The Control Of Adventitious Bone Formation In Amputation Surgery With Silicone Polymer Implants

Ralph Lusskin, M.D. Walter A. L. Thompson, M.D. Arturo Pena, M.D. Shiva Sanwal, M.D.


A preliminary report of a research program supported by Grant #2229-M from the Social and Rehabilitation Service of the Department of Health, Education, and Welfare. Laboratory studies were conducted at the Milbank Research Laboratories of the Institute for the Crippled and Disabled in New York City.

Adventitious bone formation is a problem which arises following trauma to soft parts and joints, arthroplasty, paralysis, and amputation. The problem as it relates to undesired bone formation following surgery has been of concern to skeletal surgeons for the past fifty years. This condition is the antithesis of insufficient bone formation when skeletal healing is desired.

Osteophyte formation or spurring at the end of an amputation stump frequently produces pain, creates problems with the prosthesis, and sometimes contributes to breakdown of the relatively avascular tissue at the end of the stump. Bone formation at the stump end is associated with increased cicatrix formation, diminished tissue mobility, and transfer of mechanical stress to peripheral neuromata.

The investigators' interest in this problem was aroused during the treatment of a series of amputees with painful phantom phenomena. This study was instituted to achieve modification of the unwanted hypertrophic bone production.

Modification of Healing

This study was designed as an attempt to modify the healing response of bone by the use of medical grade silicone elastomer implants. These implants were intended to act as barriers which would limit the osteogenic activity of traumatized bone and prevent the organization of surrounding tissues and hematoma by osteoblastic cells from the bone end and periosteum. The implants were used following bone resections which did not involve ablation of the extremity as well as resections which did constitute amputations. Bone necrosis resulting from the isolation of the cut bone end by the implant was studied. Since most of the blood supply to bone is derived from the medullary vessel, exclusion of the surrounding soft tissues should not produce necrosis of the entire underlying bone if the excluded segment is limited. The length of bone which can be permanently isolated from periosteal vessels without significant deep necrosis is determined by the location of nutrient vessels and the extent of intramedullary anastomoses.

Since silicone implants may be useful following joint resections and in reconstructive joint procedures such as arthroplasty, the study was designed to give some information concerning these additional uses.

Methodology

Mature cats were used in the initial experimentation. The fibulae of these animals may be transected, isolated and subsequently removed without interference with the structural integrity of the limb. This bone measures approximately 2-3 mm. in diameter and possesses intramedullary vessels. Following transection of the fibula and isolation of the bone end, tibial and femoral amputations may subsequently be performed to determine the efficacy of implants under these conditions. Adequate soft tissue masses are available and satisfactory wound healing and survival of the animal may be achieved.

The implant material used is medical grade silicone polymer (Silastic). Its properties include pliability, complete biological inertness, resistance to sterilizing heat and moisture, and slow permeability to gases. It is an ideal temporary or permanent implant material for use in living organisims.

Silicone polymers have been used for numerous types of human implants by plastic, cardiovascular, and neurosurgeons as well as other investigators. This material is readily available in sheets, and in tubular and rod forms.

Types of Implants

Four methods of construction and/or application of the experimental implants were tried: (1) Silastic sheeting was tied over the bone end; (2) Caps of silicone rubber were constructed from short segments of silicone rubber rods; (3) Silicone tubing was filled at one end with liquid self-curing silicone; (4) Larger experimental caps were constructed from silicone tubing to which silicone rod plugs were glued at one end with liquid self-curing silicone polymer. Subsequent to construction, all implants were thoroughly washed and then autoclaved.

For the preliminary study, 1 cm. segments of both fibulae of mature cats were excised. On one side a silicone cap 0.5-1.0 cm. in length was placed over the two bone ends and on the other side no cap was applied. Thus, each animal became its own control. Specimens were removed for examination at one, two, three, and four weeks; and two, four, six, and twelve months.

Longitudinal and microscopic sections of the resected specimens were compared following decalcification and staining with hematoxylin and eosin.

Upper tibial and femoral amputations were then performed and half the animals were treated by silicone capping of the transected bone or bones prior to wound closure. Specimens were removed at one, two, four, six and twelve months.

When the preceding studies demonstrated that the technique was advantageous, implants for human use were designed and utilized in selected clean cases. Except in juvenile amputation all implants have been applied following amputation revision for pain phenomena.

Results

Fibula Capping. End capping of transected fibulae was performed on 40 adult cats. A total of 93 silicone caps were implanted. Active medullary bleeding was apparent following transection of bone. While a zone of bone necrosis was present at the cut ends and along the superficial cortex, this necrosis proved to be no greater than that found in the controls. Intramedullary blood supply was maintained. Active repair and new bone formation continued under the implants (Fig. 1 , Fig. 2 , Fig. 3 and Fig. 4 ). When the implants slipped along the fibula, permitting a gap to develop between the end of the implant and the cut bone end, this gap filled with clot, callus, and, eventually, new bone (Fig. 5 ). Rather remarkable remodeling of bone took place within irregular, hand-cut implants which were first used in this study. New bone which developed within the implants conformed to the irregular contours of the implant (Fig. 6 and Fig. 7 ).

When the silicone cap was smooth and fitted closely to the bone end, smooth mature bone eventually developed within the cap (Fig. 8 ).

The control specimens showed varied amounts of hypertrophic bone formation. In some, this hypertrophy was quite pronounced (Fig. 9 ). In other animals, complete healing of the fibula with reconstruction of the bone occurred. Of great interest was the finding that the pattern and extent of necrosis in the control and capped bones was similar. Fig. 10 demonstrates the empty lacunae in a control specimen after one year.

Implants in Humans

As a result of these animal studies, implants for application to humans have been developed. These implants are designed primarily to limit osteogenic activity at the ends of the transected bones after amputation surgery and to improve the end-loading stamina of the stump. They are also being tested for their ability to prevent unwanted spur formation after other bone resections.

It is anticipated that the biologic activity of the bone ends will not be adversely influenced by the implants as the intramedullary blood supply is not damaged by the implantation procedure.

Three types of implants have been developed and designated fibular, tibial, and femoral, respectively.

The fibular implant is thin (0.32 or 1/32 in. thick), and has a series of grooves on its inner surface. These grooves reduce any tendency the implant might have to migrate distally, and permit circumferential ligation of the implant. These grooves rapidly fill with callus. The fibular implant is made in five sizes (Fig. 11 ). These implants are used to cap the fibula in below-knee amputation and may have a place in small joint arthroplasty, radial head resections and distal ulna resections when motion is desired. The largest size has also been used to cap the humerus of juvenile patients following amputation revision for spur formation and overgrowth.

The tibial implant is demonstrated in Fig. 12 . Three sizes are available. This cap has been made oval in cross section. It stretches to adapt to the triangular tibia and fits either the right or left bone. It is .062 inch (1/16 in.) in thickness. The anterior corner is constructed to fit the standard tibial resection. Contouring of the cortical surface of the tibia may be necessary in short stumps, as the tibia flares proximally.

The femoral cap has been constructed in five sizes. The end has been thickened to 5/16 inches to give some cushioning to the end of the femur (Fig. 13 ). This cap has been used for femoral resections with considerable early success. The periosteum is usually excised and is never placed over the cap. The cap is placed under tension when possible; i.e., a slightly undersized cap is used and is stretched during application. The grooves on the inner surface prevent sliding of the cap. The linea aspera can be contoured beneath the cap.

Contouring of bone to the grooves on the inner surface of the cap can be seen after six months (Fig. 14 ).

Summary

This report describes preliminary studies on the effects of silicone polymer ensheathment of transected bones and describes implants for use in human surgery. These implants are being studied for their suitability and effectiveness in preventing adventitious bone formation, reducing overgrowth in juvenile amputation and improving the load-resistive characteristics of lower-extremity amputations.

Animal studies show active bone regeneration with silicone polymer caps when medullary blood supply has been left intact. Necrosis is no greater than that found in control specimens. No hypertrophic bone formation was noted when the caps were placed extraperiosteally.

Silastic - Supplied by Dow Corning Center for aid to Medical Research.

Ralph Lusskin, M.D., Walter A. L. Thompson, M.D., Arturo Pena, M.D., and Shiva Sanwal, M.D. are associated with the New York University Medical Center New York, New York