Bone Overgrowth In The Juvenile Amputee And Its Control By The Use Of Silicone Rubber Implants
Alfred B. Swanson, M.D.
Numerous physicians involved in the care of the juvenile amputee have described the complication of distal bone overgrowth in the long bones of both the upper and lower extremities when amputation occurs through the diaphysis2,3,4,7,9,13,15,16,18. This complication does not occur in a disarticulation-type of amputation and, for that reason, the disarticulation-type amputation is recommended for all juveniles wherever possible.
Clinically we have observed the overgrowth phenomenon as consisting of a disproportionate growth between the bony stump end and the soft tissue covering. The distal end of bone elongates with gradual stretching of the distal skin which becomes reddened, irritated and secondarily infected. The sharp bone end does not push the distal skin ahead of it but rather becomes fixed to the distal soft tissues, eventually protrudes through them, and a chronic infection results17.
At the Area Child Amputee Center in Grand Rapids, it has been observed that the incidence of overgrowth in a series of 314 patients with all types of noncongenital amputations was 12.4 percent1. In order of decreasing frequency the common sites of overgrowth are: the humerus, the fibula, the tibia, the femur, and lastly, the tibia and fibula in combination. It has been observed repeatedly that amputations through the long bone in the growing child will necessitate multiple repeat operations for the overgrowth problem. The younger the child, the more frequently will the necessity arise.
In planning an epiphysiodesis operation for unequal leg length, the skeletal growth scale of Green11 is of considerable help in predicting future growth. However, this scale is of no value in predicting the amount of distal overgrowth in the amputated long bone. The chart of Duthie6 concerning the growth spurt may help during the follow-up period after amputation.
The pathogenesis of the overgrowth phenomenon is as yet undetermined. The problem is seen only in the growing child. In 1939, Vom Saal23 reported a series of epiphysiodeses in combination with amputations in children. His results clearly showed that despite epiphysiodesis of the proximal epiphysis to arrest growth, the distal overgrowth was not retarded. Utilizing chickens John Hunter12 put lead shot at both ends of the diaphysis of growing bone and demonstrated increased bone growth between the lead shot markers, thus indicating that long bone growth was also appositional. We have also noted that bone will grow distal to a marker placed at the end of a cut long bone in the juvenile amputee. This growth cannot be related to the epiphysis and hence must be regarded as appositional bone growth. Gelb10 noted that most of his patients with overgrowth problems had inadequate skin cover at the end of the stump. For this reason he used cross-leg pedicle skin flaps to obtain a very loose and well-nourished flap over the stump end. He found that recurrent bony overgrowth in his cases was corrected by this method.
It would appear that a vicious cycle starts when periosteal and endosteal bone growth at the end of the bony stump extends into an inadequate soft tissue cover which cannot move ahead. Chronic inflammation results in further stimulation of bone growth21,22. Protrusion into and further fixation of the soft tissues occur. Fibrosis and adhesions of the soft tissues around the bone then follow. External trauma further irritates the stump until the bone eventually protrudes and chronic infection ensues.
Prior Attempts at Solution
In the past, several surgical attempts have been made to solve the problem of fibula overgrowth. Duncan McKeever14 attempted a wedge osteotomy of the fibula into the lateral aspect of the tibial stump. J. Warren White24 used metal buttons with an expansion bolt over the end of the stump. However, the metal buttons were unstable and came off the end of the bone so that the results of his work were inconclusive.
Our earlier observations that the patient with a disarticulation maintained a good stump end without perforation led us to consider the possibility of avoiding the overgrowth complication by simulating this type of stump for the diaphyseal amputation. The cartilage-covered bone, as seen in the disarticulation procedure, offers no area for fixation of the bone to the skin as periosteal and endosteal bone is covered by cartilage. A protective capsule forms around the end of the bone and the skin stays loose and nonadherent. The vicious cycle occurring at the bone end with irritation of the subcutaneous tissue and epidermis with resultant fibrosis, chronic infection, and breakdown of the wound does not occur. We had also observed the results of Esslinger's8 work to develop an implant which would protrude externally to provide an attachment for a prosthetic limb. In his research Esslinger placed Room Temperature Vulcanizing Silicone Rubber on the ends of the amputation stumps of his experimental dogs. We attempted to use the same material to cover the end of the transected diaphysis, thus preventing overgrowth in the juvenile amputee. Our first procedure was performed in August 1963, and involved the humerus of a 6 1/2-year-old child. The material seemed to offer good possibilities for the prevention of bone spiking into the soft tissues. However, the technical problems of maintaining the material over the end of the bone led us to design a heat premolded intramedullary stemmed implant of Silicone Rubber (Dow Corning -Silastic(R) medical-grade elastomer). The first implant to control juvenile bone overgrowth was used on October 2, 1964. This implant has remained in situ and has helped to control the occurrence of bony overgrowth. The development of these implants, which have also been used for end-bearing amputations in the adult, was reported in the February 1966 issue of the Inter-Clinic Information Bulletin19.
An Improved Implant
Our experience with these cases led us to develop an improved implant with better stability and in appropriate sizes for both the upper- and lower-extremity amputee (Fig. 1 ). The latest design of our mushroom-shaped implant has both an intramedullary stem and a collar which fits over the distal portion of the bone (Fig. 2 ). Experience in the use of the implants in both animal and human applications suggests that both endosteal and periosteal bone formation may be retarded if both the intramedullary canal and the distal external surface of the bone are covered with silicone rubber. Intramedullary stemming of the implants, from our observations, does not affect the viability of the shaft to its distal end. However, covering more than one centimeter of the external bone surface may result in aseptic necrosis of the distal bone. Consideration of the extent to which the external surface of the bone can be covered without resulting in aseptic necrosis is the subject of a continuing research evaluation in animals (Fig. 3 ). A firm fibrous capsule is formed around the distal end of the Silastic® implant and a smooth, nonadherent surface is presented to the skin at the distal end of the stump.
Fourteen children with long-bone overgrowth problems have been studied at the Area Child Amputee Center since June 1963 (see Table 1 ). The majority of these patients had had previous surgical procedures for recurrent overgrowth problems. These patients had transdiaphyseal amputations either for trauma or for revisions of congenital amputations (Fig. 4A/B , Fig. 4C/D , Fig. 5A-C , Fig. 6A/B , Fig. 7A/B ). A total of 16 limbs was involved. Most of the patients in this study fell either into the midgrowth spurt group (4 to 6 years of age), or into the adolescent growth spurt group (10 to 14 years of age). It is interesting to note that three patients whose overgrowth extended over a period of seven years had their traumatic amputations during the midgrowth spurt years. Also, young patients, amputated between the ages of three and four years, experienced overgrowth during the midgrowth spurt regardless of whether the amputation was traumatic or was a revision of a congenital defect. The average age of the patients was 9.7 years, the youngest being three years of age and the oldest 19 years. Of the 14 patients, 12, or 83.4 percent, were boys. This ratio probably reflects the higher incidence of trauma in males. The average duration of follow-up was 2.1 years, the shortest being seven months and the longest five years. The sites of overgrowth are: humerus -six; fibula - seven; tibia - two; and tibio-fibula (i.e., involving both bones) - one.
Most complaints stemming from the overgrowth problem refer to such factors as superficial skin breakdown, tenderness at the pointed end of the stump, bursa formation, and protrusion of the bone with chronic infection. The majority of the patients fitted with Silastic® implants in our series were users of artificial limbs. These patients have been followed closely in the clinic. Serial X-rays have been taken and limb lengths measured. Of the 16 operated limbs, five developed complications with infection and skin breakdown. In two of these patients, the implants were removed and then replaced at a later date. Only one patient had a recurrent overgrowth problem following removal of the implant because of infection. The high incidence of complication is explained by the fact that many of these patients had chronically infected stumps and grossly inadequate soft tissue cover. Two had bone growth around the implant, but no spiking or invasion of the soft tissues. Three of the implants were unstable at the bone end. However, overgrowth did not occur through the implant as the implant continued to push the soft tissue away from the bone end.
Our surgical experience in the cases with chronic infection would suggest that the distal end of the bone should be removed first and wound healing secured. Then in three to six months a secondary procedure could be done to install the implant in a cleaner field.
Silicone rubber (Silastic® 372 medical-grade elastomer, Dow Corning) has been used as the implant material because it has good elastic properties, does not fatigue quickly with repeated compression, and its tissue acceptance has been firmly established(5,20).
The implants are preformed from molds, then heat vulcanized. They can be sterilized by autoclaving. A variety of sizes is necessary, so that minimum reaming of the intramedullary canal of the stump bone(s) is required. Roentgenograms, dissections, and microscopic sections have demonstrated biological acceptance of the implants. The bone is in close contact with the shaft of the implant. An insignificant amount of foreign body reaction has been found. In fact, a simple mesothelial-lined fibrous capsule forms over the silicone. No bursa formation has been noted.
The surgical technique in revising amputation cases with overgrowth has consisted of a transverse anterior incision approximately one inch from the end of the stump, resection of the overgrown distal bone, careful minimal reaming of the intramedullary canal, and insertion of a stemmed implant of appropriate size. Periosteal stripping is avoided. The implant should be well seated. The intramedullary stem should not be fixed as this will weaken the implant or may result in wobble. If possible, the implant is held in place by myofascial closure. Ideally, the implant should have a layer of soft tissue between it and the skin. The need for the provision of this covering should be considered when the dissection is done. Catgut sutures have also been used to fix the implant until the appropriate capsule is formed around it. This formation takes approximately three weeks. The skin at the end of the stump is left loose and redundant. The prosthesis may be used as soon as soft tissue healing occurs.
The overgrowth problem in the transdi-aphyseal amputation stump of the growing child has been presented together with a possible solution of the problem using a Silastic® intramedullary stemmed implant at the time of revision amputation. It is our feeling that the overgrowth problem can be prevented if fixation of the soft tissues to the end of the bony stump is prevented and if the periosteum and endosteum at the cut end of the bone are covered.
Of the 16 amputation revisions done using the implant, one had a recurrence of the overgrowth problem after the implant had been removed because of infection. The complications found in these cases were mainly due to postoperative infection as a result of operating through a chronically infected area. A technique for the treatment of these cases is recommended. To avoid the overgrowth problem, the use of disarticulation rather than transdiaphyseal amputation in children is again emphasized.
The author wishes to thank his colleagues, Dr. Charles H. Frantz and Dr. Raphael Recto, for their invaluable assistance in this study.
Alfred B. Swanson, M.D. is associated with the Area Child Amputee Center Grand Rapids, Michigan
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