Stem

14th Annual Vail Orthopaedics Symposium
[Orthopaedics Course Reports - © 2000 Medscape, Inc.]

The Use of Intramedullary Stems in Total Knee Replacement

Michael A. Kelly, MD

Introduction

As the number of total knee arthroplasty (TKA) procedures being performed in an active population increases, so too does the number of failed implants requiring revision.^[1] Preoperative planning for revision TKA should include obtaining accurate radiographs that permit both the evaluation of bone loss in the metaphyseal region of the femur and tibia and visualization of the intramedullary (IM) canal to determine appropriate stem size and length. The surgeon should assess bone loss and ligament stability, correlating the need for bone graft or prosthetic augmentations, modular TKA with a variety of IM stems, and versatility of prosthetic constraints.^[2]

Options for stemmed components have been improved in recent years. Modular components, including both cemented and uncemented styles, are available in a variety of sizes for a variety of purposes. Some stemmed components have offset dimensions. At the 14^th Annual Vail Orthopaedics Symposium, Alfred J. Tria, Jr, MD, discussed the different type of stems used in total knee revision. Owing to the fact that the primary femoral component does not usually include a stem, when planning the revision surgery, the surgeon must decide whether to add an IM stem on the femoral or tibial side.^[3]

Intramedullary Stems

Tria described 3 types of IM stems: a dangle stem, a cortical contact stem, and a press-fit stem. The dangle stem has a small diameter and can be added to either side of the implant. The stem does not touch the distal cortices of the metaphyseal/diaphyseal area. The cortical contact stem is larger in diameter and barely touches the cortex of the femur or tibia without mechanical impaction. The press-fit stem is impacted into the cortex of the bone and has substantial firm contact.

Figure 1 shows the dangle stem in place. Figures 2 and 3 show the cortical contact and press-fit stems in place. The differences between the stems lie in the amount of contact (or lack thereof) between the stem and the cortex of the bone. All 3 stems may be uncemented, partially cemented, or fully cemented.^[3]

Figure 1 A and B. Dangle stem in place. Provided courtesy of Alfred J. Tria, Jr., MD.

Figure 2 A and B. Cortical contact stem in place. Provided courtesy of Alfred J. Tria, Jr., MD.

Figure 3 A and B. Press-fit stem in place. Provided courtesy of Alfred J. Tria Jr., MD.

Planning Replacement

To prepare for TKA, it is helpful to have a classification system to determine the defects in the femur and the tibia. For purposes of this discussion, the classifications, developed by Engh and Ammeen^[2] are explained on Table 1. The defects described are most often seen in revision surgery but may occur with severe varus or valgus deformity in the primary knee.

Table 1. Classification of Tibial (T) and Femoral (F) Defects in TKA*

CLASSIFICATION

DESCRIPTION

T1

Defect is completely contained

T2

Cortical deficiency on either one (T2A) or both (T2B) tibial plateau surfaces

T3

Includes substantial segmental bone loss, requires bone grafting, and is associated with ligamentous laxity or deficiency

F1

Defect is completely contained

F2A

Cortical loss of one condyle

F2B

Cortical loss of both medial and lateral femoral condyles

F3

Segmental loss of bone with occasional ligamentous laxity

* Adapted from Engh and Ammeen.^[2]

Primary Knee Arthroplasty

Although most primary knee replacements are fully cemented, there are some strong proponents of cementless designs, such as Leo Whiteside. Whiteside^[4] has demonstrated results with cementless designs that are comparable to those of cemented knees with good long-term follow-up. When discussing primary knee replacement, however, the fully cemented Insall-Burstein posterior stabilized knee is held as one of the gold standards in replacement surgery.

In primary knee replacement, it is usually not necessary to use an IM stem unless there is a T2 or F2 type deformity with substantial bone loss. The deformity is most common on the tibial side. Defects can be filled with cement, metal, or bone. If the defect involved the entire medial or lateral plateau, the metal tray of most components will need IM stem augmentation. This stem can "dangle" in the canal or have cortical contact without impaction and does not need cementing. This will allow for easier revision if it becomes necessary.

In some cases, it may be necessary to use a constraint in the primary setting. In these cases, IM stems may be added to the construct, to distribute the forces across the knee joint more evenly and prevent implant loosening. Dangle or cortical contact stems can be used in this situation without cement.

Revision TKA

Revision knee deformities are usually more advanced and fixation is of greater concern. T2A and F2A deformities include the entire condyle area and are augmented with IM stems that can be press-fit or cortical contact type. More difficult problems of T2B or F2B level require reconstruction of the plateau or condylar surfaces before adding IM stems. The use of IM stems is even more important in the case of T2B or F2B reconstruction, since the tibial surface may not be totally stable and the construct sitting on the tibial metaphyseal may not be rigidly fixed. Ideally, these stems should be load sharing to avoid substantial stress shielding.

Most advanced defects will require further fixation of the stems. For this reason, press-fit stems are not as successful in total knee replacement as they are in total hip replacement. Instead, in these cases, a cortical contact stem augmented with bone graft or cement is a viable choice. Cancellous bone grafts impacted distally around the stem can also be augmented by cement proximally to provide additional support. This technique allows for further repair and facilitates removal of the stems if necessary.

Tria believes surgeons should not be afraid to use IM stems, should use less cement rather than more, and should not complete revision surgery without stable fixation.^[3]

Offset Stems in TKA

In his presentation, "The Use of Offset Stems in TKR," Wayne G. Paprosky, MD, listed the rationale for using offset stems in planning total knee replacement. Offset stems can be press-fit or partially cemented. Preoperative radiograph assessment should show 4 views, to help determine the location of the joint line and the preferable size of the offset stem. Just as templating is used in hip replacement, it is important for radiographic views to be longer than normal for the tibial portion to provide the surgeon with a template for tibial stem placement. More often than not, the offset stem is used as a tibial stem, rather than the femoral stem.^[5]

Rationale Use of Offset Stems

The main problems with the distal femur and proximal tibia in revision surgery are (1) geometry determines the pathway of the stem and (2) deformity of the proximal tibia may not allow for standard press-fit stem components. Offset stems are generally used in the femur to help improve the flexion/extension gap mismatch. The offset stem allows the femoral component to be placed more posteriorly, adding to improved flexion gap stability. In the tibia, offset stems are usually used to match the anatomic deformity, thereby compensating by correcting the intramedullary alignment and permitting the tibial component tray to be placed correctly.

It has been about 5 years since the advent of "crooked stems" (offset stem). In that time, we have seen no component loosening and no progression of radiolucencies. Only one patient had to undergo revision and that was due to infection. Restoring tibial alignment may require the use of stem extenders, even after re-resection of the tibia. The offset bushing on the tibial component has numbers, and as the tray is rotated, the number will indicate what the matching number of the offset stem should be.

Offset stems are used in the femoral side to help alleviate flexion/extension gap mismatch. Figure 5 shows the insertion of the femoral offset stem. If there is malalignment of the femur, then recutting is necessary. If the femur is recut, then the surgeon is starting with a flush surface. After inserting a neutral press-fit stem into the canal, one can use the offset guide to bring the femoral component into anteroposterior alignment. Once again, the offset bushing number on the tray will match the number of the stem to be used. There may be a need for fine-tuning adjustments in the medial/lateral plane, but once the lock nut is in place, the final component impaction is possible.

Summary

Planning for total knee revision surgery includes the use of radiographic templating to allow accurate assessment of bone loss and to help decide on the optimal stem size. For the most part in revision surgery, stems are used in the tibia, They can, however, be used in the femur to assist in bridging the flexion/extension gap mismatch. Deformities and bone loss can exist in both the tibia and the femur at revision. Classifications for the severity of these deformities help the surgeon determine what steps to take to revise the knee replacement. In general, the stem fixation of choice in knee revision surgery is partially cemented, with or without bone graft ballast. Offset stems can compensate for tibial deformities and permit appropriate tibial tray rotation, and in femoral revision improve alignment for anteroposterior movement and matching the flexion/extension gaps.

References

Engh GA, Parks NL. The management of bone defects in revision total knee arthroplasty. Instr Course Lect. 1997;46:227-236.
Engh GA, Ammeen DJ. Classification and preoperative radiograph evaluation: knee. Orthop Clin North Am. 1998;29:205-217.
Tria AJ Jr. Intramedullary stems in total knee replacement. Presented at the 14^th Annual Vail Orthopaedic Symposium; January 23-28, 2000; Vail, Colo.
Whiteside LA. Cementless total knee replacement: nine- to 11-year results and 10-year survivorship analysis. Clin Orthop 1994;309:185-192.
Paprosky WG. The use of offset stems in TKR. Presented at the 14^th Annual Vail Orthopaedic Symposium; January 23-28, 2000; Vail, Colo.