of seconds. Boundary lubrication by the synovial gel is established if sliding motion follows until a fresh SF is entrained into the contact. This theoretical prediction is open for experimental verifications.

In vivo comparison of hip separation after metal-on-metal or metal-on-polyethylene total hip arthroplasty

BACKGROUND

Twenty subjects were analyzed in vivo with use of video fluoroscopy to determine if the femoral head separates from the acetabular component during normal gait and to determine if the amount of separation differs between metal-on-metal and metal-on-polyethylene total hip prostheses.

METHODS

Ten subjects had been treated with a metal-on-metal total hip arthroplasty and ten, with a metal-on-polyethylene total hip arthroplasty. All of the prostheses were implanted by the same surgeon utilizing the same surgical technique, and all were judged to be clinically successful (a Harris hip score of >90 points). Each subject walked with a normal gait on a level treadmill while under fluoroscopic surveillance. The two-dimensional fluoroscopic videotapes were then converted into three-dimensional images with use of a computer-automated model-fitting technique. Each implant was analyzed at various flexion angles to assess the amount of femoral head sliding.

RESULTS

No femoral head sliding was observed in the subjects with a metal-on-metal implant, whereas all ten subjects with a metal-on-polyethylene implant had sliding that was greater than our threshold value of 0.75 mm. The average amount of femoral head sliding in these subjects was 2.0 mm, and the sliding was observed during the swing phase of gait. The sliding was typically seen medially while the femoral head remained in contact with the acetabular component superolaterally.

CONCLUSIONS

Femoral head sliding commonly occurred following traditional metal-on-polyethylene total hip arthroplasty but not after metal-on-metal arthroplasty. These kinematic data may be of value in future hip-simulation studies to better duplicate wear patterns observed in retrieval analyses, assist in the understanding of the lubrication and wear rates of metal-on-metal designs, and facilitate designing of prosthetic components that minimize wear and optimize hip kinematics.

Impact biomechanics and pelvic deformation during insertion of press-fit acetabular cups

Five fresh cadaver pelves were cleaned of soft tissue and instrumented with strain gauges. The acetabula were reamed, and a cementless cup, oversized at the periphery, was inserted. The applied force and cup acceleration were measured during insertion and used to calculate an effective mass of the cup, insertion device, and pelvis during each impact. Periacetabular strains increased variably during cup seating. After the cups were seated, strains continued to increase with postseating impacts. The effective mass remained constant throughout the test, indicating that cup seating is not associated with a change in acceleration. This finding implies that an accurate assessment of cup seating cannot be inferred by surgeon proprioception during impaction, and use of an apical hole in the cup is necessary to determine when the cup has seated.

Comparison of a mechanical acetabular alignment guide with computer placement of the socket

We hypothesized that use of mechanical acetabular guides for intraoperative alignment leads to variations between the actual and desired implant orientation. Acetabular implant orientation using only the mechanical guide was studied in 78 patients (82 hips) undergoing primary total hip arthroplasty. A computer-assisted navigation system was used to measure alignment and to monitor the orientation of the pelvis during surgery. When using the mechanical guide, there was significant variation in cup alignment from the desired goal of 45 degrees of abduction and 20 degrees of flexion, and this would have resulted in unacceptable acetabular alignment in 78% of hips. With the support system used, there was significant variability in pelvic orientation during surgery. The mean anteversion of the pelvis was an average of 18 degrees from the optimal orientation. These results show a clear need to develop more reliable tools than were used or anatomically based alignment strategies to provide reproducible and accurate acetabular alignment.

Slide track analysis of the relative motion between femoral head and acetabular cup in walking and in hip simulators

Joint simulators are important tools in wear studies of prosthetic joint materials. The type of motion in a joint simulator is crucial with respect to the wear produced. It is widely accepted that only multidirectional motion yields realistic wear for polyethylene acetabular cups. Multidirectionality, however, is a wide concept. The type of multidirectional motion varies considerably between simulators, which may explain the large differences in observed wear rates. At present, little is known about the relationship between the type of multidirectional motion and wear. One illustrative way to compare the motions of various hip simulators is to compute tracks made on the counterface by selected points of the surface of the femoral head and acetabular cup due to the cyclic relative motion. A new computation method, based on Euler angles, was developed, and used to compute slide tracks for the three-axis motion of the hip joint in walking, and for two hip simulators, the HUT-3 and the biaxial rocking motion. The slide track patterns resulting from the gait waveforms were found to be similar to those produced by the HUT-3 simulator. This paper is the first to include a verification of the computed simulator tracks. The tracks were verified in the two simulators using sharp pins, embedded in acetabular cups, engraving distinct grooves onto the femoral heads. The engravings were identical to the computed tracks. The results clearly differed from earlier computations by another research group. This study is intended to start a thorough investigation of the relationship between the type of multidirectional motion and wear.

Role of the limbus in femoral-head deformation in developmental dislocation of the hip: findings of two-directional hip arthrography

Two-directional arthrographic findings made during conservative treatment of developmental dislocation of the hip were compared with the femoral-head configurations and radiological results obtained from long-term follow-up examinations in this retrospective study. Sixty hips were followed until at least age 14. Arthrography was carried out according to Terazawa's method. The shape of the superior, anterior, and posterior limbus was evaluated based on a modified Fujii's classification. The femoral-head configuration was classified into 4 groups, and the radiological results were evaluated using Severin's classification at the final observation. There was a statistically significant relationship between the shape of the anterior limbus, the number of portions of deformed limbus (superior, anterior, posterior), and the femoral-head configuration. Also, a statistically significant relationship between the shape of the limbus and Severin's classification was observed. These results suggest that the deformed limbus seems to play an important role in triggering femoral-head deformities, possibly via mechanical compression, and negatively affects development of the hip joint.

Press-fit stability of uncemented hemispheric acetabular components: a comparison of three porous coating systems

Cementless acetabular components require good initial fixation to allow bony in-growth. The initial press-fit stability, important for designs that do not rely on supplemental fixation, was examined for three designs with different porous coating systems: beads, fiber mesh, and plasma spray. The ability to withstand tangential loads was determined (maximum rim loads: 122-1730 N). The plasma-sprayed acetabular cups withstood the greatest tangential load within the specified range of motion of 150 microm (P<0.01). Differences in surface preparation of titanium acetabular cups may significantly affect the initial stability of the implants in rim loading.

The material properties of the bovine acetabular labrum

The compressive and tensile material properties of the bovine acetabular labrum were measured. Confined compression testing was used to determine the aggregate compressive modulus and the permeability of the labrum. The compressive modulus of the labrum (0.157 +/- 0.057 MPa) is comparable to that of the morphologically similar meniscus, and approximately one-quarter to one-half that of the adjoining acetabular cartilage. The permeability of the labrum (4.98 +/- 3.43 x 10(-16) m4/N s) was lower than that of the meniscus and cartilage. With a significantly higher resistance to interstitial fluid flow across the acetabular rim than along the rim. Specimens from the posterior and superior regions of the labrum were tested to failure in uniaxial tension. The maximum stress at failure (11.9 +/- 6.1 MPa), maximum strain at failure (26.5 +/- 7.6%) and tangent modulus (74.7 +/- 44.3 MPa) were similar to those reported for the bovine meniscus, and to other tissues composed of highly oriented collagen fiber bundles. In tension, the labrum is much stiffer (10-15 x) than the adjoining articular cartilage, and the posterior region of the labrum is significantly stiffer (45%) than the superior region. The labrum's low permeability may contribute to sealing of the hip joint. The high circumferential tensile stiffness of the labrum, together with its ring structure, reinforce the acetabular rim and may contribute to joint stability.

A technique for measuring the compressive modulus of articular cartilage under physiological loading rates with preliminary results

On the issue of ultrasound measurement of cartilage thickness, in his Reply to my initial Communication [1] Dr Shepherd asserts that we ‘assume’ a velocity for the speed of sound in cartilage in our ultrasonic measurement of acetabula cartilage thickness and topography [2]. Page 254 of our 1981 Journal of Biomechanics article [2] states: ‘Sound velocity in the respective media was experimentally determined using samples of known geometry.’ A prior publication of the Leeds group in 1989 [3] cited reference [2] and stated ‘Efforts by the present authors to measure cartilage thickness using ultrasound indicate… Thickness measurements were difficult to obtain and even with immaculate cartilage, signal attenuation was pronounced. The unreliability in obtaining measurements prevented even a rudimentary repeatability study from being performed.’ Dr Shepherd also cites in his Reply to my Communication [1] a brief abstract [4] by his group which also dismisses ultrasound measurement with ‘ultrasonic measurement … its accuracy in measuring the thickness of human articular cartilage in situ has not been rigorously tested, nor has the potential of ultrasonic measurements for measuring mechanical properties of cartilage been evaluated’; abstract [4] cites no references.