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Cancellous bone grafting at plate removal to counteract stress protection

Arnold S. Lesser, VMD, Diplomate ACVS

Fractures of the distal portions of the radius and ulna in dogs of small breeds can result in dual problems. During the healing process, bone atrophy is expected, especially if there is no weight placed on the limb. In toy breeds, there is little bone; therefore, atrophy leaves the bones fragile and susceptible to refracture. A second repair may necessitate bone grafts, and occasionally these cases may result in nonunion or amputation. To circumvent these complications, compression plating has been recommended to allow early weight-bearing, thus reducing bone atrophy caused by excessive disuse; however, stress protection from the plate also may result in bone atrophy. Because stress protection weakens the bone via osteoporosis, the timing of plate removal is important to reduce risk of refracture.

In the following series of cases plates were removed early when bone union was evident on radiographs. Experimental work has shown that healed fractures are one third as strong as normal bone immediately after plate removal. This is reversible, with the healed bone doubling in strength in one month. To shorten this period, an autogenous cancellous bone graft was placed in the screw holes and along the cortex where the plate had been in contact with bone. The bone graft was harvested from the proximal portion of the contralateral tibia or proximal portion of the ipsilateral humerus.

A 10-month-old, 7-kg, spayed female Miniature Poodle (dog 1) was referred with fractures of the distal third of the left radius and ulna. A 5-hole ASIF dog plate was used to repair the radius (Fig 1).

A cotton bandage was placed on the limb for 10 days to retard postoperative edema and give added support. Eight weeks after surgery, the plate was removed and autogenous cancellous bone chips from the proximal aspect of the contralateral tibia were placed in the empty screw holes and along the plate track of the radius (Fig 2). A splint was placed on the limb for 10 days, after which the dog was allowed to put full weight on the limb. Radiographs were taken 20 days after plate removal and grafting (Fig 3).

An 8-month-old, 5-kg, female Miniature Poodle (dog 2) was referred with fractures of the distal third of the left radius and ulna. The radial fracture was repaired with a 6-hole ASIF cat plate, applied without compression. After surgery, a cotton bandage was applied for support and to reduce swelling. The bandage and sutures were removed 2 weeks later. At 6 weeks, no fracture line was visible on radiographs, and the 2 fragments were connected by mature callus. The plate was removed 9 weeks after surgery, and an autogenous cancellous graft from the contralateral proximal tibia was placed in the empty screw holes and along the previous bed of the plate. The limb was supported with a cotton bandage for one week, and follow-up radiographs were obtained 20 days later.

A 6-month-old, 8-kg, male Italian Greyhound (dog 3) was admitted with bilateral fractures of the distal third of the radius and ulna. A 5-hole ASIF cat plate was placed on the left radius. Because of anesthetic complications, the repair of the right limb was postponed for 2 days, at which time a 6-hole semi-tubular self-compressing cat plate was placed on the right limb. Because both limbs were involved, fiberglass splints were used for extra support for the antebrachium. To allow weight-bearing forces to be transmitted to the radius and ulna, the feet were not included in the splint.

The dog was released 5 days after the first surgery and returned 6 days later for suture removal. At that time, the splints were removed and bilateral cotton bandages were applied for support. The dog would bear full weight on the left forelimb and partial weight on the right forelimb at the time.

Six weeks after surgery, the left forelimb appeared healed; radiography revealed complete union on the left radius and ulna, with mature bridging callus, but a radiolucent area remained at the fracture site of the right radius. There was no fracture line visible on the right ulna. Three months after surgery, radiography revealed complete bridging of all fracture lines by mature callus. Because both radii were plated and thinning of the distal portion of each ulna was evident bilaterally (4 mm at fracture; 1.5 mm at healing), it was decided that plate removal would be postponed and the screws retrieved in 2 stages. Proximad to distad, the 1 st , 3 rd , and 4 th screws were removed from the left radius, leaving the 2 nd and 5 th screws. The 1 st , 5 th , and 6 th screws were removed from the right radius. This order was chosen to allow future comparison of removal of the middle screw with removal of the end screws. In the left radius, the 1 st screw was removed instead of the 2 nd screw because the 1 st screw had loosened.

Seven weeks later, the plate and remaining screws were removed from the left limb, and an autogenous cancellous bone graft from the proximal aspect of the ipsilateral humerus was placed in the open screw holes and along the plate bed of the radius (Fig 4). Three weeks later, the left limb was radiographed (Fig 5), and the above procedure was performed on the right limb. The owners failed to return the dog for follow-up radiographs of the right limb, but phone conversations revealed that the dog was walking well on both limbs one month after the final procedure.

Radiography 10 to 20 days after grafting revealed improvement in the bone mass of the radius at the fracture site in 2 cases, and increased density of bone in the third (dog 2). In dog 1, there was an increase from 5 mm at time of plate removal to 7.5 mm 20 days later, a gain of 50%. In dog 3, there was an increase from 5 mm at time of plate removal to 7 mm 21 days later, a gain of 40%. The second limb in dog 3 also was grafted, but the owners failed to return the dog for follow-up radiographs.

In dog 3, staggered screw removal was used in hope of further reducing stress protection by loosening and shortening the bone plate contact. By loosening the plate, it was hoped that some forces would be redirected through the bone rather than through the plate alone, hence stimulating the bone to remodel and strengthen. Work has shown that an area of ischemia persists where the plate is in close, firm contact with the underlying bone; therefore, loosening the plate might relieve the ischemia. This was accomplished by removal of 3 of the 6 screws on one limb and 3 of the 5 screws on the other. The remaining screws and plate were removed 7 and 10 weeks, respectively, from the first partial screw removal. However, when the plate was removed completely from both limbs, radiography did not reveal any increased bone mass or density, as compared with the limbs in dogs 1 and 2, from which all the screw were removed at one time. There also was no measurable difference radiographically between the right and left limbs of dog 3, where the loosened plate remained for 3 additional weeks. Because no stress testing for failure was possible with a clinical case, it may be that the staggered screw removal did add to the strength without obvious radiographic changes.

The question of if and when to remove a plate from a healed fracture is still under debate. Recommendations of 1 to 1 ½ years for plate removal was proposed originally by the ASIF. Other recommendations for removal at the time of bone union or on the basis of the age of the animal have been made. It has been shown experimentally that plate fractures (plate removal at 10 weeks) are weaker than similar fractures treated with IM pins or half-Kirshner splintage. A number of experiments using both osteotomized and intact bone have revealed a reduction of strength directly under a plate.

Up to one third of the strength of bone can be lost from stress protection in human femurs and tibias.

The changes responsible for weakness under the plate appear histologically as osteopenia, cortical thinning, increased medullary cavity diameter, and increased woven bone. They are caused by increased bone resorption, with a smaller increase in bone formation. Stress protection has been blamed for these changes responsible for local loss of strength. There is a 9:1 difference in the elastic coefficient between 316 stainless steel plates and bone. This difference allows the stiffer plate to absorb bending and shearing forces transmitted along the limb. Therefore, the bone under the plate is protected from these forces. The mechanism for the histologic changes is attributed to Wolff's law. Wolff's law states that all changes may occur in the function of a bone are attended by definite alterations in its internal structure. This is seen as bone deposition in areas of compression and bone removal in areas of tension. Therefore, bone will be thicker and well organized where stress is placed on it and thinner and less organized where there is no stress. This phenomenon allows bone to remodel in response to load. Another theory incriminates a compromise to the local blood supply within the bone because of the presence of the plate itself.