Computer-assisted Surgery in Revision Total Knee Arthroplasty: Early Experience With 46 Patients

Abstract

Use of the OrthoPilot navigation system (B. Braun Aesculap, Tuttlingen, Germany) in navigated revision surgery allows for precise alignment of components, restoration of the joint line, balance of the gaps, and filling of the bony defects by facilitating selection of appropriate implant sizes and wedges. Our study results confirm these benefits through clinical and radiologic evaluation of the first 46 cases in our series in which the system was used. Continuous feedback from the system during surgery confirms that the surgeon is using proper technique. The OrthoPilot navigation system simplifies revision surgery and helps produce successful patient outcomes.

The primary goals of revision in total knee arthroplasty (TKA) are 1) extracting knee components with minimal bone loss; 2) restoring the cavitary and segmental bony defects; 3) restoring the joint line; 4) balancing knee ligaments; and 5) stabilizing knee components.^1-3

In revision knee surgery, it is important to identify why previous arthroplasty had failed. If the mechanism of failure is not clearly understood, revision is not likely to succeed.^4,5 Computer-assisted surgery helps in the reconstruction of the neutral mechanical alignment of the limb and its joint line, which improves the chance of successful revision TKA.^6,7

We have used the OrthoPilot software-assisted navigation system (B. Braun Aesculap, Tuttlingen, Germany) in 46 patients since May 2006. The purpose of this study is to evaluate the reliability and usefulness of the application.

Materials and Methods

Preoperative Assessment

Preoperatively, all cases must be reviewed carefully for relevant history and clinical findings. Ligamentous stability must be assessed in extension and flexion, and the appropriate prosthesis must be selected.

Infection must be excluded by aspiration and complete evaluation. Results of previous surgeries and incisions should be included in the evaluation, as well as determination of existing ligamentous stability. A complete radiologic workup that includes anteroposterior (AP), lateral, and patella views, as well as standing whole-leg radiographs is vital. High-quality radiographs allow for proper identification of all necessary landmarks and assessment of the overall limb alignment. The amount and location of bone loss must be quantified. If there is suspicion of malpositioning, computed tomography (CT) should be carried out to analyze the rotational configuration of the implants (Figure 1).

Figure 1: Preoperative finding with a significant instability caused by malpositioning and gross elevation of the joint line: Preoperative radiographs (A), Transepicondylar CT scan showing 3.7° of internal rotation (B), and Evaluation of ligamentous instability (C).

Preoperative Planning

The mechanical axis of the tibia should show no critical bowing of the bone. Planning templates help the surgeon preselect the appropriate size of the femoral and tibial components. The tibial joint line should be identified based on radiographs of the opposite knee and should be correlated to a landmark of the involved knee (eg, tip of the fibula, surface of the insert, or undersurface of the plateau). The tibial joint line can also be identified in preoperative radiographs from the same side.

In our procedure, the distal valgus cut angle for the revision implant was determined according to the mechanical and anatomic axis of the femur. If its position was not altered, the femoral joint line was taken from the distal surface of the femoral component. Alternatively, if the position of the femoral component was altered, the femoral joint line was measured from the contralateral knee. Another option involves the correlation of the femoral joint line to the epicondyles.

The rotation of the femoral component should be measured according to CT scans in relation to the epicondylar axis.

All information obtained during perioperative planning was then input to the navigation system software.

Surgical Procedure

For exposure of the prosthesis in revision surgery, the initial incision lines should be used whenever possible. Scarring in the extensor mechanism often prevents eversion of the patella without compromising the patella ligament attachment to the tibia. A stepcut of the tuberosity provides secure refixation of the patella tendon after implantation and is an alternative to the quadriceps snip technique.

Debridement of the thickened synovia and intraarticular adhesions exposes the entire joint.

After fixation of the reference frames to the distal femur (using a clamp), proximal tibia (monocortical screw), and ankle (elastic band), acquisition of the joint centers was performed and landmarks palpated.

The tibial component should be removed, sparing as much bone as possible. The intramedullary canal of the tibia was opened with sharp reamers under guidance of the navigation system. Using the tibial mechanical axis shown by the navigation system, the correct position of the tibial plateau was defined.

The configuration of the medullary canal of the tibia determined the depth and diameter of the reamer. The tibial cutting block was fixed to the tibia, and resection performed until viable bone was reached. In this way, the new tibial component was placed orthogonal to the tibial mechanical axis. The exact orientation and location of the bone cut were recorded and stored in the navigation system. In turn, the navigation software provided information on the need for augmentation wedges and on polyethylene insert thickness.

The femoral component and the extension gap were recorded by the navigation system.

The removal of the femoral component was done with small chisels to help preserve bone stock. The femoral canal was enlarged with navigated reamers for the orientation of the distal femoral cutting block. The navigation system provided the valgus cut angle, thereby providing an orthogonal orientation of the block to the mechanical axis of the femur. Fixation of the cutting block on the anterior surface of the femur allowed the surgeon to save bone during distal resection.

Before measuring the flexion gap, the level of the anterior cortex of the distal femur was recorded. By using a template, the correct size of the femoral cutting block was selected. The trial block was mounted to the reamer, which was left in place during opening of the intramedullary canal. The flexion gap was recorded with the use of a calibrated spreader (100-150 N) with the knee flexed at 90°.

During the planning and simulation phase, different options for balancing the gaps were shown by the navigation system, and the best one chosen for ligamentous stability and preservation of the femoral joint line. The system provided information on depth of the distal femoral cut, size of the femoral component, and femoral rotation in relation to the epicondylar line (Figure 2).

Figure 2: OrthoPilot screen shot of the joint line and cut simulation.

After the reamer was removed, a distal femoral cut was made with the use of a navigated cutting block. A navigated assessment of this cut, with respect to the planned femoral joint line, shows whether there was a need for distal femoral wedges. The wedge-augmented four-in-one cutting block was applied to the reamer’s rod and fixed to the distal cut in the correct rotational position in relation to the epicondylar line provided by the navigation software. Anterior and posterior condylar osteotomies and chamfer cuts were carried out.

The trial implant with wedges for the femur was assembled using information from the navigation system. The trial components were reduced with a trial spacer. The soft tissue balance, limb alignment, and range of motion (ROM) were checked, and the final thickness of the polyethylene insert chosen. Varus and valgus stress on the tibia will test for ligament stability in full extension and during 10° to 20° of flexion. If the result showed satisfactory stability the definitive implant was assembled and implanted using bone cement. Patella tracking was checked, and any maltracking corrected with a lateral retinaculum release. Loose cement was removed and osteotomy of the tibial tubercle fixed with two cerclage wires.

Postoperative Evaluation

Postoperative protocol was the same as that with primary implantations with full weight bearing. Partial weight bearing is done only if tuberosity refixation is doubtful, or if an additional bone graft was used.

AP and lateral radiographs with long films and patella views were taken to analyze postoperative result. Full-length weight-bearing radiographs were used to evaluate limb alignment. Spiral CT was performed and the posterior condylar line correlated to the epicondylar line of the femur to ensure correct femoral rotational alignment (Figure 3).

Orientation of the tibia component was checked in relation to the middle of the tuberosity.

Clinical evaluation includes wound healing, ROM, stability during the entire ROM, and patellar tracking.

Figure 3: Postoperative radiological result in AP (A), lateral (B), and patella (C) views, and CT scan of the epicondyle region (D). Correct limb alignment and femoral rotation are shown.

Results

Our series included 46 cases, including 29 women. Mean age was 72 years (range, 67-81). There were nine septic two-stage revisions, 17 aseptic-loosening cases, and 20 revisions because of a malpositioned femoral component.

There were no femoral or tibial fractures, loosening of fixation devices, or infections related to the use of the fixation devices for the reference frames.

The OrthoPilot navigation system functioned well in all cases and added approximately 15 minutes (range, 12-20 min) to the procedure.

In all cases, the mechanical axis was reconstructed within a range of ±3°, as confirmed by the navigation system and postoperative full-length standing radiographs.

The joint line was reconstructed in 38 cases (83%) within a range of 0 to 4 mm proximalization. In eight cases (17%), a proximalization up to 8 mm was indicated by the system, and had to be accepted because of a bone stock defect.

There was no patellar maltracking, and the correct femoral rotation (±2°) was confirmed by CT.

All patients achieved more than 90° of flexion, and five patients complained of mild anterior knee pain (Figure 4).

Figure 4: Postoperative pictures of the patient after 2 months.

Discussion

The navigation system provided the surgeon with real-time feedback during surgery.⁸

In the literature, the loosening rate increases significantly, with a deviation of 4° or more from the neutral leg axis.⁹ Long-term results are superior with a reconstructed distal femoral valgus alignment of 5° to 7°.¹⁰

In one study using a navigation system in primary TKAs, 88% of cases did not deviate more than 3° from normal mechanical axis.⁷ Our preliminary results show a correct alignment according to AP radiographs. The reconstruction of the tibial and femoral mechanical axis was successful in all of our cases within a range of less than 2°. The navigated stem extensions provide better orientation and stability of the implants.

Elevation of the joint line of more than 7 mm indicates an unsuccessful outcome of revision TKA.^11,12 With the revision software, a joint line can be planned. All of our cases showed satisfactory reconstruction of the joint line. In cases of unexpected bone deficiencies or ligamentous laxicity, the femoral joint line was reconstructed with a thicker polyethylene insert. The revision software made the position and size of the femoral component easier to select and more comfortable for the surgeon. Correct rotational alignment of the components is important for a successful revision TKA.^13,14 In the e.motion revision system (B. Braun Aesculap), a mobile-bearing tibia insert is used, which offsets problems that may arise with incorrect alignment.

The preoperative analysis of the rotation of the femoral component using CT provided information about component position and location of the epicondylar line. The revision software allows correct rotational positioning of the femur in relation to the epicondylar line. We achieved correct rotation in all of our cases within 2° tolerance.

As with results reported in other studies, our findings confirm the improvement of mechanical alignment.¹⁵ The reconstruction of the joint line could be improved using additional information from preoperative planning. Rotational alignment was reconstructed in relation to the epicondylar axis with high precision.

Conclusion

The results presented here are from our first experience with OrthoPilot-assisted revision knee arthroplasty. The use of a navigation system with specially adapted software is helpful in revision TKA. Determining the cause of failure of the initial implant and carrying out preoperative evaluation are essential for surgical planning. Reconstruction of mechanical alignment of the joint line, as well as correction of the rotational femoral alignment, can be controlled and documented intraoperatively. By using the navigation system, balancing of flexion and extension gaps and selecting femoral size and position are less confusing for the surgeon. Software design based on experience with primary TKAs and modified for revision surgery helps the surgeon avoid having to make difficult calculations and helps to keep the surgeon’s attention on the surgical technique. The system provides continuous feedback on limb alignment, angles, and thickness of bone cuts and augments throughout the procedure.

References

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Authors

Drs Thielemann and Hadjicostas are from Schwarzwald Baar Klinikum Villingen-Schwenningen, Villingen-Schwenningen, Germany; Dr Clemens is from Helios Kliniken Breisach, Breisach, Germany.

Correspondence should be addressed to: F. W. Thielemann, MD, Röntgenstraße 20, Schwarzwald Baar Klinikum Villingen-Schwenningen GmbH, D-78054 Villingen-Schwenningen, Germany.

Dr Thielemann and Clemens are members of the B. Braun Aesculap speaker’s bureau. Dr Hadjicostas had not provided financial disclosure information.