The Management of Malalignment in Hip Revision Surgery

Abstract

When facing revision surgery, the deformation of the femur must be considered for the important implications in the strategy of the operation and the choice of the implant. Deformation can occur in varus and retroversion, or both, in different degrees. For severe varus deformations a straightening osteotomy is generally indispensable and different types of distal fixations of the implant are necessary; in milder deformations other solutions are possible. Retroversion can be generally managed with proximally modular stems, stems without metaphyseal component, or, rarely, with a derotation osteotomy.

Surgeons should have an understanding of the proximal femur deformation that occurs as a result of failed implant fixation, regardless of the cause of failure of the previous stem (in both cemented and uncemented stems).

The load on the femoral head and the lever arm of the femoral neck force the stem in varus and retroversion. Until bone implant fixation becomes effective, the proximal femur maintains its original shape and resists stress. When the fixation fails and micromotion occurs, a variable degree of progressive bone deformation follows the migration of the prosthesis. In revision surgery, the complication of implant removal and bone loss becomes more challenging as a result of the abnormal shape of the proximal femur.

Both the varus and retroversion deformities influence the strategy that will be adopted during revision surgery. The varus deformity prevents a surgeon from using a long stem that reaches or bypasses the isthmus. Using a long stem is often mandatory because proximal bone loss dictates purchasing fixation more distally. Even if proximal bone loss does not require distal fixation, a surgeon should resist the temptation of using a short stem to prevent the varus deformity from progressing after the implantation of the new stem.

Varus Deformation

A surgeon has two options for overcoming the varus deformity (Figures 1 and 2). The first option is to undersize the implant. Undersizing is suitable if the deformity is mild and the surgeon is willing to accept a non-canal filling implant, for example, when using fluted stems that can obtain rotational stability with a three-point fixation. If a cylindrical fully coated stem is used, then scratch fit and full canal fill at the isthmus is mandatory to obtain rotational stability; in this situation undersizing should be discouraged because it would endanger the stability of the implant. The amount of undersizing can be reduced by taking advantage of the flexibility of the femur, especially in the proximal part where the cortical bone is thinned by the failure of the previous implant. Two risks may occur as a result of this technique. The first risk is that the tip of the stem can cause too much pressure on the lateral cortex when the new stem is inserted, especially where the tip of the failed stem formerly rested and where the bone is weakened; this may result in a femoral fracture or a perforation. The second risk is that the stress caused by excessive deformation can lead to subsequent femoral fracture when the patient starts to load on the operated limb. This may occur even if the implantation has been completed successfully.


Figure 1: Failed cementless stem with varus deformity. Figure 2: After implantation of an S-ROM stem; in this case cerclage wires were not felt indispensible.

A surgeon’s second option for overcoming the varus deformity is to perform a straightening osteotomy (Figure 3). A straightening osteotomy should be performed at the apex of the deformity, sometimes 2 cm or 3 cm more proximal, to preserve a longer distal fragment. This technique relies on the axial and rotation al stability guaranteed by the long stem into the distal femur. Some degree of rotational stability can be obtained with an oblique or a step-cut osteotomy. However, these techniques make the reduction more difficult at final stem implantation. For these reasons, the author suggests performing a transverse osteotomy.

Osteotomies can be performed for reasons other than straightening the femur. Osteotomies can also help to remove the cement or enable trephining the distal part of an ingrown stems after having cut it. If the cement must be removed, then the stem should be removed first. Next, a surgeon should remove the proximal cement using a chisel, a high-speed burr, an ultrasonic instrument, or a combination of all three instruments.

After the osteotomy is performed, the remaining cement can be removed in a retrograde fashion in the proximal fragment and in an antegrade fashion in the distal femur, facilitating this part of the surgery and reducing the likelihood of femoral perforation. Another advantage of the transverse osteotomy is the preservation of vascularity, which accelerates the healing process. Vascularity is preserved because the muscles are not stripped from the proximal femur.

When a transverse osteotomy is performed, the stem length is paramount. Although the stem should advance beyond the osteotomy at least 2 femoral diameters, the author prefers to advance >2 diameters. Even if the distal part of the stem enters the femoral flare without cortical contact, a longer stem guarantees a lower risk of fracture when a patient loads the femur excessively before the osteotomy has healed.

In terms of cerclage wires at the site of the osteotomy, a surgeon can place a prophylactic wire on one or both sides of the osteotomy (Figure 4). The distal side is more important due to the high stress concentration. Or a surgeon can place the wires after implanting the stem in case of fractures. Cerclage wiring after the fracture has occurred may not be as effective as prophylactic wiring and sometimes requires removal of the implant to obtain a satisfactory reduction of the fracture.


Figure 3: Failed cemented stem with varus deformity of the femur. Figure 4: After straightening osteotomy with prophylactic cerclage wires and trochanteric washer and screw.

Deformation in Retroversion

The challenge of managing the deformation in retroversion of the metaphysis is generally not as important as managing the varus deformity. The deformation of the metaphysis is often not too evident due to the proximal bone loss but has an important role in determining the implant a surgeon will use. If a short stem is used, then the options are a distally fixed implant with a narrow proximal part (such as the Wagner Conus), which will allow for the deformed metaphysis to be ignored, or a proximally modular implant in which the neck of the prosthesis can be inserted in the correct anteversion regardless of the shape of the bone.

If proximal bone loss dictates the use of a long stem that bypasses the isthmus, then multiple options are available. If a straight stem is used, then the canal cannot be filled unless an osteotomy is performed. However, performing an osteotomy will cause the loss of the natural bow of the femur, and the knee will have a recurvatum of 5° to 7°. Although these complications are well-tolerated, they can be avoided by using a bowed stem. The advantage of a straight stem is that it can be rotated in any degree of anteversion because no fixed relationship exists between the neck and the bow of the distal part.

To overcome this challenge, modular stems were developed in which the neck, methaphysis, and distal part are independently adjustable from each other. It is unfortunate that the locking mechanism of these devices are Morse tapers, which work in compression and flexion and cannot rely on positive locking. To alleviate this complication, a screw was added to force the two Morse tapers in compression. The addition of the screw causes the proximal part of the prosthesis to become bulky (this is generally not an issue in revision surgery because the missing bone provides enough space for the prosthesis due to bone loss) and bone fragile, as indicated by the failures of devices such as the ZMR Hip System-XL (Zimmer, Inc, Warsaw, Ind).

Straight stems rely on flutes for rotational stability and are cylindrical in the proximal part to ignore proximal deformity. If a bowed monoblock stem is used, then it has a fixed relationship between the plane of the bow and neck anteversion (generally 15°). If these stems are designed to have a degree of proximal fixation, then the metaphysis cannot fill the proximal femur unless the stem is put in retroversion. Retroversion will force the distal bow in a different plane from the bow of the femur, causing premature locking and making fractures more likely. If the stem used is proximally modular (I consider this option to be an advantage in this situation), then the metaphyseal component should be free to rotate independently from the neck and the distal part so that it can be placed in the most favorable position to fill the metaphysis.

The metaphyseal component should also be free so that a seal can be provided to prevent particulate debris from migrating distally into the femur and causing osteolysis. Stems in which the neck and the metaphysis are made of one piece and not free to be adjusted independently are unsuitable in proximally deformed femurs because they will cause the neck to be placed in retroversion.

Conclusion

An understanding of bone deformation that occurred during the failure of the old implant, as well as accurate preoperative planning, is important in performing faster, more effective revision surgery that leads to less complications. The surgical approach, implant and cement removal, implant selection, management of bone loss, bone preparation, and implantation and soft tissue repair require similar knowledge and preparation.

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Author

Dr Marega is from Casa di Cura S. Anna in Brescia, Italy.