Introduction

Total knee arthroplasty (TKA) through a standard approach produced good long-term results [1–3]. The procedure, however, traditionally required an extensile exposure with much soft-tissue disruption and eversion of the patella [4, 5]. Although the good long-term results achieved with a standard TKA, the rehabilitation was often arduous and painful [2, 4, 5].

In recent years, minimally invasive TKA (MI-TKA) has become increasingly popular. A variety of previously published studies have demonstrated superior results for the minimally invasive surgical (MIS) technique [6–8], while others warn of problems associated with this technique (e.g., wound healing, incorrect component orientation, etc.) [9–11]. The industry supports the new MIS technique and has developed special instruments, and finally, it is asked for by the patients, who ask for smaller incisions, better cosmetics, and earlier rehabilitation [9, 11, 12]. However, MI-TKA exposures provide reduced visualization of critical surgical anatomy [13, 14]. Inferior results in restoration of the leg axis and a malalignment of components are established as the most common adverse consequences of the reduced visualization associated with minimally invasive total knee exposures [13, 15].

Computer-assisted techniques were developed to overcome the inherent limitations of manual instrumentation. The reports of performing TKAs with computer-assisted techniques using conventional, extensile surgical exposures indicate that increased accuracy of limb and implant alignment and reduced incidence of alignment outliers occurs [16, 17]. Therefore, the rationale for combining computer-assisted and MIS techniques is that the reduction in perioperative morbidity and the improvement in early postoperative function that are achieved with less invasive exposures can be realized while retaining the accuracy of implant and limb alignment that can be achieved with computer techniques, even when crucial surgical anatomic landmarks are not visible [18–24].

Despite the encouraging evidence in the literature [18–24], the clinical and functional benefits of MICA-TKA are still unclear and remain to be defined on a larger scale. Moreover, up to now, limited prospective, randomized comparison between MICA-TKA and C-TKA has been documented [23]. The purpose of the current study is to present the 12-month results of a prospective, randomized clinical trial comparing the short-term results of MICA-TKA with C-TKA. The hypothesis is that MICA-TKA may achieve at least the same level of clinical improvement, compared with C-TKA.

Materials and methods

We conducted a prospective randomized short-term clinical study, which was approved by our local ethic committee. All subjects who were candidates for TKA at our institution from January 2008 to December 2011 were considered for inclusion. The inclusion criteria were <70 years old, a diagnosis of osteoarthritis, <20° flexion contracture, and <20° varus or valgus deformity. Exclusion criteria included patients with rheumatoid arthritis, previous knee surgery, infection, revision TKA, severe cardiopulmonary comorbidities, body mass index (BMI) >40 kg/m2, or neurological problems. The study was explained to each patient. Written and informed consent was taken by all subjects.

After an informed consent, subjects were randomly assigned to either the C-TKA group (Group A) or MICA-TKA group (Group B) choosing one of two closed envelope by a nurse (YQD) not involved in the study, just prior to the skin incision. All TKAs were conducted by the same senior surgeon (Z.X.Z). All subjects were implanted with the same cruciate-substituting design (P.F.C Sigma, DePuy International Ltd, Warsaw, In, USA). The computer-assisted surgery total knee replacements were performed using the Stryker Knee Navigation system (Stryker, Marwah, New Jersey, USA) according to the manufacturer’s recommendations. In Group A, a mini-medial parapatellar approach was performed. In the C-TKA group, a standard medial parapatellar approach was performed.

Anteroposterior, long-leg, weight-bearing undigitized radiographs (a 320-mA, 0.03-s exposure at 80–100 kV, depending on soft-tissue thickness) were taken. The coronal mechanical axes of the long leg just after operation and at 12-month follow-up were taken and evaluated. In addition, axial radiographs of the distal femur were taken at 12-month follow-up. The radiograph is an accepted method of evaluation of femoral component rotation with comparable reproducibility and correlation with CT results. The angle between the clinical epicondylar axis (a line that connects the medial and lateral epicondylar prominences) and the posterior condylar axis was defined as the condylar twist angle (CTA) in the radiographs. These evaluations were performed at least three times in each subject by two authors (LXZ and QSL) blinded to clinical information, and the final judgment was defined based on these data.

Tourniquet time and total blood loss were analyzed. ROM was measured preoperatively and at postoperative 12-months by one orthopedic surgeon. The Knee Society clinical rating system, including evaluations of Knee Society score (KSS) and Knee Society functional score (KSFS), was evaluated at a preoperative visit as well as at postoperative 12-month follow-up. Moreover, adverse events, such as any complications or need for revision surgery, were recorded. These evaluations were performed by one of the authors, who was independent of the treatment.

Results were analyzed statistically using a statistical software package (Stat Mate III; ATMS Co., Ltd., Tokyo, Japan). The differences in the clinical and radiographic results between the two groups were analyzed using the nonpaired Student’s t test. Results in the same group at different time points were analyzed using the paired Student’s t test. Differences in P < 0.05 were considered statistically significant.

Results

A total of 87 subjects who were enrolled in this study were randomized to two groups, and 82 patients were available for the 12-month follow-up. In Group A, there were 42 subjects (21 men and 21 women) and the mean age was 64.5 years (range 62–69 years). The dominant leg was affected in 22 patients. In Group B, there were 40 subjects (19 men and 21 women) and the mean age was 65.9 years (range 60–67 years). The dominant leg was affected in 19 patients. No significant differences with regard to age, gender, affected leg, or BMI were detected when the two groups were compared (Table 1). No cases of infection, no neurovascular complications, and no patients needed revision surgery in either group.

Table 1 Comparison of patient characteristics
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The mean tourniquet time was 79 min (range 71–106 min) in Group A and was 83 min (range 69–113 min) in Group B; no significant difference was detected. The estimated total blood loss was 588 ml (range 478–876 ml) in Group A versus 345 ml (range 276–643 ml) in Group B, and a significant difference was investigated in terms of total blood loss between the two groups.

As far as Knee Society clinical rating system is concerned, the mean KSS significantly increased (P < 0.05) from 42.4 ± 2.22 before surgery to 71.0 ± 3.12 at 12 months in Group A and significantly increased (P < 0.05) from 41.9 ± 3.09 before surgery to 92.1 ± 7.33 at 12 months in Group B; a significant difference was noted between Group A and Group B in terms of KSS (P < 0.05) at 12-month follow-up; the mean KSFS significantly increased (P < 0.05) from 53.1 ± 4.22 before surgery to 64.1 ± 4.2 at 12 months in Group A and significantly increased (P < 0.05) from 54.1 ± 3.9 before surgery to 80.4 ± 4.6 at 12 months in Group B, a significant difference was investigated between Group A and Group B with regard to KSFS at 12-month follow-up (P < 0.05) (Table 2).

Table 2 Clinical outcomes within and between Group A and Group B preoperatively and at 12-month follow-up
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The mean ROM before surgery was 101.3 ± 6.9 followed by a significant increase (P < 0.05) to 112.4 ± 5.9 at 12 months in Group A and was 103.1 ± 9.1 before surgery followed by a significant increase (P < 0.05) to 129.2 ± 5.1 at 12 months in Group B; a significant difference was detected between Group A and Group B regarding mean ROM at 12-month follow-up (P < 0.05) (Table 2).

The radiographic evaluations (Table 2) revealed that the preoperative coronal mechanical axis between the two groups was almost the same. The alignment of Group A at 12-month follow-up was similar in terms of varus to Group B (P > 0.05). The mean angle differences between postoperation and 12-month follow-up were 0.9° and 0.8° in Group A and Group B, respectively. No statistically significant difference was noted between the two groups. The CTA in Group A was similar to that of Group B at 12-months follow-up (P > 0.05). The obtained results indicated that the femoral prosthesis in Group A was implanted with similar internal rotation in relation to the clinical epicondylar axis to Group B. Long-leg mechanical axis at 12-months follow-up demonstrated that the rate of outliers over 3° varus/valgus from the mechanical axis was 20.1 % in Group B and 19.8 % in Group A (P > 0.05).

Discussion

To our knowledge, there are a quite few prospective, randomized studies comparing MICA-TKA with C-TKA in the literature up to now [23]. The purpose of the current study is to present the results of a prospective, randomized study which compare the 12-month results of MICA-TKA with those of C-TKA. The most important finding of the present study is that the two groups were found to be similar in terms of coronal mechanical axis. Similarly, the femoral rotational profile revealed that the prosthesis in MICA-TKA was implanted with similar internal rotation to C-TKA. It is also demonstrated that the average blood loss in patients of MICA-TKA group was significantly reduced as compared to patients of C-TKA group. No significant difference was detected in terms of tourniquet time. Moreover, clinical results of MICA-TKA, in terms of ROMs, KSSs, as well as KSFSs, were significantly superior to that of C-TKA. Our results are in line with the findings published by Lüring et al. [23].

Minimally invasive joint replacement has become increasingly popular in the orthopedic community. Proponents of this technique in TKA surgery claim benefits, including faster recovery, reduced blood loss, and lower costs. Often, less invasive surgery has been identified only as a shorter surgical approach to implant a traditional total knee arthroplasty performing the so-called key-hole surgery. Likewise, malalignment, avulsion fractures, and wound problems are all potential dangers encountered when performing joint replacement surgery through small incisions. Nestor et al. [13] reported that four of 30 mini-midvastus patients had tibial component varus malalignment greater than 3°, while none of 30 limited medial parapatellar patients had such surgical outliers. Seon et al. [18] reported a postoperative mechanical alignment within 3° varus or valgus in 96 and 100 %, respectively, in navigated implants. Obviously, computer-assisted surgery should address the more difficult components’ position in a correct alignment even with smaller surgical exposure. In the current study, we found that the accuracy of the implantations in relation to the coronal mechanical axis in MICA-TKA group was similar to that of C-TKA group, and the femoral rotational profile revealed the prosthesis in MICA-TKA group that was implanted with similar internal rotation to C-TKA group. The senior author of the present study is a joint arthroplasty surgeon with 4-years experience in navigation TKA. The mini-medial parapatellar approach has been routinely used in our hospital in an effort to minimize the incision size, patellar eversion, and tibia translation. The senior author also performed approximately 50 MICA-TKAs using the mini-medial parapatellar approach before the present study in order to eliminate bias due to the learning curve.

Blood loss in minimally invasive TKA is an essential issue but is commonly underestimated [25, 26]. All the subjects enrolled in the present study are older than 60, and concomitant pathological conditions, such as hypertension, diabetic disorders, and heart disease, are frequently investigated. The operative and postoperative risks can be increased by blood loss during and after operation. Researchers have recommended a variety of solutions to minimize intraoperative bleeding, such as use of a tourniquet, the insertion of a bone block to plug the entry hole made by the femoral intramedullary alignment rod, diathermy coagulation, prophylactic administration of antifibrinolytic agents, control of knee position, and, more importantly, minimally invasive surgery [9, 25, 27, 28]. Haas et al. [27], using an autologous bone graft to plug the femoral hole, demonstrated a significant difference in postoperative suction drainage between plugged and unplugged groups, but no difference in the requirement for transfusion. In accordance with these findings, Schroer et al. [9] indicated same results using an acrylic cement plug to seal the femoral hole. Tenholder et al. [28] in a similar study displayed that sealing the femoral canal is effective in reducing hemoglobin decrease and transfusion requirement. As a matter of fact, the reason for the smaller amount of blood loss in Group B in our study is probably the less invasive approach to the intramedullary canal with the computer-assisted technique even if it involves the drilling of multiple bicortical pins. This is very important because using a standard technique the intramedullary femoral hole can be easily plugged with bone; in contrast, using the MICA-TKA technique, the deepest part of the bicortical hole cannot be reached due to the small incision. The bicortical pin approach seems to be a safe procedure even if the risk of hematoma over the thigh cannot be excluded.

There are some limitations of the present study. First, results obtained in this work are not able to be generalized to all minimally invasive incisions, prosthesis types, computer-assisted alignment systems, etc. Second, the number of the patients is limited, and the follow-up is quite short. However, between the groups, there are some significant and important findings, which we want to report now. We are planning to follow these patients for many years, so that we will find out the long-term result of these patients.

Conclusion

In summary, this prospective, randomized study demonstrated that MICA-TKA can produce similarly accurate restoration of leg alignment and component orientation as well as reduced blood loss than C-TKA. Moreover, MICA-TKA produces superior clinical results to that of C-TKA. The MICA-TKA technique is more demanding than the C-TKA technique. That is why, so far, MICA-TKA is recommended only for the hands of the most experienced surgeons. We hope that this study will encourage other investigators to use MICA-TKA data in assessing functional outcomes. Larger studies, however, are required to further understand and assess the relationship between alignment through the functional arc of the knee and patient functional outcomes.