https://www.thelancet.com/pdfs/journals/lancet/PIIS0140673607604577.pdf

The operation of the century: total hip replacement

Bone remodeling in response to in vivo fatigue microdamage

The pressures on human articular cartilage have been measured in vivo. An instrumented femoral head prosthesis that telemeters interarticular pressure at 10 discrete locations 253 times per second was implanted in apposition to natural acetabular cartilage. Data were acquired during surgery, recovery, rehabilitation, and normal activity, for longer than 1 year after surgery. Pressure magnitudes were synchronized with body-segment kinematic data and foot-floor force measurements so as to locate transduced pressure areas on the natural acetabulum and correlate movement kinematics and dynamics with local cartilage pressures. The data reveal very high local (up to 18 MPa) and nonuniform pressures, with abrupt spatial and temporal gradients, that correlate well both in magnitude and distribution with in vitro data and computer simulations of synovial joint mechanics. Peak pressures in vivo are, however, considerably higher than pressures measured in vitro under the putative forces experienced by the joint in life, particularly in normal movements where cocontraction occurs in agonist and antagonist muscles across the hip joint. Thus, extant gait-analysis studies which apply inverse Newtonian calculations to infer joint forces establish the lower limit on such forces, since such analyses include only the net muscular torques about the joint and cannot account for the contribution of the increment in joint force due to muscular cocontraction. Our data also contribute to the understanding of normal synovial joint tribology and the possible role of mechanical factors in the deterioration evident in osteoarthritis. Further, design criteria for both partial and total hip replacement prostheses and specific aspects of rehabilitation protocols following hip surgery (e.g., the extent to which crutches and canes unload the hip joint) warrant reconsideration in light of the extraordinary high pressures measured during the activities of daily living.

https://www.pnas.org/content/pnas/83/9/2879.full.pdf

Contact pressures in the human hip joint measured in vivo

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

Uniaxial fatigue tests were conducted of devitalized cortical bone specimens machined from human femora. Specimens were tested at strain ranges from 0.005 to 0.010 under physiologic loading rates. The influence of compressive, zero, and tensile mean strains on fatigue life and on the stress/strain histories during fatigue were examined. Results showed that bone fatigue is a gradual damage process accompanied by a progressive increase in hysteresis and a loss of bone stiffness. The total number of cycles to fatigue failure was influenced only by the total strain range and was not affected by mean strain. Bone was shown to have extremely poor fatigue resistance. Fully reversed cyclic loading to one half of the yield strain caused fatigue fracture in 1000 cycles. Biological implications. The bone regions which experience the highest strain ranges in vivo generally have a compressive mean strain. The results of this study indicate that mechanical fatigue damage accumulates more rapidly in these "compressive" areas than in "tensile" areas of bone.

https://www.tandfonline.com/doi/pdf/10.3109/17453678108992136

Fatigue behavior of adult cortical bone: the influence of mean strain and strain range.

The purpose of the work described was to find the average pressure on each of several areas of the acetabular cartilage of the cadaver hip under physiological loads. By obtaining load-deflection curves for one chosen area of cartilage, firstly with all the cartilage present and then after the successive removal of other areas, the fractions of the original load carried by the several areas were found, and hence the average pressures on those areas. Seventeen hips (age range twenty. two to eighty-seven years) were examined. Local pressures varied from zero to 3.4 times the average pressure in each hip. The highest pressures in the series (about 4 to 5 megaNewtons per square metre) were on areas of thin fibrocartilage which were identified at the zenith of certain acetabula. The results are too few to establish whether or not the pressure distribution was age-related. The higher pressures found are within the range which in other experiments has led to fatigue failure of femoral head cartilage, and it is suggested that hips in which such pressures exist under loads of three times body weight may be predisposed to osteoarthritis.

Contact pressures in the loaded human cadaver hip

The Scientific basis of joint replacement

Rotating cantileverfatigue tests were performed on specimens extracted longitudinally from the cortices of human femora. The usual inverse relationship between stress amplitude and fatigue life was found; the median life was 1·3×104 cycles at a stress amplitude of 12,200 1bf/in2 (84·1 MN/m2) and 4·6×106 cycles at 6750 1bf/in2 (46·6 MN/m2). Arguments are advanced which suggest that these results are compatible with what is known of the stresses applied in life and the clinical occurrence of fatigue fractures.

The fatigue properties of human cortical bone

Cadaveric knees replaced with the Geomedic, ICLH, Marmor and Total Condylar prostheses were tested in axial compression, in rotation and in hyperextension in order to observe the strength of fixation of the tibial components. In axial compression the strengths at failure varied widely, both with any one prosthesis and between prostheses. This is attributed largely to the strength of the cancellous bone of the tibia, which was measured in each case and also varied widely. Three natural knees failed at loads of 7300, 7600 and 8300 newtons respectively, whereas the strengths of replaced knees ranged from 3000 to 15750 newtons. At least one example of each design failed at less than 7300 newtons, suggesting little or no reserve of strength. The strength of fixation was greater when the tibial prosthesis was large enough to rest on the whole cross-section of the tibia. In rotation the three prostheses embodying rollers in troughs were stiffer than the Marmor which had a nearly flat tibial-bearing surface. The presence or absence of the cruciate ligaments had a negligible effect on torsional stiffness. In hyperextension, knees replaced with the ICLH, Marmor and Total Condylar prostheses failed by rupture of the posterior capsule at moments of about 60 newton-metres, compared with about 100 for natural knees. With the Marmor prosthesis the anterior cruciate ligament was avulsed at about 20 newton-metres compared with about 75 in natural knees, suggesting that in this respect the retention of the cruciate ligaments contributes little. None of the four knees tested after inserting a Geomedic prosthesis showed strengths as high as those replaced with the other three designs.

/pdf/mechanical-tests-on-the-tibial-components-of-non-hinged-knee-2lrqricg9w.pdf

Mechanical tests on the tibial components of non-hinged knee prostheses

1. Thirty-seven specimens of the proximal third of the human femur were subjected to cyclically varying loads applied in a physiological direction to the femoral head, having maximum values of from four to thirteen times body weight.

2. Ten of these specimens sustained subcapital fractures of the femoral neck after numbers of cycles of loading varying from 123 to 8,193.

3. The maximum value of cyclic load needed to give fatigue fracture after 10,000 or fewer cycles was found to vary from about twelve times the body weight at ages twenty to fifty to about five times the body weight at age seventy or more.

4. In youth and in middle age the load levels mentioned above are greater than those encountered in normal living, but are comparable to those which may be applied to the femoral head during activities known to produce "fatigue" fractures clinically in young adults.

5. In the elderly the load levels mentioned above are within the range that can be applied in normal living. It is inferred that some femoral neck fractures in the elderly may be fatigue fractures caused by the cyclic loading of normal walking.

M. A. R. Freeman

Papers

Contact pressures in the loaded human cadaver hip

The Scientific basis of joint replacement

The fatigue properties of human cortical bone

Mechanical tests on the tibial components of non-hinged knee prostheses

Experimental fatigue fracture of the human cadaveric femoral neck