Biologically compatible bone cements and orthopedic methods

Details Biologically compatible bone cements and orthopedic methods Biologically compatible bone cements and orthopedic methods

2001-03-01

2004-04-20

Padmanabhan, Sreeni (Department: 1619)

Drug, bio-affecting and body treating compositions

Preparations characterized by special physical form

Implant or insert

C424S602000, C424S665000, C424S678000, C424S679000, C424S680000, C424S682000, C523S116000

Reexamination Certificate

active

06723334

ABSTRACT:

FIELD OF INVENTION
This invention relates to biocompatible bone cements and orthopedic methods of use.
BACKGROUND OF THE INVENTION
Orthopedic repairs, required, for instance, as the result of trauma, surgical removal or skeletal changes, are in need of improvement. Existing methods and materials present many unsatisfactory characteristics. Existing orthopedic implants are walled off by the body by a fibrous capsule as the result of the foreign-body protective response of the tissue in contact with the implant. This prevents a strong physical bond between the tissue and the implant. Failure of a joint repair or replacement is often attributed to movement made possible by the presence of the soft fibrous capsule. The capsule gets progressively thicker as the implant ages in the body and the implant becomes more mobile and the motion exceeds a critical level.
Presently, the average service life of a prosthetic implant is about 12 years. About 60% of implants need revision during the lifetime of the patient, subjecting the patient to additional surgery and the risks that accompany such procedures. The success rate is even lower for revision implants. Furthermore, a second revision is often impractical.
Commonly an orthopedic repair includes bone or joint replacement. One important reason for joint replacement is arthritic deterioration that produces pain and loss of mobility and fracture as the result of cartilage and bone deterioration. Another is the result of osteoporetic bone, especially in post-menopausal females, where bone resorbtion produces weak, brittle, and porous bone. Another reason for bone or joint replacement is the deterioration of specific areas of bone resulting from the failure of the circulatory system related to that specific area of bone. When the blood supply is occluded, the loss of blood produces necrotic bone in the affected area. This can lead to bone deterioration and collapse. For example, the spongy bone supporting the articular cortex and cartilage of a proximal femoral head can collapse if its blood supply is lost to disease, injury, or surgical trauma.
Known joint replacements typically incorporate metal, plastic, or ceramic components that are sufficiently biologically-inert that they seldom cause tissue reactions other than the fibrous capsule from the foreign-body response mechanism. A typical hip replacement utilizes a dense polyethylene cup cemented into the reamed acetabulum, and a metal stem cemented into a reamed hole in the femur proximal to the greater trochanter. The ball rotating in the polyethylene cup can be metal, integral with the stem, or ceramic attached to the stem. The metal components are usually a cobalt-chrome alloy, or Titanium Al
6
V
4
alloy. In animals, it is often 316-L stainless steel. The ceramic ball, and sometimes the cup, can be polycrystalline alumina or, more rarely, a zirconium composition. Failure of these systems is often the result of wear debris; particles of polyethylene and particles of metal often are separated from the prosthetic and invoke an inflammatory response, bone resorbtion, and pain, ultimately resulting in a loosening of the entire prosthesis. This usually occurs within the joint capsule. Wear of the polyethylene is the most common source of the debris. The tissue response includes granulation of tissue by a persistent foreign-body reaction, transforming the articulating capsule into a mass of fibrous tissue that can extend to the ligaments and muscles. Large areas of bone can become poorly vascularized and necrotic. The final stages of deterioration include resorbtion of the supporting bone.
The cement used to attach joint components to surrounding tissue is typically a polymethylmethacrylate (PMMA) cement, which may be modified by chemical additions for radio-opacity or short-term antibiotic activity, for instance. PMMA cements set by an exothermic polymerization reaction. Full strength is obtained quickly, so the cement has the advantage of providing support immediately after setting. The working time and setting time can be controlled to provide the surgeon with a surgically practical cement. It was the development of PMMA cement that made joint replacement possible.
For aged patients with short life expectancy the replacement of “broken hips” with a PMMA-cemented prosthesis was an improvement when it was first invented. For patients having longer lifetimes, there are serious problems as discussed below. The American Society for Testing and Materials specifies the following requirements (ASTM F-451) for PMMA cement:
Working time
5 minutes maximum
Setting time
5-15 minutes
Strength
70 MPa minimum
Solubility
0.05 mg/cm
3
maximum
Temperature rise
90° C. maximum
Intrusion
2.0 mm minimum
The solubility is limited to reduce both local tissue and systemic responses (e.g., when the monomer is distributed systemically it can lower blood pressure and affect organs.). The temperature rise is limited to reduce the cauterization and death of tissue overheated by the exothermic setting reaction. The hazards associated with solubility and temperature rise are well recognized. Other affects are not.
The cement must fill the space between the prosthesis component and the bone. The geometry of the prosthesis component is shaped to aid the load-bearing requirement. The prosthesis-to-PMMA bond and the PMMA-to-tissue bond participate in this. The prosthesis-to-PMMA bond is controlled by the bond chemistry and prosthesis geometry. The PMMA-to-tissue bond is controlled by the tissue reactions and the body's physiological response. Initially this response includes tissue resorbtion and then reconstruction through wound-healing mechanisms to repair the damage produced by the surgical trauma and the temperature rise. When first inserted, the PMMA is smooth and undesirable tissue response is limited. With time, the PMMA becomes rough and brittle, also cracking and releasing fragments. The fragments of cement invoke an inflammatory response in the surrounding tissue and the cracks provide fresh surfaces for chemical exchange. The PMMA is weakened by the cracks. Inflammation and tissue resorbtion further weakens the PMMA-to-tissue bond and, ultimately, failure of the prosthetic device occurs. The most common reason for device replacement is pain, usually occurring with inflammation and device loosening under stress at one of the two bond sites.
Another concomitant problem is that there is no chemical bond between the PMMA and the bone tissue. Instead a mechanical bond is achieved by forcing the fluid PMMA cement, under pressure, into the bone to penetrate pores and irregularities in the bone geometry. Sometimes a dam is inserted in the intramedullary space to restrict the longitudinal flow of the PMMA cement and obtain higher pressure and more radial flow. As an example, the epiphysis region is an important load-bearing area composed of trabecular bone with the trabeculae oriented to transmit the load from one load-bearing region to another. The trabeculae are strong, thin regions of bone, forming the mesh-like interiors of spongy bone, commonly growing along stress lines. Their blood supply comes from the pores (also oriented by the trabeculae orientation) and from the intramedullary region, from attached tendons and from surrounding muscle, although the latter is usually less important. When a blood supply is removed by surgery, it must be compensated by other sources. This is not possible if the pores supplying blood are blocked by the PMMA cement.
Thus, inherent in the use of PMMA cement is an undesirable interference with blood supply. Even in healthy bone, fracture of the hydroxyapatite (“HA”) occurs locally and must be repaired or remodeled. Although PMMA cements contribute strength to the bone by filling the pores and supporting the trabeculae, such cements do not have enough strength when the trabeculae become seriously weakened, which is all but inevitable. Therefore the use of PMMA cement presents a basic limitation to the longevity of an implant. The cement breakdown and the PMMA-induced tissue respo

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