Robert E Booth and David G Nazarian

Distal femoral fractures associated with a total knee arthroplasty are mercifully rare, as they are arguably among the most difficult osseous infractions to treat. Fractures occurring outside the "no-man's land" between the femoral epicondyles and the femoral diaphysis (some 12 cm proximal) are less problematic. Femoral diaphyseal fractures have good bone, less comminution, and sufficient distance from the joint to be minimally affected by the arthro-plasty itself. Fractures distal to the femoral epicondyles do not involve the collateral ligaments of the knee, and they can be treated with simple revisional augmentations.

Periprosthetic total knee fractures within 3 to 15 mm of the joint line, however, hold several distinct hazards. First, they can occur with surprisingly little trauma to the limb yet with severe bony comminution. in fact, as a general rule, the less the trauma, the worse the fracture. This is explained by the second point, which is that the supracondylar area of the femur is extremely osteo-porotic in these patients, with thin cortices and practically no intramedullary cancellous bone. Once the "eggshell" of the distal femur has cracked, reconstructive efforts will be frustrated by the simple lack of substance proximal to the arthroplasty.

Third, it is rarely appreciated that one of the contributing factors to the fracture is the unsatisfactory nature of the original arthroplasty. This is particularly true of stiff total knees, most commonly the result of a tight posterior cruciate ligament or oversized components. The stress that this stiff arthroplasty places on the femoral bone not only predisposes to fracture, but also confounds attempts at stable fixation. While one would prefer to treat either the fracture or the failed total joint individually, it is often necessary to address these problems simultaneously, since they are so interrelated.

For biologic as well as sociologic reasons, conservative treatment of supracondylar femoral fractures is almost impossible today, and open intervention of some variety is usually necessary.

Many techniques of internal fixation are available, but all share significant technical difficulties as well as a surprisingly high incidence of nonunion and malunion. The medial mechanical axis of the lower limb, the concerted action of the posterior knee musculature, and the sagittal plane of motion of the joint itself all conspire to destabilize even the most rigid internal fixation. This is compounded by the effects of bony comminution, severe femoral osteopenia, and a stiff knee arthroplasty. It is not surprising, therefore, that many fractures develop nonunions or go on to a tardy malunion with the typical deformity of adduction, flexion, and internal rotation of the distal femoral fragment.

Rush rod fixation, as espoused by Ritter1 is economical and expeditious, but in most cases has provided insufficient stabilization. Better results have been found with distal condylar plate and screw devices,2 although even good surgical results will often deteriorate into nonunion or malunion and the bone available for distal screw fixation is often compromised by the intercondylar design of the femoral prosthetic component. New plating systems with abundant supracondylar screw options may improve this situation, but the biologic issues of bone quality and joint dynamics will remain.

The competing principles of fracture immobilization in the face of joint mobility require ever more rigid fixation. The use of intramedullary rods, introduced through the intercondylar notch of most prostheses, is an attractive option that requires minimal disturbance of the arthroplasty. Excellent results have been reported with this technique,3 although several important technical issues should be considered. First, the precise design of the prosthetic femoral component must be known, so that a rod of sufficient diameter to achieve intramedullary stabilization of the fracture can be introduced through the open box of the femoral component. The diameters of these components are well known (Table 14.1). There have been apocryphal reports of the need for a "prosthetic notch plasty" using a Midas Rex burr to enlarge the metallic intercondylar space, although this is clearly not to be recommended.

Second, one should be prepared for the necessity to open the fracture site above the femoral prosthesis and place an intercalary allograft—sculpted from a distal femur—to surround the intramedullary rod, fill the metaphyseal void, maintain femoral limb length, and provide support for the comminuted host cortical bone, which can be wired about the graft. Without this graft material, the rod alone may be insufficient to maintain length and promote healing of the fracture. Finally, one may enhance the function of the stiff arthroplasty after stabilization of the fracture by

Table 14.1. Intercondylar distances of commonly used total knee

implants*

Intercondylar

Implant

distance (mm)

Miller-Galante (Zimmer, Warsaw, IN)

12

Insall-Burstein (Zimmer)

14-19

Biomet (Warsaw, IN)

22

Intermedics (Austin, TX)

18

AMK (DePuy, Warsaw, IN)

14-17

Osteonics (Allendale, NJ)

19

PFC (Johnson & Johnson, New Brunswick, NJ)

20

Kirschner wires (Timonium, MD)

20

Genesis (Smith & Nephew Richards, Memphis, TN)

20

Duracon (Howmedica, Rutherford, NJ)

12-16

* Reproduced, with modification, from: Engh and Ammen4

sectioning an excessively tight posterior cruciate ligament or downsizing an excessively thick patellar button.

All too frequently, none of these options will suffice. The total knee arthroplasty may be too bad to salvage, compromising the fracture healing and yielding a dysfunctional limb even if union should occur. The "personality" of the fracture may be unattractive, with such problems as profound comminution, insufficient distal bone for screw or rod fixation, periprosthetic bone loss secondary to prefracture osteolysis, or intercondylar fragmentation and compromise of collateral support. In these and other severe situations, simultaneous revision of the arthroplasty and stabilization of the fracture must be considered. This can be a heroic endeavor, to be undertaken only by those with a full array of revi-sional prostheses and tools, an adequate supply of allograft material, and extensive total knee revisional experience.

In the operating room, one must be prepared for an extended surgical procedure, with sufficient anesthetic to last several hours. Some thighs will be too short to permit a proper tourniquet, although a sterile tourniquet can be used to maintain hemostasis through much of the procedure. The preferred incision is an extension of the knee arthrotomy midline incision well up into the proximal thigh. This approach will even allow the removal of prior failed fixation devices from the medial or lateral side of the femur without the use of parallel skin incisions. All prior prosthetic materials must be removed, and it is generally preferable to address the previous fracture materials before removing the femoral com ponent of the total knee. This will protect the fragile distal femoral bone as long as possible during the surgery.

The optimal stabilization of the fracture usually involves a long intramedullary rod, extending several inches at least above the fracture site. This must be compatible with the new total knee arthro-plasty and systems such as the constrained condylar knee remain the industry standard. Curved rods may be necessary to match the femoral bow, and they have the additional advantage of conferring some rotational stability upon the ultimate construct. Rods of 150 to 200 mm of length are most helpful. Offset rods may additionally allow for accommodation of previous fracture malunions.

The mechanism of failure of the index arthroplasty must be clearly understood and reversed. Most frequently this involves conversion from a cruciate retaining to a cruciate substituting design, downsizing of prosthetic components, and correction of internal rotational malalignment of the femoral and tibial components. Extra hands are often needed during surgery even to place trial components, since the distal femoral fragment in particular will be difficult to control, tending to flex and internally rotate in response to muscle influences about the knee.

Once a prosthetic device has been selected and trials implanted, the fracture can be addressed. Particular attention should be paid to the proper rotation of the limb, often using palpation of the posterior linea aspera to confirm position. Whether fresh fracture, malunion, or nonunion, the interface between the proximal and femoral and distal femoral fragments may need to be simplified and freshened. This is preferably performed in an oblique fashion, avoiding butt or step cuts. An oblique osteotomy provides greater bone surface for healing, partial correction of flexion deformities, and significant stability against rotation. Occasionally, supplemental cortical plating of the fracture may be necessary, although only unicortical screw fixation will be available if the intramedullary rod is of appropriate substance.

Extensive grafting of the fracture may be required. At the very least, small bone fragments or paste will be helpful at the termination of the procedure to enhance healing in an area of extreme osteopenia. An intercalary graft, fashioned to surround the intramedullary rod but provide bulk and fill for the supracondylar area may be extremely helpful, as previously described in the retrograde rodding technique. Occasionally, the distal bone is of such poor quality that an entire distal femoral allograft may be needed. An arthroplasty of the graft can be performed on a back table, then mated with the host femur within the operative field. The junction of the massive allograft can be accomplished either by invagination of the graft within the residual femoral canal or—as described previously—by an oblique osteotomy. In either situation, the graft surmounting the intramedullary rod should be made intentionally too long, then whittled to proper limb length once the arthroplasty has been balanced in flexion. This allows the secondary adjustment of proper extension balance in the same way that one would balance a simple revisional arthroplasty using prosthetic augments.

If an intercondylar fracture should occur during the procedure, the bony fragments should be preserved with their attached collateral ligaments. These can be cemented at the time of fracture reduction within the "pockets" of the femoral component, secured with methylmethacrylate and held temporarily by a bone clamp, potentially reinforced with mersilene tape. The medial collateral ligament is, of course, of the highest priority, since even a constrained condylar knee system will display rotational instability in its absence. Onlay cortical plates or struts may be wired about the host/graft junction to augment bone stock, contain residual cortical fragments, and confer further rotational stability. The late incorporation of these grafts is quite good, much as has been observed in proximal femoral hip reconstructions.

Finally, it is the obligation of the surgeon to confer stability upon both the fracture and the arthroplasty at the time of surgery. Occasionally, this may require cementation of the intramedullary stem. This should be done with caution because of potential future revisional difficulties as well as possible sequestration of some of the fracture fragments. Intramedullary rods appropriate for cementation should be used, as well as cement restrictors. Internal stabilization of these fractures is far preferable to subsequent external bracing, although this adjunctive therapy may be helpful in the early mobilization of some reconstructions. Protected weightbear-ing is at the discretion of the surgeon, influenced by the extent of the allograft, the stability of the reconstruction, the need for postoperative motion, and patient compliance.

When successful, simultaneous revision of an unsatisfactory knee arthroplasty and fixation of the fracture it precipitated can be an extremely satisfying and costeffective procedure.

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