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Two-year Outcomes of Computed Tomography-based and Computed Tomography Free Navigation for Total Knee Arthroplasties

Martin, Arno; von Strempel, A

Clinical Orthopaedics and Related Research: August 2006 - Volume 449 - Issue - p 275-282
doi: 10.1097/01.blo.0000218738.69247.d8
SECTION II: ORIGINAL ARTICLES: Knee
Free
SDC

Optimal component position in all planes and well-balanced soft tissues facilitate a good clinical outcome and long-term survival after total knee arthroplasties. We investigated the accuracy of implantation of navigated total knee arthroplasties at 3 months followup and the influence on the clinical outcome at 2 years followup. Forty-four patients (44 procedures) were enrolled in our prospective study. One half of the surgeries were performed using a computed tomography-based navigation system, and half were performed with im-ageless navigation. Outcomes were based on the Insall knee score parameters, anterior knee pain, patient satisfaction, feeling of instability, and step test. The radiographic parameters were the mechanical axis, tibial slope, lateral distal femoral angle, and medial proximal tibial angle. The radio-graphic measurements were similar in both groups (patients within ± 3° inaccuracy range in computed tomography-based/imageless groups; mechanical axis 86%/81%, tibial slope 95%/91%, lateral distal femoral angle 95%/91%, medial proximal tibial angle 91%/95%). The imageless system provided equal radiographic results, but we found improved ligament balancing in the computed tomography free group. The computed tomography-based approach has a good pre-operative planning procedure, but is more expensive and time consuming.

Level of Evidence: Therapeutic Study, Level II. See the Guidelines for Authors for a complete description of levels of evidence.

From the Department of Orthopedic Surgery, Academic Teaching Hospital, Medical University of Innsbruck, Feldkirch, Austria.

Received: January 26, 2005

Revised: September 12, 2005; January 26, 2006

Accepted: February 27, 2006

Each author certifies that he has no commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article. Each author certifies that his or her institution has approved the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research, and that informed consent was obtained.

Correspondence to: Arno Martin, MD, Academic Teaching Hospital, Department of Orthopedic Surgery, Carinagasse 47, A-6800 Feldkirch, Austria. Phone: 43-5522-303-1600; Fax: 43-5522-303-7520; E-mail: Arno.Martin@lkhf.at.

The number of total knee arthroplasties (TKAs) continues to increase each year,27 while the mean patient age is decreasing.29 Increased long-term survival of TKAs is becoming more important. Several studies have confirmed that optimal positioning and alignment of prosthetic components is crucial for the best long-term results.2,10,13,26 Investigators of these studies suggest a postoperative mechanical alignment of no greater than 3° varus or valgus to achieve the best possible outcome at 2 years followup.15,18

Failure to achieve mechanical alignment within 3° varus or valgus during TKAs using conventional intramedullary or extramedullary instruments has been reported in as much as 26% of patients.19,24,33,34 Inaccuracies in pre-operative planning may occur because of inconsistent radiographic techniques or from preoperative flexion contractures, which affect radiographic projections.32,36

One study on nine cadaveric specimens performed with a computed tomography (CT)-based image-guiding system showed improved alignment of the prosthetic components.21 Initial comparative radiographic studies showed increased accuracy using an imageless navigation system for TKAs.5,20,30 A study comparing TKAs using a fluoroscopic-based navigation system versus a conventional implantation technique showed no differences in range of motion (ROM) and International Knee Society knee and function scores at 3 months followup.35

Our hypothesis was that a CT-based navigation system leads to superior component positioning in all planes compared with an imageless navigation technique (CT-free). We asked whether clinical outcome and ligament balancing were affected by the different navigation system modules at 3 months and at 2 years followup. We also wondered whether there would be differences in time and cost between the two navigation techniques.

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MATERIALS AND METHODS

We prospectively evaluated the first 22 patients who had primary TKAs from October 2001 to April 2002 using a CT-based navigation system. Our selected series used specific inclusion and exclusion criteria (Table 1). From May to September 2002, primary TKAs were performed on another 22 patients with an imageless computer-assisted surgical technique when that became available (Table 2). There were no differences in age, gender, diagnosis, leg deformity, Insall knee score, and ROM (Table 2). The VectorVision® knee navigation system (Brain-LAB Inc, Munich, Germany) was used in both groups. This prospective nonrandomized comparative study was approved by the hospital ethics committee.

TABLE 1

TABLE 1

TABLE 2

TABLE 2

The VectorVision® system consists of a workstation and an infrared light-emitting camera. An infrared light-reflecting dynamic reference base (DRB) was fixed to the tibia and the femur to detect leg movements. The pointer and the cutting block adapter were assembled with passive fiducial markers to reflect the infrared light back to the camera. The workflow was passed through by using the sterile-draped touch screen.

One week before surgery, CT scans of the ipsilateral hip, knee, and ankle were taken of the CT-based group. We took 5-mm axial CT slices for the hip and ankle and a 1.5-mm helical scan for the knee. All preoperative parameters relevant to TKAs can be planned with the CT data set. For each tibial and each femoral navigation, a separate registration of the patient's CT data set must be done intraoperatively.

Patient registration in the imageless group was performed by defining anatomic landmarks and extended surface matching after rigidly attaching a DRB to the femur and tibia. The planning procedure can be done intraoperatively. The intraoperative planning procedure is automated but can be modified by the surgeon. An accuracy check of the navigated bone cuts provides information about deviation in all planes. A last computer-assisted verification of reconstructed mechanical axis, ROM, and ligament balancing can be performed with the trial components before implanting the prosthetic components.

The position of the femoral component in the sagittal plane was oriented parallel to the distal anterior cortex in both groups. The tibial component was positioned in a 7° posterior slope in relation to the tibial mechanical axis. The tibial component rotation in the axial plane was adapted to the medial ⅓ of tibial tuberosity. The rotation of the femoral component was 3° external to the posterior condylar line or parallel to the epicondylar line.

All operations were performed by one senior consultant orthopaedic surgeon (AVS). We used a standard medial parapatellar exposure. A lateral approach was used for severe valgus deformities (angle between femoral and tibial mechanical axis > 15°) in conjunction with the preoperative ligament balancing requirements. In the CT-based group, 21 NexGen® mobile-bearing knee prostheses (MBK, Zimmer Inc, Warsaw, IN) were used. In one patient, a Legacy® constrained condylar knee pros-thesis (Zimmer Inc) was implanted because of a severe preoperative valgus deformity and insufficient collateral ligaments. In the imageless group, each patient was implanted with a NexGen® mobile-bearing knee prosthesis. No patella prosthesis was implanted. In both study groups, one patient required conversion from the navigated surgical technique to the conventional implantation technique because of loosening of the femoral DRB. Therefore, statistical analysis was calculated with 21 patients in each group.

Radiographs were taken preoperatively and at each followup. A long-leg weightbearing anteroposterior (AP) radiograph was taken for measuring alignment of the limb in the coronal plane. A lateral radiograph of the knee was taken for sagittal component alignment. The varus/valgus angle (mechanical axis) was measured by using the mechanical axis of the femur and tibia (Fig 1A). Frontal alignment of the components was evaluated according to the lateral distal femoral angle (LDFA) and the medial proximal tibial angle (MPTA) (Fig 1B). The tibial component posterior slope was measured in relation to the mechanical tibial axis (Fig 1C). The tolerable inaccuracy range was ± 3° for all radiographic measurements. The cumulative error of alignment was calculated based on how many patients achieved optimal alignment for all four measured radiographic variables (mechanical axis, tibial slope, LDFA, MPTA) in a range of ± 3°.

Fig 1A

Fig 1A

We examined the following parameters during physical examination: ROM, medial/lateral ligament balancing in full extension and 30° flexion (instability: + = 6°-9°, ++ = 10°-14 °, +++ = ≥ 15°),14 drawer test in 90° flexion (instability: + = 5-7 mm, ++ = 8-10 mm, +++ ≥ 11 mm),14 patellar pressure, and movement pain. We recorded patients' responses to feeling instability during walking, rising from a chair, or climbing stairs. A step test (with the surgically treated leg up first and the contralateral untreated leg down first) also was done. The patient was given a questionnaire regarding satisfaction with the TKA (1 = perfect, 2 = very good, 3 = good, 4 = fair, 5 = poor) and regarding feeling any anterior knee pain (1 = no, 2 = yes).

The patient filled out the questionnaire and an independent person blinded to the results (CS) analyzed the answers. The Insall knee score14 was recorded preoperatively and at the 3-month and 2-year followups. The radiographic and clinical measurements were done by an independent orthopaedic surgeon (CS) who was blinded to the results.

Statistical analysis was performed using SPSS version 11.5 software (SPSS Inc, Chicago, IL). Frequencies were obtained for all variables. Descriptive statistics including mean, standard deviation (SD), minimum value, maximum value, and 95% confidence intervals (CI) were obtained. The differences between CT-based and CT-free data and differences with time were of particular interest. For normally distributed data (eg, surgery time and ROM), paired t tests with one-way analysis of variance (ANOVA) and repeated measures ANOVA were used. For data that statistically were not normal and for categorical data (eg, preoperative and postoperative limb alignment, tibial slope, LDFA, MPTA, cumulative error of alignment, ligament balancing, and patient satisfaction), nonparametric tests such as chi square and Mann-Whitney U tests were used as appropriate. A Friedman test was used to compare the knee scores between the groups and changes in scores with time in each group. Significance was set at the p < 0.05 level.

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RESULTS

Neither group showed differences in the accuracy of implantation of the components in the coronal and sagittal planes or the accuracy of reconstruction of a neutral mechanical axis of the limb. No differences in the accuracy of the reconstructed mechanical axis were seen between the two study groups at 3 months and at 2 years postoperatively. In the CT-based group, 18 of 21 patients showed a maximum inaccuracy of 3° varus/valgus versus 17 of 21 patients in the imageless group. In each group, 20 of 21 patients had a varus/valgus range of 4°. One patient in each group had a maximum inaccuracy of 5° (Table 3) (Fig 2). Analysis of the planned 7° posterior tibial slope showed both groups had comparable results at the two followups for the achieved posterior slope and for the measured inaccuracies. In the CT-based group, 20 of 21 patients had ± 3° inaccuracy. In the imageless group, 19 of 21 patients had ± 3° inaccuracy. For the planned 90° of the LDFA and the MPTA, the inaccuracies were similar between both groups. All patients in the CT-based group had ± 3° inaccuracy for LDFA versus 20 of 21 patients in the imageless group. For the MPTA in the CT-based group, 19 of 21 patients had ± 3° inaccuracy, and 20 of 21 patients in the imageless group were in the tolerable range. The MPTA outliers had a maximum inaccuracy of 4° in both groups. The cumulative error of alignment showed no difference between the study groups. Seventeen of 21 patients fulfilled four criteria in the CT-based group, and 15 of 21 patients fulfilled four criteria in the comparison group. Nineteen of 21 patients in both groups achieved three criteria in an optimal manner.

Fig 2

Fig 2

TABLE 3

TABLE 3

Both groups had similar results for all clinical parameters except the findings for ligament balancing in 30° and 90° flexion. An increased (p < 0.001) Insall knee score was found with time in both study groups but the increases were similar (Table 4) (Fig 3). The postoperative ROM in both groups also was similar at the 3-month and at the 2-year followups. Both groups had an increase (p ≤ 0.002) in ROM between the 3-month and the 2-year followups. The improvement of ROM was similar in the CT-based (13%) group compared with the imageless group (7%). Patients' satisfaction was rated as very good or good in 14 of 21 patients in the CT-based group and in 19 of 21 patients of the imageless group (Fig 4). One patient in each study group had anterior knee pain. Navigation-related perioperative and postoperative clinical complications could not be detected. During clinical examination, two of five ligament-balancing parameters showed a better soft tissue situation in patients in the imageless group (Table 4). Our examination of ligament balancing in full extension was similar in the groups. In 30° flexion, we detected better (p = 0.004) medial and lateral stability in the imageless group. The anterior drawer test showed better (p = 0.035) stability in the imageless navigation group. Patients in both groups answered the question regarding instability in a similar manner. Two patients needed help from the examiner to do the step test: one in the CT-based group because of insufficient ROM of 85° and one in the imageless group because of pain.

Fig 3

Fig 3

Fig 4

Fig 4

TABLE 4

TABLE 4

Using the CT-based navigation module is approximately 42 minutes more time consuming preoperatively, requires more organization, and is more expensive. Performing the preoperative CT scan took 20 to 30 minutes. The mean time for the preoperative planning procedure for CT-based navigation was 17 ± 3 minutes (range, 13-21 minutes). The durations of surgery for the CT-based and imageless navigation techniques were similar (100 ± 17 minutes and 107 ± 19 minutes, respectively).

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DISCUSSION

Several clinical studies showed there was an increased incidence of aseptic loosening and long-term failure of TKAs in patients with a reconstructed mechanical axis after TKAs exceeding ± 3° inaccuracy in the coronal plane.2,13,15,26 A range of 3° inaccuracy (varus or valgus) of an implanted mobile-bearing tibial component was confirmed in a simulated study18 as maximum in vivo tolerability. A rate of 24% for aseptic loosening15 was reported in patients with an alignment in the coronal plane exceeding ± 3°. In comparison, the loosening rate was 3% when the deviation from a neutral axis was within ± 3°. A 90% 10-year survival rate was reported after TKAs, with a tolerable range of 4° for varus/valgus deviation in the coronal plane.25 The survival rate decreased to 73% in patients with varus deformities and 71% in patients with valgus deformities when the mechanical axis inaccuracy exceeded ± 4°. Failure to achieve mechanical alignment of the limb within 3° varus/valgus using conventional implantation instruments during TKAs have been reported in as much as 26% of patients.19,24,33

Our study has several limitations. The first is the absence of randomization because at the beginning of the study, only the CT-based navigation module was available for us. However, all preoperative measured parameters were similar between the two groups (Table 2). Therefore, the preconditions in the two patient groups are comparable. The surgeon was well trained in using the two different modules of the navigation system (cadaver tests and 20 initial patients) before patient recruitment. Therefore, the conclusions might not be applicable to someone less familiar with the techniques. The postoperative radio-graphic measurements and clinical examinations were performed once by one orthopaedic surgeon (CS) not involved in the surgical procedures and who was blinded to the results. Although one observer eliminates variability, one observer may introduce systematic bias. Our study also does not allow for conclusions regarding tibial and femoral component rotational position (postoperative CT was not possible within the agreement of the ethical committee). However, during the preoperative planning procedure in the CT-based navigation group, there were variations in the definition of the transepicondylar axis of as much as 4°. Therefore, there is variability in the CT-assisted definition of the surgical transepicondylar line, indicating it is not an absolute clearly definable anatomic landmark. This finding was confirmed in independent studies that showed unreliable precision of transepicondylar axis definition.1,11,16,17 The intraoperative definition of the femoral component rotation is best based on a combination of all available landmarks (posterior condylar line, surgical transepicondylar axis, AP line, and the actual medial and lateral ligament balancing environment in 90° flexion).

Our data for limb alignment in the coronal plane, tibial component positioning in the coronal and sagittal planes, and coronal femoral component position are comparable to those in previous studies5,9,12,20,22,23,28,30,31,35 using CT-based or imageless navigation systems (Table 5). The findings for postoperative varus and valgus inaccuracies were confirmed in an experimental cadaver study21 using a CT-based navigation system. The cumulative error of alignment showed a high rate of optimal component position in all measured planes in both groups. We found no differences using the CT-based or imageless navigation module. These findings were in agreement with the results of another comparative study.3,4 In addition, we could not identify alignment differences in either study group between the two followups. The accuracies of the mechanical alignment of the limb reported in the literature using conventional intramedullary or extramedullary instruments without a navigation system are less than in both of our study groups.6,7,19,24,33,34

TABLE 5

TABLE 5

The femoral component should be implanted according to the distal femoral anterior cortex, avoiding cortical notching. This requirement also can be achieved using conventional implantation instruments (Fig 5A). The latter was confirmed by a prospective randomized study9 that established no difference for the femoral component orientation using a navigation system or a conventional jig-based surgical technique. With the exception of the CT-controlled studies,8,9,31 a scientific statement of the orientation of the femoral component in the sagittal plane is not possible because of using plane radiographs with a lateral view of the knee (the femoral head is not imaged). It also is variable to define a reference line5,23 tangential to a curve (curve = anterior bow of the femur). Two studies8,31 judged the optimal flexion/extension position of the femoral component perpendicular to the femoral mechanical axis. Implanting the femoral component strictly parallel to the mechanical axis can lead to an anterior cortical notching, especially in a femur with a large anterior bow (Fig 5B).

Fig 5A

Fig 5A

We found no difference in surgery time between groups. When using the imageless navigation module, no time consuming CT scan and extensive preoperative planning procedure had to be performed. Surgery time can be reduced to a mean of 70 to 80 minutes after an improvement in the special navigation cutting guides and optimization of the software work flow.

One study35 comparing a fluoroscopic-based navigation technique with a conventional implantation method for TKA found no differences for the ROM and the International Knee Society knee and function scores at 3 months followup. Additional clinical parameters were not included in the study protocol. At 2 years followup, we found no differences between study groups for ROM, Insall knee score, subjective feeling of instability during walking, rising from a chair or stair climbing, ligament balancing in full extension, step test, and anterior knee pain. The ROM and the Insall knee score showed clear improvement 2 years after TKA. More stable and symmetric ligament balancing in 30° flexion and for the drawer test was found in the imageless navigation group. This can be attributed to the available ligament-balancing module of the imageless navigation system. Information regarding ligament stability in every knee position can be obtained. The different significance levels for ligament balancing in 30° flexion and full extension can be attributed to the small sample size. The greater satisfaction of patients in the imageless navigation group can be attributed to improved ligament balancing.

We found no differences in radiographic alignment of the implanted total knee prostheses using the CT-based or imageless navigation module. A high precision of implantation can be achieved by using CT-assisted surgical techniques for TKAs. At 2 years followup, we found better ligament balancing results in patients in the imageless study group. All other clinical variables showed no differences. Both methods offer comparable radiographic results and clinical outcomes. We prefer the imageless module because it is less time consuming and less expensive, whereas patients prefer the imageless module because they receive less radiation.

Acknowledgments

We thank C. Sukopp, MD, Munich, Germany, for radiographic and clinical measurements and J. Hayden, MSN, RN, Department of Orthopedic Surgery, Rush University Medical Center, Chicago, IL, for statistical analysis.

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