The Use of Porous, High-Density Polyethylene Caps in the Prevention of Appositional Bone Growth in the Juvenile Amputee: A Preliminary Report

Leslie C. Meyer, M.D. Barry W. Sauer, D.V.M.


Six cases of appositional bone growth in the amputation stumps of juvenile amputees have been treated by the insertion of porous, high-density polyethylene caps. In vivo studies indicate ingrowth of bone into the porous material, thereby preventing extrusion of the cap. The only complication has been one infection, which has been controlled with antibiotics without removal of the cap. All caps have remained intact, are stable, and have prevented appositional growth.


Appositional bone growth is the most common complication requiring surgical correction in the juvenile amputee. This complication is found in both the acquired and the congenital amputee. Aitken1 recorded an incidence of 12.4 percent in a series of 314 cases. Hall2 reported stump revision for terminal skeletal overgrowth as the most common surgical procedure in the U.C.L.A. Child Amputee Clinic. Lambert3 found bony overgrowth in 8.4 per cent of 853 cases with 927 acquired amputations. In 1,115 congenital amputees he found four patients who developed bony overgrowth.

Although periodic revisions keep these problem stumps manageable, many efforts have been made to prevent appositional growth and thereby avoid repeated surgical procedures. Early observers logically considered this problem a matter of overgrowth because many of the involved amputation stumps had proximal epiphyses which contributed substantially to the elongation of the respective limbs, i.e., humerus, tibia, and fibula. In 1939 Vom Saal7 thought that appositional growth could be controlled by proximal epiphysiodesis. Aitken1, however, proved this concept wrong by placing metallic clips in the resected bone end. Subsequent roentgenographic studies indicated that bone was added distal to the implanted metal marker.

Histological studies indicate that appositional bone growth occurs as the result of direct metaplasia of fibrous tissue, or by way of enchondral ossification. Remodeling, with extensive osteoclastic activity at the cortical margins, ultimately produces a tapered or spiked end which protrudes through the terminal end of the stump, making revision necessary. Failure of this phenomenon to occur in all juvenile amputees, and its disappearance once the epiphyses have closed physiologically, are observations which have not been explained.

Prosthetic fitting may hasten the appositional-growth phenomenon; but it is not the only cause, since this growth has been observed in limbs that have not been fitted. Poorly fitted sockets are a factor, yet careful repetitive fitting with total-contact sockets has not prevented overgrowth in young acquired amputees in our clinic.

Logically, the bone end also has been attacked; and as early as 1940 Dr. I. Warren White, former Chief Surgeon of the Shriners Hospital, Greenville, South Carolina, fashioned a metallic button with a split stem (Fig. 1 ) to prevent protrusion of the bone end in a young girl with an acquired midthigh amputation for a tumor. This metallic implant was extruded and could not be retained in the bone end in spite of its split stem. Swanson6 renewed the principle of capping the bone end with silicone implants, but retention in the bone remained a problem. Marquardt4 presently is capping the bone end with homologous cartilaginous implants in an effort to create a condition similar to a joint disarticulation. His work may well provide a physiological barrier to appositional growth.

Intramedullary Cap Development

We believe that appositional growth can be lessened and protrusion through the terminal end of the stump can be prevented if the bone end can be capped satisfactorily with some type of material. A pliant skin can easily accommodate increased length produced by proximal epiphyseal growth in the amputation stump.

The search for a suitable material which could be used to cap the bone end without extrusion resulted in animal experimentation with porous, high-density polyethylene. Polyethylene has a fairly long history as an implant material. Early implants were fabricated from low-molecular-weight polyethylene, but more recently high-density polyethylene and ultra-high-molecular-weight polyethylene have been used. The latter two materials are stronger and are more resistant to chemical degradation than the low-molecular-weight polyethylene. The porous, high-density polyethylene used in fabrication of the bone caps has a molecular- weight range of 500.000 to 1,000,000 and a pore size of approximately 250 microns. Animal studies using this material have demonstrated rapid bone ingrowth. Pellets of polyethylene implanted in the lateral cortex of the canine femur showed a bony ingrowth to a depth of 2 mm in 14 days (Sauer, et al.5).

Subsequent to the animal studies, high-density, porous polyethylene caps were fabricated for insertion into the juvenile amputation stump. Caps used in this study were of two types. Those used in our first three cases were machined from rod stock supplied by Porex Materials Corporation of Fairburn, Georgia. The caps used in three later cases were molded by the same company. The machined caps and the molded caps were similar in design, with a porous intramedullary stem and a pericortical cuff with a solid end piece. The machined caps had end pieces of silicone rubber, while in the molded caps the end piece was solid, high-density polyethylene. Caps currently are available in five sizes having intramedullary stem diameters of 5,7,9, 11 and 13 mm (Fig. 2 ).

Clinical Experience

Six juvenile amputees with appositional bone growth formed the basis of this preliminary study in which caps were used to prevent appositional growth. Three amputations were congenital, two were traumatic, and one was the result of gangrene due to chicken pox (purpura fulminans) (Table 1 ). Four occurred in the tibia, one in the humerus, and one in the femur (Table 2 ) (It should be noted that in the below-knee amputations the fibula, when present, was consistently overgrown). The implanted caps have been in place for an average of 14 months, with a maximum of 23 months and a minimum of eight months.

Fig. 3 and Fig. 4 are preoperative and postoperative radiographs showing cap implantation in a white male, aged 12, with a traumatic above-knee amputation. The cap remains firmly embedded in the intramedullary canal, and there is no tapering of the bone end. The cortex is thin but not eroded. There is a triangular-shaped area of periosteal reaction at the upper margin of the stem of the cap.

In our six cases there has been one complication consisting of an infection. In spite of this infection the cap was not extruded, and with the administration of antibiotics the child continues to retain the cap in the bone end and is wearing his prosthesis satisfactorily.

It is inadvisable to insert the implant if protrusion of the bone has occurred and secondary infection is present. It is best to remove the extruded bone, excise the granulation tissue and scar, and allow the wound to heal prior to insertion of the cap.

Surgical Procedure

Insertion of the implant is not difficult and is performed under tourniquet. A terminal incision is used to excise the scar and bursa and to expose the appositional bone. The bony spike is resected sufficiently to allow for insertion of the stem of the cap into the intramedullary canal and to provide skin closure without tension. Care should be taken not to shred or tear the periosteum during the bone transection, and an a-periosteal cuff is not constructed. The intramedullary canal is reamed to stem size. Because a tight fit is essential, the cap should fit tightly over the bone end with firm impaction.

We have not intended to make these stumps end-bearing, although the rapidly growing child frequently outgrows his prosthesis and becomes end-bearing without apparent difficulty.


Appositional bone growth in the juvenile amputee occurs sufficiently often to warrant control of its occurrence, thereby avoiding surgical revisions.

It appears from our preliminary studies that the insertion of a single cap may well control the phenomenon of overgrowth. The high-density, porous polyethylene cap has many characteristics which make it suitable for this implant.

Shriners Hospital for Crippled Children, Greenville, South Carolina

Dr. Sauer is Associate Professor of Bioengineering at Clemson University, Clemson, South Carolina.

1. Aitken, G. T., Osseous overgrowth in amputations in children. In Swinyard, C. W. (Editor), Limb Development and Deformity-Problems of Evaluation and Rehabilitation. 448-456, Charles C. Thomas, Springfield, Illinois, 1969.
2. Hall, Cameron B., Recent concepts in the treatment of the limb-deficient child. Artif. Limbs, 10:1:36-51, Spring 1966.
3. Lambert, Claude N., Amputation surgery in the child. Orthop. Clin. No. Amer., 2:2:473-482, July 1972.
4. Marquardt, E., Personal communication, 1975.
5. Sauer, Barry W., A. M. Weinstein, J. J. Klawitter, S. F. Hulbert, R. B. Leonard, and J. G. Bagwell, The role of porous polymeric materials in prosthesis attachment. 145-153, J. Biomed. Material, Res. Symposium No. 5, (Part 1), 1974.
6. Swanson, Alfred B., Improving the end-bearing characteristics of lower extremity amputation stumps: a preliminary report. Inter-Clin. Inform. Bull., 5:5:1-7, February 1966.
7. Vom Saal, F. H., Epiphysiodesis combined with amputation. J. Bone and Joint Surg., 21:442-443, 1939.