P ITFALLS

• Wide, osteopenic IM canals often require cemented fixation.

   

• Uncemented stems must be long enough to bypass cortical defects.

   

• Uncemented stems offer a limited ability to deliver antibiotics in the setting of infection.

   

• Fluted titanium stems have been shown to produce less bone pain at the stem tip.

Uncemented Stems in Revision TKA

Indications

• Failed primary total knee arthroplasty (TKA).

• Adequate structural integrity of femoral and tibial intramedullary (IM) canals to support press-fit stems.

• Large metaphyseal defects and soft tissue laxity can be addressed with press-fit stems in a revision setting.

• A “hybrid” fixation uses cement at the metaphysis and a press-fit or ongrowth stem.

  Examination/Imaging

• Complete imaging of the femur and tibia is required to assess bone quality and defects.

  Controversies

  • Advantages of uncemented stems: anatomic alignment, early inherent stability, the ability to bypass metaphyseal defects and achieve load sharing with good cortical contact

  • Disadvantages of uncemented stems: risk of fracture during insertion, inability to deliver antibiotics

   

  • The preoperative radiograph in Figure 1A demonstrates a large lytic lesion of the distal femur.

  • At the time of surgery, the lesion encompassed most of the distal lateral femoral condyle and required a femoral augment and bone graft (Fig. 1B).

• Preoperative images, clinical examination, and history are used to determine the mechanism of failure of the primary TKA.

  • Figure 2 shows a preoperative radiograph of a failed primary TKA. Significant medial tibial collapse has occurred. The need for medial tibial augments and soft tissue balancing should be anticipated.

  • Figure 3 shows a failed primary TKA demonstrating rotational and anterior-posterior instability, polyethylene wear, and lysis of the medial and lateral femoral condyles. This revision will require femoral augments, bone grafting or screws and cement, and possibly a higher degree of constraint.

• Additional existing hardware (e.g., a total hip replacement) should be noted. This will indicate the limitation of stem length and will reduce the risk of disrupting existing implants.

• Stem lengths and the need for augments are determined from the preoperative radiographs.

• Long-leg films help evaluate limb alignment, extra-articular deformities, and the need for offset stems.

   

  Uncemented Stems in Revision TKA

   

   

   

   

   

   

   

   

   

  313

   

  A

  FIGURE 1

   

  B

   

  FIGURE 2

   

  FIGURE 3

   

   

   

   

  314

   

  Uncemented Stems in Revision TKA

   

  FIGURE 4

   

  Treatment Options

  • Small defects in metaphyseal bone (less than 5 mm depth) can be treated using screws in the defect, followed by cement on the metaphysis at the time of insertion of the final component (Fig. 5), or by particulate bone graft.

  • Larger defects require the use of augments on the prosthesis.

   

• Anteroposterior and lateral radiographs are templated to determine the “exit” of the diaphysis relative to a compromised metaphysis. This will help prevent perforation of the IM canal during reaming and assures anatomic positioning of the components. Figure 4 shows a failed left TKA with malpositioned femoral and tibial components secondary to tibial collapse and osteolysis. Preoperative templating of the exit of the diaphysis at the metaphysis will help guide reaming and prevent perforation of the diaphyseal cortex.

FIGURE 5

P EARLS

• A tourniquet should be placed as proximally as possible on the thigh to allow extensive exposure if needed.

Medial epicondyle

Distal augment

Anterior

Posterior

3 cm

3 cm

Equipment

• Use a bump or a leg holder to facilitate working on the knee in flexion.

FIGURE 6

Surgical Anatomy

Posterior augment

315

FIGURE 7

• The tibial canal is usually 2–4 mm smaller than the femoral canal at 80–120 mm from the joint line.

   

  P EARLS

  • A tibial tubercle osteotomy or quadriceps snip can help improve a difficult exposure.

   

  Uncemented Stems in Revision TKA

   

• The joint line can be difficult to identify where massive bone loss exists. In general, the joint line will be approximately 3 cm distal to the medial femoral epicondyle in flexion and extension (Fig. 6).

  Positioning

• Standard supine positioning is used.

   

   

  Portals/Exposures

  • An extensile medial parapatellar approach is used.

    Procedure

    Step 1

• Following exposure and implant removal, reaming is undertaken up to a thickness such that the cortical bone of the diaphyseal isthmus is encountered (“chatter”).

• Cortical purchase of the uncemented stem is needed to achieve rigid stability. The press-fit femoral stem in Figure 7 was not wide enough to achieve good bicortical contact and rests only on the lateral femur. This prosthesis became loose and required revision using a wider stem.

• Intramedullary jigs are used to prepare the metaphyseal bone ends to accept the components. An effort should be made to achieve maximal rim contact.

• Offset stems may be needed to accomplish this if there is significant bone loss or in cases in which there is a deformity of the diaphysis (such as following a fracture).

   

  Uncemented Stems in Revision TKA

   

  • Instruments are available to help determine the direction and magnitude of stem offset based in IM reamers. Figure 8 shows examples of offset instrumentation (Vanguard; Biomet, Inc., Warsaw, IN).

• The position of the tibial stem can be adjusted by turning the offset ring, allowing anatomic fit of the

316

A B

C D

FIGURE 8

317

P EARLS

• Longer (80–160 mm) and larger diameter stems should be used in an uncemented application to achieve rigid stability in the IM canal.

   

  P ITFALLS

• Care must be taken when inserting and removing trial stems to prevent perforation or fracture of the diaphysis.

Uncemented Stems in Revision TKA

stem into the diaphysis while optimal coverage of the metaphyseal end is maintained (Fig. 8A–8C).

• An offset stem on the femoral component can accommodate a wide range of angular deformity (Fig. 8D).

  • Offset stems can generally be inserted “loose” so that they can rotate (without detaching from the trial) to achieve bicortical contact while the

    metaphyseal end is positioned to achieve maximal bony coverage.

• Figure 9 shows an example of femoral and tibial offset stems. In this case, the bone quality was deemed insufficient to accommodate press-fit stems, so the stems were instead cemented.

• Note how the offset permits the stems to be anatomically placed in the diaphysis while the components achieve optimal coverage and position at the metaphyseal ends of the bones.

• When the optimal metaphyseal position is determined, the stem can be locked or tightened in place and the trial removed. The orientation of the trial stem relative to the metaphyseal portion should be noted and re-created in the final prosthesis.

FIGURE 9

318

P EARLS

• The slope of the stem in relationship to the tibial baseplate should be considered. Small changes in axial rotation of the tibial component may place the implant in relative varus or valgus position.

   

• The tibia should be placed in line with the flexion and

extension axis of the knee, which will enhance neutral alignment in the coronal plane.

Uncemented Stems in Revision TKA

Step 2

• Once the tibial surface has been reconstructed, the extension gap is balanced by adjusting how far the stemmed femoral component is impacted into the femur or by adding augments to the distal femoral trial. In Figure 10, the size of distal femoral augments is determined by balancing the knee and restoring the joint line in extension. Augments are added to fill the space between the prosthesis and bone.

• Next, with the femoral trial component resting on

  the anterior femoral cortex, the flexion gap is balanced by adding augments to the posterior condyles of the trial or by increasing the size of the femur.

• Once the knee is balanced in extension and flexion and the joint line is restored, the length, width, and orientation of the stems is confirmed, assuring that there is excellent bicortical purchase.

   

  P EARLS

  • Hybrid fixation requires that at least 25% cancellous bone be available for cement

  interdigitation. Otherwise, bone grafting must be undertaken to support the implant.

   

  Step 3

• Irrigation is followed by application of screws and bone graft, if needed.

• If hybrid fixation is undertaken, cement is applied to the metaphyseal bone ends.

• Final components are inserted carefully to reproduce the anatomic orientation established during trialing and to prevent cortical perforation and fracture.

  Instrumentation/ Implantation

  • Stems must be long enough and wide enough to achieve rigid fixation of the implant.

  • Stem-to-canal fill ratios of 0.8 for the femur and 0.85 for the tibia are associated with excellent results.

   

• Radiographs confirm the adequacy of cortical contact of the stems and the integrity of the bone after

   

   

   

  FIGURE 10

   

   

   

  319

   

  Uncemented Stems in Revision TKA

   

  FIGURE 11

   

   

  P EARLS

  • Radiolucent lines appear more frequently around uncemented stems in long-term follow-up, but they do not appear to correlate with radiographic or clinical failure.

   

  insertion. Figure 11 shows a postoperative revision TKA with press-fit stems. Note that both stems are wide enough and long enough to achieve excellent cortical contact.

  Postoperative Care and Expected Outcomes

  • Patients are permitted to weight-bear as tolerated.

  • We have found no benefit to using a continuous passive motion device postoperatively.

  • The overall reoperation rate of revision TKA is 20% at 10 years.

  • Short-term results comparing cemented versus uncemented stems in revision TKA are comparable, with a reoperation rate of 2–8% at 2–3 years.

  • Long-term results of uncemented stems appear to be highly dependent on technique.