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

The operation of the century: total hip replacement

Bone resorption around hip stems is a disturbing phenomenon, although its clinical significance and its eventual effects on replacement longevity are as yet uncertain. The relationship between implant flexibility and the extent of bone loss, frequently established in clinical patient series and animal experiments, does suggest that the changes in bone morphology are an effect of stress shielding and a subsequent adaptive remodeling process. This relationship was investigated using strain-adaptive bone-remodeling theory in combination with finite element models to simulate the bone remodeling process. The effects of stem material flexibility, bone flexibility, and bone reactivity on the process and its eventual outcome were studied. Stem flexibility was also related to proximal implant/bone interface stresses. The results sustain the hypothesis that the resorptive processes are an effect of bone adaptation to stress shielding. The effects of stem flexibility are confirmed by the simulation analysis. It was also established that individual differences in bone reactivity and mechanical bone quality (density and stiffness) may account for the individual variations found in patients and animal experiments. Flexible stems reduce stress shielding and bone resorption. However, they increase proximal interface stresses. Hence, the cure against bone resorption they represent may develop into increased loosening rates because of interface debonding and micromotion. The methods presented in this paper can be used to establish optimal stem-design characteristics or check the adequacy of designs in preclinical testing procedures.

/pdf/the-relationship-between-stress-shielding-and-bone-4o0laxx9ds.pdf

The relationship between stress shielding and bone resorption around total hip stems and the effects of flexible materials

The Social Transformation of American Medicine is one of the most comprehensive studies on the rise of the medical profession and the development of the health care industry published to date, Starr is able to span the fields of medicine so that he discusses intelligently the economic, political, and historical developments in medical care. His wri ting is clear and succinct, his arguments are copiously footnoted, and the inferences he draws are sound. In Book I, he covers \"the rise of medical authority and the shaping of the medical system\"; in Book II, \"doctors, the State, and the coming of the corporation.\" Reviews by Daniel Bell of Harvard and George Silver of Yale call Starr's work brilliant-I would agree. I would recommend this book to anyone who wants to understand the current health care system. Starr describes the movement of the medical profession from one that was initially viewed skeptically to one that was later embraced; and now, coming full circle, to one that is viewed critically. Starr maintains that the current status of American medicine is the result of our history of accommodating professional interests while failing to exercise control over health programs, and then needing to adopt piecemeal regulations and cut-backs on programs that become too inflationary, One of the primary messages I take from this book is the importance of a com bination of forces: a profession's authority, the political climate, and the current philosophy about health care. Starr illustrates how these forces coalesce to defeat or achieve medical improvements. As occupational therapists, we are dependent on the development of the health care industry. It is wise for us to understand the forces that have an impact on medical care and what they could mean for our profession, Kay Barbara Schwartz, M.S., OTR

The social transformation of american medicine.

Acrylic bone cement occupies a distinctive place in the hierarchy of synthetic biomaterials, because it is the only material currently used for anchoring the prosthesis to the contiguous bone in a cemented arthroplasty. However, the cement is not without its drawbacks. The main one is the role that it has been postulated to play in the aseptic loosening and, hence, clinical life of the arthroplasty. In turn, this role is directly related to the mechanical properties of the cement, especially the resistance to fracture of the cement in the mantle at the cement-prosthesis interface or the cement-bone interface. The present work is a detailed critical review of the recent literature on the properties of bone cement that are considered germane to its use in the stated application. The relevant properties are identified and a case is made for including each of them. Compilations of the values of these properties, obtained under clearly identified conditions, are presented for the six commercial formulations of bone cement in current popular orthopedic use. The gaps and unresolved questions in the current data base, efforts that should be made to address these issues, and research directions are covered.

Properties of acrylic bone cement: state of the art review.

A balloon for use in compressing cancellous bone and marrow (also known as medullary bone or trabecular bone) against the inner cortex of bones whether the bones are fractured or not. The balloon comprises an inflatable, non-expandable balloon body for insertion into said bone. The body has a shape and size to compress at least a portion of the cancellous bone to form a cavity in the cancellous bone and to restore the original position of the outer cortical bone, if fractured or collapsed. The balloon is prevented from applying excessive pressure to the outer cortical bone. The wall or walls of the balloon are such that proper inflation the balloon body is achieved to provide for optimum compression of all the bone marrow. The balloon is able to be folded so that it can be inserted quickly into a bone. The balloon can be made to have a suction catheter. The main purpose of the balloon is the forming or enlarging of a cavity or passage in a bone, especially in, but not limited to, vertebral bodies.

Inflatable device for use in surgical protocol relating to fixation of bone

Cylindrical porous-coated implants were placed in the distal femoral metaphyses of twenty dogs and were subjected to zero, twenty, forty, or 150 micrometers of oscillatory motion for eight hours each day for six weeks with use of a specially designed loading apparatus. The in vivo skeletal responses to the different magnitudes of relative motion were evaluated. Histological analysis demonstrated growth of bone into the porous coatings of all of the implants, including those that had been subjected to 150 micrometers of motion. However, the ingrown bone was in continuity with the surrounding bone only in the groups of implants that had not been subjected to motion or that had been subjected to twenty micrometers of motion; in contrast, the implants that had been subjected to forty micrometers of motion were surrounded in part by trabecular bone but also in part by fibrocartilage and fibrous tissue, and those that had been subjected to 150 micrometers of motion were surrounded by dense fibrous tissue. Trabecular microfractures were identified around three of the five implants that had been subjected to forty micrometers of motion and around four of the five that had been subjected to 150 micrometers of motion, suggesting that the ingrown bone had failed at the interface because of the large movements. The architecture of the surrounding trabecular bone also was altered by the micromotion of the implant. The implants that had stable ingrowth of bone were surrounded by a zone of trabecular atrophy, whereas those that had unstable ingrowth of bone were surrounded by a zone of trabecular hypertrophy. The trabeculae surrounding the fibrocartilage or fibrous tissue that had formed around the implants that had been subjected to forty or 150 micrometers of motion had been organized into a shell of dense bone tangential to the implant (that is, a neocortex outside the non-osseous tissue). CLINICAL RELEVANCE: The findings of the present study quantitate the in vivo patterns of bone ingrowth and remodeling that occur in association with different magnitudes of micromovement of porous-coated implants. Small movements (zero and twenty micrometers) are compatible with stable ingrowth of bone and atrophy of the surrounding trabecular bone, whereas larger movements (forty and 150 micrometers) result in less stable or unstable ingrowth of bone, the formation of fibrocartilage or fibrous tissue around the implant, and hypertrophy of the surrounding trabecular bone. This study not only quantified the magnitudes of relative micromotion that cause these different skeletal responses but also may help in the interpretation of radiographs of patients who have a porous-coated prosthesis.

In Vivo Skeletal Responses to Porous-Surfaced Implants Subjected to Small Induced Motions*

We evaluated the initial stability of cemented and uncemented femoral components within the femoral canals of cadaver femurs during simulated single limb stance and stair climbing. Both types were very stable in simulated single limb stance (maximum micromotion of 42 microns for cemented and 30 microns for uncemented components). However, in simulated stair climbing, the cemented components were much more stable than the uncemented components (76 microns as against 280 microns). There was also greater variation in the stability of uncemented components in simulated stair climbing, with two of the seven components moving 200 microns or more. Future implant designs should aim to improve the initial stability of cementless femoral components under torsional loads; this should improve the chances of bony ingrowth.

Micromotion of cemented and uncemented femoral components

Eleven whole anatomic specimens of the femur were retrieved at autopsy from patients who previously had cemented total hip arthroplasty. Implant duration ranged from 0.5 to 210 months. Clinically and roentgenographically the implants were stable. A detailed biomechanical analysis evaluated bone strains and implant stability in both the single-limb stance and stair-climbing positions using a 100-pound spinal load. The stability offered by cement in these well-fixed prostheses was remarkable, with the maximum axial micromotion being 40 mu. This is a reflection of intimate osseointegration at the bone-cement interface with only rare intervening fibrous tissue. The strain gauge and photoelastic strain-coating studies revealed that marked stress shielding in the proximal medial femoral cortex persists long after a cemented femoral component is inserted. Even 17 years after surgery, the strain in the calcar region did not normalize.

Biomechanical and histologic investigation of cemented total hip arthroplasties. A study of autopsy-retrieved femurs after in vivo cycling.

In this study centrifugation dramatically reduced the porosity and substantially increased the mechanical properties of bone cement. Monotonic tensile tests to failure of centrifuged specimens of cement demonstrated an increase of 24 per cent in the mean ultimate tensile strength compared with the control value. Mean ultimate tensile strain was improved by 54 per cent. In fully reversed tension-compression fatigue-testing, centrifugation resulted in a mean increase in fatigue life of 136 per cent. These strong advantages in mechanical properties were obtained without any detrimental changes. There was no change in elastic modulus, setting time, or peak temperature. Handling properties were improved. There was no increase in systemic toxicity as demonstrated in dogs by assessment of arterial blood-pressure response and peak levels of monomer in the serum during simulated total hip arthroplasty. We also present a practical system of cement centrifugation and delivery that is suitable for use in the operating room.

Centrifugation as a method of improving tensile and fatigue properties of acrylic bone cement.

Nerve damage takes place during surgery. As a consequence, significant numbers (10%-40%) of patients experience chronic neuropathic pain termed surgically induced neuropathic pain (SNPP). The initiating surgery and nerve damage set off a cascade of events that includes both pain and an inflammatory response, resulting in "peripheral and central sensitization," with the latter resulting from repeated barrages of neural activity from nociceptors. In affected patients, these initial events produce chemical, structural, and functional changes in the peripheral and central nervous systems (CNS). The maladaptive changes in damaged nerves lead to peripheral manifestations of the neuropathic state-allodynia, sensory loss, shooting pains, etc, that can manifest long after the effects of the surgical injury have resolved. The CNS manifestations that occur are termed "centralization of pain" and affect sensory, emotional, and other (eg, cognitive) systems as well as contributing to some of the manifestations of the chronic pain syndrome (eg, depression). Currently there are no objective measures of nociception and pain in the perioperative period. As such, intermittent or continuous pain may take place during and after surgery. New technologies including direct measures of specific brain function of nociception and new insights into preoperative evaluation of patients including genetic predisposition, appear to provide initial opportunities for decreasing the burden of SNPP, until treatments with high efficacy and low adverse effects that either prevent or treat pain are discovered.

/pdf/surgically-induced-neuropathic-pain-understanding-the-ynqu9mkmaz.pdf

Dennis W. Burke

Papers

In Vivo Skeletal Responses to Porous-Surfaced Implants Subjected to Small Induced Motions*

Micromotion of cemented and uncemented femoral components

Biomechanical and histologic investigation of cemented total hip arthroplasties. A study of autopsy-retrieved femurs after in vivo cycling.

Centrifugation as a method of improving tensile and fatigue properties of acrylic bone cement.

Surgically induced neuropathic pain: understanding the perioperative process.