Comparison of functional results with navigation-assisted minimally invasive and conventional techniques in bilateral total knee arthroplasty

Abstract

This study was undertaken to compare the clinical and radiological results achieved using navigation-assisted minimally invasive surgery (NA-MIS) and conventional (CON) techniques in 42 bilateral total knee arthroplasty (TKA) patients with a minimum follow-up of one year. Clinical evaluations were performed using range of motion (ROM), Hospital for Special Surgery (HSS) scores, and Western Ontario and McMaster University (WOMAC) scores (pain, functional, and total) at 3, 6 and 9 months and one year postoperatively. Patients’ subjective preferences and radiological indices, including mechanical axis and coronal inclinations of the prostheses, were compared at one year postoperatively. NA-MIS TKA yielded better HSS and WOMAC total scores than CON TKA up to six months, and a better WOMAC pain score up to 9 months. However, these differences were not significant at one year postoperatively. ROM was comparable in both groups at all times, but more patients preferred the NA-MIS side to the CON side. Radiological results showed no differences in mean values between the two surgical groups, although the NA-MIS group contained fewer outliers than the CON group. In conclusion, NA-MIS TKA was associated with better clinical results up to 6 or 9 months after surgery, giving more accurate leg alignment than CON TKA.

• Navigation
• minimally invasive
• total knee arthroplasty

Introduction

Good results have been reported for minimally invasive total knee arthroplasty (TKA) with regard to early postoperative recovery and reduced soft tissue trauma [1–5]. However, this approach has potential issues with component malpositioning arising from a limited surgical view, and questions about this have been prompted by some operative series [6–7].

Computer-assisted navigation systems were introduced to improve component alignment accuracies [8–11], and a number of studies have concluded that leg and component alignment are improved in TKA performed using navigation systems. Therefore, to solve the challenges caused by the limited surgical view in minimally invasive TKA, we considered combining the accuracy of navigation systems with minimally invasive surgery (MIS). In our preliminary study [12], the MIS technique using a navigation system was associated with a more rapid functional recovery without compromising component and leg alignments. To the best of our knowledge, no previous study has compared the results of MIS using a navigation system with the conventional technique in bilateral TKA, or determined whether improved early postoperative results are sustained in the long term. Moreover, results based on comparisons in the same patients eliminate the variability introduced by differences in gender, age, body mass index, co-morbidities, and activity levels. In addition, we considered that this bilateral comparative approach might expose subtle differences between the two modalities.

Using a bilateral approach, we compared the short-term clinical and alignment accuracies of legs and components that were achieved using navigation-assisted minimally invasive (NA-MIS) TKA with those achieved using conventional (CON) TKA, and examined the durability of the early postoperative improvements reported for NA-MIS TKA.

Materials and methods

Forty-five patients scheduled for simultaneous NA-MIS TKA and contralateral CON TKA were enrolled in this prospective study, after obtaining approval from our institutional review board and written informed consent from all participating patients. In all patients, one knee was assigned to NA-MIS TKA and the other to CON TKA on an alternating basis. However, one knee scheduled for NA-MIS TKA was converted to CON TKA because of registration failure, and two patients were lost to follow-up, leaving 42 patients with a minimum follow-up of one year available for study.

All TKAs were performed by the senior author (E.K.S.), who had performed over 100 CON TKAs using a navigation system, 30 MIS procedures, and 20 NA-MIS TKAs prior to initiation of this trial.

Minimally invasive surgery (MIS) was defined as surgery performed via a curvilinear skin incision medial to the patella from 2 cm proximal to the superior pole of the patella to 2 cm below the joint line, using a mid-vastus approach without patella eversion (Figure 1). The length of the skin incision varied between 9 and 13 cm (Figure 2). The Orthopilot® navigation system (Aesculap, Tuttlingen, Germany) employed in the operations is an image-free system that uses kinematic analysis of hip, ankle, and knee joints and anatomical mapping of the knee joint to build a working model of the patient's knee, as described previously [8], [9]. When fixing the tibial and femoral cutting jig to bone, we used a freehand technique under navigation control.

Figure 1. Skin incision and position of sensor fixing screws.

Figure 2. A 68-year-old woman with bilateral TKA (CON TKA on the left side; NA-MIS TKA on the right) at the one-year follow-up showing the smaller incision on the NA-MIS side.

On the contralateral sides, CON TKA was performed using a medial parapatellar arthrotomy, extending approximately 3–4 cm into the quadriceps tendon, with patella eversion. Intramedullary instrumentation was used for femoral alignment, and a 6° valgus cut was selected for all knees. The tibial cut was performed with extramedullary instrumentation, with the goal of making the cut perpendicular to the tibial shaft in both the coronal and sagittal planes. Alignment was checked with extramedullary rods referenced to the anterior superior iliac spine (ASIS) and placed 5–10 mm medially from the midpoint of both malleoli. In both surgical groups, the posterior cruciate ligament was retained and the patella was not resurfaced. e-motion® (Aesculap, Tuttlingen, Germany) prostheses were used for arthroplasty and all were cemented.

Patients in both groups were treated with the same postoperative rehabilitation protocol, incorporating active range of motion (ROM) exercise within the first few hours post-surgery. Weight bearing with an assisting device (a walker or crutch) and active and passive ROM exercises were started on the first postoperative day and were progressed according to patient tolerance.

Average patient age was 64.2 years (range: 48–82 years), and the study group comprised 9 males and 33 females. Primary diagnoses included osteoarthritis in all patients, and no patient had undergone a previous knee operation.

Clinical evaluations were performed preoperatively and at 3, 6 and 9 months and one year postoperatively. Clinical results analyzed included range of motion, Hospital for Special Surgery (HSS) scores, Western Ontario and McMaster University (WOMAC) scores (for pain and function), and complications. In addition, the subjective preferences of patients were evaluated at one year postoperatively. There were no significant differences in the preoperative ROM, HSS score, and deformity between the two groups (Table I).

Table I.  Comparison of the preoperative data of the NA-MIS and CON sides.

	NA-MIS side	CON side	p
Preoperative  HSS (point)	68.5 (51–83)	66.5 (48–81)	0.324
Range of motion (°)	115.5 (30–140)	117.0 (35–135)	0.777
Femorotibial  angle (° ± SD)	−0.5 ± 7.2*	−0.4 ± 7.2*	0.968

* = varus

Radiological indices included the mechanical axis (optimum 0°) and the coronal inclinations of the femoral and tibial prostheses (optimum 90° for both) on standing anteroposterior (AP) radiographs of knees taken at one year after surgery, and were measured by a resident independent of the surgical unit. Outcome was defined as acceptable when within ± 3° of the optimum and as an outlier when ≥ ± 3° of the optimum.

Independent and paired Student's t-tests and Chi-square tests were used to compare the two groups. All analysis was performed using SPSS software (SPSS for Windows Release 11.0, Chicago, Illinois) with significance set at 95%.

Results

Preoperative average HSS scores were 68.5 (range: 51–83) in the NA-MIS group and 66.5 (range: 46–81) in the CON group, and these scores improved to 93.6 (range: 85–100) and 92.5 (range: 77–100) respectively, at one year postoperatively (p < 0.01 for both groups). Knees had a higher average HSS score in the NA-MIS group than in the CON group until 6 months postoperatively (p = 0.042), but not after 9 months postoperatively (p = 0.111). These clinical results are shown in Tables I and III.

In terms of WOMAC scores, pain scores in the NA-MIS group showed better results up to 9 months postoperatively (p = 0.020), but not at one year postoperatively (p = 0.122), and the total score showed better results up to 6 months (p = 0.031), but not after 9 months postoperatively (p = 0.297) (Tables II and III).

Table II.  Comparison of results at 3 and 6 months postoperatively for the NA-MIS and CON sides (all values shown as mean ± standard deviation).

	NA-MIS side	CON side	p
3 months postop.
ROM	125.8 ± 9.8	121.2 ± 7.9	0.074
HSS score	90.0 ± 4.0	87.3 ± 5.7	0.040
WOMAC
Pain	8.1 ± 1.0	9.3 ± 2.1	0.024
Function	27.0 ± 3.5	29.0 ± 3.1	0.059
Stiffness	3.6 ± 1.3	4.0 ± 1.1	0.230
Total	38.8 ± 4.8	42.0 ± 4.6	0.029
6 months postop.
Range of motion	128.4 ± 9.3	126.8 ± 10.8	0.633
HSS score	92.7 ± 3.8	89.8 ± 5.5	0.042
WOMAC
Pain	7.1 ± 1.7	8.3 ± 1.9	0.031
Function	25.2 ± 3.8	27.2 ± 4.2	0.116
Stiffness	2.8 ± 1.1	3.0 ± 1.3	0.609
Total	35.1 ± 5.0	38.5 ± 4.9	0.031

Table III.  Comparison of results at 9 months and one year postoperatively for the NA-MIS and CON sides (all values shown as mean ± standard deviation).

	NA-MIS side	CON side	p
9 months postop.
Range of motion	128.3 ± 6.8	127.0 ± 10.7	0.737
HSS score	93.1 ± 3.8	90.4 ± 5.3	0.111
WOMAC
Pain	6.2 ± 1.2	7.6 ± 2.1	0.020
Function	24.3 ± 6.5	25.2 ± 6.7	0.678
Stiffness	2.9 ± 1.0	3.0 ± 1.2	0.603
Total	33.5 ± 7.0	35.5 ± 7.8	0.545
1 year postop.
Range of motion	129.5 ± 8.0	127.1 ± 8.8	0.364
HSS score	93.3 ± 4.0	92.8 ± 4.3	0.730
WOMAC
Pain	6.1 ± 1.5	7.2 ± 2.3	0.122
Function	23.0 ± 4.5	25.0 ± 7.4	0.573
Stiffness	2.8 ± 1.0	2.9 ± 1.0	0.504
Total	31.9 ± 5.6	35.2 ± 9.8	0.277

Range of motion (ROM) preoperatively averaged 115.5° in the NA-MIS group and 117.0° in the CON group, and these measurements improved to 131.6° and 127.1°, respectively, at the one-year follow-up (p < 0.05 for both groups). However, these results were not significantly different at any stage during follow-up (p > 0.05) (Tables II and III). Patients’ subjective preferences were for the NA-MIS side in 24 cases and for the CON side in 10 cases, while 8 patients indicated no preference (p = 0.003).

No case of deep infection or loosening requiring revision occurred in either group during follow-up.

Mean mechanical axes were not significantly different in the two groups (varus: 0.7 ± 1.6° and 0.8 ± 2.5°, respectively) (p = 0.815). However, there were more outliers in the CON group (8) than in the NA-MIS group (2) (p = 0.043). In the coronal alignment of the femoral component, the CON group (9) had more outliers than the NA-MIS group (3). There were significant difference (p = 0.061) in outliers and in mean value (88.9 ± 2.6° in the NA-MIS group; 88.7 ± 2.8° in the CON group; p = 0.769). Neither was any difference observed between the two groups in terms of means (p = 0.365) or outliers (p = 0.645) for coronal tibial alignment (Table IV).

Table IV.  Comparisons of radiological outlier results for the two groups.

	NA-MIS side	CON side	p
Mechanical axis angle
Average value (°)	−0.7 ± 1.6*	−0.8 ± 2.5*	0.815
No. of outliers	2	8	0.043
Coronal inclination  of femoral prosthesis
Average value (°)	88.9 ± 2.6	88.7 ± 2.8	0.769
No. of outliers	3	9	0.061
Coronal inclination  of tibial prosthesis
Average value (°)	90.5 ± 1.7	90.1 ± 2.3	0.365
No. of outliers	2	3	0.645

* = varus

Discussion

Minimally invasive TKA may be a near-ideal option for reducing blood loss, postoperative pain, and hospitalization, and for promoting recovery. Investigators have reported that TKA using a MIS technique leads to rapid recovery from surgery and short hospital stays, and an improved ROM during the early postoperative period [1–5]. However, in MIS TKA, component malpositioning and neurovascular injury may arise due to a limited surgical exposure [6], [7]. Moreover, computer navigation has been demonstrated to reduce the number of component coronal plane outliers in TKA [8–11]; thus, we considered that the coupling of computer navigation and MIS TKA was likely to be beneficial.

In a preliminary report on this topic [12], we reported that NA-MIS TKA resulted in better pain scores, shorter times to achieve 90° flexion and straight-leg raise, and a smaller extension lag during the early postoperative period (within 2 weeks). However, no previous study has presented the functional results of NA-MIS TKA.

Therefore, we undertook this study to determine the comparative longevity of early postoperative results. To the best of our knowledge, this is the first study to compare the functional results obtained with minimally invasive techniques using a navigation system and with a conventional technique in bilateral knee joints after TKA. The postoperative duration of the benefit of MIS in TKA is debatable. Laskin et al. [5] reported that it disappears before 3 months postoperatively, whereas Haas et al. [4] found that the benefit was still evident at one year postoperatively. In the present study, we found that the NA-MIS group showed better results in terms of HSS and WOMAC (pain and total) scores until 6 months postoperatively, but no benefit in terms of ROM at 3 or 6 months postoperatively. At one year postoperatively, no measurable differences were found between the NA-MIS and CON sides. However, more patients expressed satisfaction with the NA-MIS side.

Our data confirm the findings of previous investigations that found a reduction in the number of coronal plane alignment outliers for computer navigation versus traditional manual instrumentation without navigation.

We conclude that NA-MIS TKA using a “mini-midvastus” approach results in better knee functional scores after TKA than CON-TKA up to 6 or 9 months postoperatively. However, no differences in any functional parameters were evident at one year postoperatively. NA-MIS TKA had fewer prosthetic alignment outliers than CON-TKA. A larger cohort and longer term studies will be needed to determine if this reduction in outliers results in an improvement in prosthetic survival.