Using Accelerometer-Based Portable Navigation to Perform Accurate Total Knee Arthroplasty Bone Resection in Asian Patients

February 27, 2017- Hideki Ueyama, MD ; Yoshio Matsui, MD , PhD; Yukihide Minoda, MD , PhD; Masanori Matsuu ra, MD ;
Hiroaki Nakamura, MD , PhD

Total knee arthroplasty (TKA) is one of the most successful surgeries for patients with degenerative joint dis­ease, and many studies have shown good results from TKA.1-3 Implant alignment is an important factor for good clinical out­comes and survivorship.2 Bone resection in TKA has conventionally been performed with intra- and extra-medullary guides. However, severe femoral bowing can in­crease bone cut errors in TKA performed with conventional techniques.3 The ana­tomical features of the lower limb in Asian individuals have been investigated in pre­vious reports.3-6 These studies suggest that Asian individuals, including Japanese indi­viduals, have shorter and stronger femoral bowing than do other races.7


Computer-assisted navigation surgery (CAS) can help ensure more precise TKA implant alignment than the conventional method can.8-10 Even in cases of severe femoral deformities, navigation systems contribute to accurate bone resection.11 However, most CAS requires numerous large devices and is associated with high capital cost, increased operative time, and complex procedures.12 Portable navigation may be a solution for these problems due to its small size and easy handling (Figure 1).13 However, there have been no reports on the utility of this device for Asian pa­tients with strong femoral bowing. The aim of the current study was to evaluate the ac­curacy of this new device for bone resec­tion compared with that of the conventional method in Asian patients with strong femo­ral bowing.

Materials and Methods

This retrospective, comparative study included 142 knees from 102 Japanese patients who underwent TKA at the au­thors’ hospital between July 2013 and April 2015. Informed consent was ob­tained from all patients. The authors began performing TKA with a portable navigation system, including an acceler­ometer (KneeAlign2 system; OrthAlign Inc, Aliso Viejo, California), in Septem­ber 2014. Between September 2014 and April 2015, sixty-seven patients (8 men, 59 women) underwent TKA using the KneeAlign2 device (navigation group). Between July 2013 and August 2014, seventy-five patients (14 men, 61 women) underwent TKA using the conventional method (conventional group) (Table 1).  All surgeries were performed by 2 senior surgeons (Y.M., M.M.). An air tourniquet was inflated to 250 mm Hg during sur­gery. The medial parapatellar approach was used for all surgeries. The Vanguard posterior stabilized implant (Biomet Inc, Warsaw, Indiana) was used for all cases. Postoperative therapy was the same for all patients.

In the navigation group, the surgeon inserted a narrow screw into the center of the exposed distal femoral end on Whi­teside’s line and attached the femoral jig with the screw.14 The reference sensor and cutting block were then attached to the jig. The KneeAlign2 system included an accelerometer as a reference sensor (Fig­ure 1A). This small navigation device did not require drilling, as is required with the conventional method. The center of the hip was registered by maneuvering the knee with navigation, and the mechanical axis was detected (Figure 1B). After suc­cessful registration, the KneeAlign2 de­vice was able to adjust the resection angle digitally in the coronal (varus/valgus) and sagittal (flexion/extension) planes (Fig­ure 1C). The authors adjusted the cut­ting angle vertical to the mechanical axis as closely as possible to the ideal cutting angle.

The KneeAlign2 registered the tibial mechanical axis based on the insertion of the anterior cruciate ligament, anterior cortex of the distal tibia, and the lateral and medial malleoli. Once registration was successful, the surgeon adjusted the cutting angle for the tibia. The perpendic­ular cutting angle was adjusted to the me­chanical axis on the coronal plane (varus/valgus), and a 3° posterior slope on the sagittal plane was set as the ideal resec­tion angle.15,16

In the conventional group, the surgeon planned the angle for  distal femur and proximal tibia resection using plain radiog­raphy, with the ideal resection angle of the femur and tibia perpendicular to the me­chanical axis. The surgeon drilled a hole at the center of the distal femur to insert an intramedullary rod during the operation. Then, a distal femur cutting guide was set depending on the preoperative planning of the coronal plane (varus/valgus). The sag­ittal plane resection angle (flexion/exten­sion) was estimated to be perpendicular to the anatomical femoral axis. An extra­medullary cutting guide was used for the proximal tibia. The tibia was cut perpen­dicular to the tibial mechanical axis in the coronal plane (varus/valgus) and with a 3° posterior slope in the sagittal plane (flex­ion/extension). The rotational axis of the tibial component was determined to be a line connecting the medial one-third of the tibial tuberosity and the center of the poste­rior cruciate ligament insertion.17

Clinical information, including labora­tory data, was obtained from institutional patient records. Age, sex, diagnosis, and body mass index (BMI) were obtained. Intraoperative blood loss was minimal because of tourniquet use; therefore, the authors calculated blood loss during the perioperative period based on the patient’s hematocrit levels and estimated blood volume using the Gross formula.18 Blood loss was defined as estimated blood loss. The angle of knee flexion and the Knee Society knee and functional scores were estimated as functional outcomes.19 These data were measured preoperatively at the outpatient clinic. In addition, postopera­tive functional outcomes were also mea­sured at the final outpatient visit.

Radiographic evaluation was per­formed as follows. Standing lower-extremity radiographs were taken using long films preoperatively and at 2 weeks after TKA. The femoral mechanical axis was defined by a line drawn from the cen­ter of the femoral head to the most dis­tal point of the intercondylar notch of the femur, and the tibial mechanical axis was defined by a line drawn from the center of the tibial plateau to the center of the tibial plafond. The angle between the femoral and tibial mechanical axes was measured as the mechanical axis (MA) pre- and postoperatively (Figure 2A).20,21 In ad­dition, the authors estimated femoral and tibial bowing on coronal plain radiographs using Yau’s method.3 Bowing of greater than 3° was defined as marked femoral or tibial bowing (Figure 2B).3 The posi­tioning of each component was confirmed with postoperative frontal and lateral ra­diographs.12,22 The authors measured the angle between the mechanical axis and the tangent of the component on the long radiographs to use as the coronal femoral and tibial component alignment (Figures 3A-B).5,23 On lateral radiographs, the authors measured the angle between the femoral anatomical axis and the femoral distal cut surface as the sagittal femoral component alignment, and they measured the angle between the tibial shaft axis and the tibial proximal cut surface as the sag­ittal tibial component alignment (Figure 3C). Ideal bone resection was defined as 90º from the mechanical axis. However, for the tibia, a 3° posterior slope in the sagittal plane was defined as ideal. The ac­ceptable position of each component was within 3° of the ideal position, as in previ­ous reports.15,24,25 Outliers were defined as angles of less than 87° (varus) or greater than 93° (valgus) in the femoral and tibial coronal planes, less than -3° (extension) or greater than 3° (flexion) in the femoral sagittal plane, and less than 84° (back­ward tilt) or greater than 90° (forward tilt) in the tibial sagittal plane.

Continuous variables were analyzed with the Student’s t test, and discrete vari­ables were analyzed with Fisher’s exact test. To determine an adequate sample size, a power analysis using the hypoth­esis test with a power of 80% and a sig­nificance of .05 was performed. It showed that 36 knees were required per group to detect a difference of 1 point and 1.5 stan­dard deviations. These analyses were per­formed with R software (Vienna, Austria).


A total of 142 patients were included in the current study; 67 underwent TKA with the KneeAlign2 (navigation group) and 75 underwent TKA with the con­ventional method (conventional group). Patients’ mean±SD age at operation was 77.5±5.5 years (range, 53-88 years), with a mean BMI of 25.1±4.7 kg/m2 (range, 16.4-35.5 kg/m2). The mean±SD follow-up periods for the navigation and con­ventional groups were 10.8±1.8 months (range, 6.1-16.9 months) and 21.1±4.8 months (range, 5.9-30.5 months), respec­tively (P<.01). The follow-up periods were significantly different because the authors started using portable navigation only in September 2014. The preoperative mean±SD angle for knee flexion and the Knee Society knee and functional scores were 115.6°±14° (range, 80°-135°), 43.3±9.6 (range, 16-72), and 43.8±8.9 (range, 30-65), respectively. Preopera­tive MAs were 169.1°±4.5° (range, 157°-179°). The only significant between-groups difference was the length of the follow-up period (Table 1).

The mean±SD operation time overall was 114±12 minutes (range, 85-180 min­utes). Estimated blood loss after TKA was calculated to be 622±268 mL (range, 224-1670 mL). Postoperative Knee Society knee and functional scores were 75.7±8.3 (range, 57-95) and 74.7±6.0 (range, 60-85), respectively. There were no signifi­cant differences between the navigation and conventional groups in clinical out­comes (Table 2).

Regarding radiographic outcomes, the postoperative mean±SD MA was 178.9°±1.3° (range, 176°-182°). Regard­ing the coronal positioning of compo­nents, the mean±SD femoral coronal angle was 89.4°±1.6° (range, 84°-94°) and the mean±SD tibial coronal angle was 89.9°±1.4° (range, 87°-94°). Re­garding the sagittal positioning of com­ponents, the mean±SD femoral sagittal angle was 0.42°±2.6° (range, -6° to +7°) and the mean±SD tibial sagittal angle was 87.1°±2.0° (range, 81°-92°). The rate of outliers for the femoral coronal com­ponent was 1.5% (1 knee) in the naviga­tion group, significantly lower than that of 13.3% (10 knees) in the conventional group (P=.01). Other outliers were not significantly different between the 2 groups (Table 2).


Results were then analyzed by sub­groups based on the presence or absence of femoral bowing. The femoral coronal angle and the outlier of femoral coronal component were significantly different between the navigation and conventional groups in the marked femoral bowing sub­group (P<.05). The femoral coronal align­ments of the prosthesis were not deviated in the portable navigation groups (Figure 4).

There was no significant difference in the subgroup without marked femoral bowing (Table 3).


The current study showed more accu­rate implant alignment for TKA using an accelerometer-based portable navigation system than for TKA using the conven­tional method in Asian individuals with marked femoral bowing. This new device has been verified as useful for accurate prosthesis alignment in TKA.13,26,27 How­ever, there has been no evaluation of its utility in cases of anatomical variations, such as marked femoral bowing. This is the first report to show the utility of a portable navigation in Asian patients with marked femoral bowing.

Total knee arthroplasty is one of the most successful procedures for treat­ing degenerative knee joint disease, and many reports have already indicated its good long-term results.2,28 However, malpositioning of the prosthesis is one of the most serious complications, and it necessitates revision. Ritter et al28 re­ported that obtaining neutrality on the mechanical axis in TKA is important for component survival. Some have reported that setting the component within ±3° from the mechanical axis predicts good results, including function and quality of life.2,15 Femoral bowing influences the accuracy of bone resection using an in­tramedullary nail.6 One anatomical fea­ture that is stronger in Asian individuals than in Caucasian individuals is a strong femoral bow.3,4,7 Therefore, it is impor­tant to present the results of TKA for Asian patients using the portable naviga­tion system.

Computer-assisted navigation surgery helps in the planning of accurate bone re­section for ideal prosthesis setting.29 Some randomized controlled trials showed that TKA with CAS led to more accurate post­operative component placement than did conventional methods.29,30 It had been reported that CAS is useful for avoiding outliers in TKA for patients with strongly deformed legs.31 However, it was unclear whether a portable navigation system with an accelerometer was beneficial as well.

In the current study, the rate of outli­ers for the femoral coronal component in navigation group was 1.5%. This is simi­lar to results of TKA with the same por­table navigation system.13,26,27 It is impos­sible to compare the results with previous reports in patients with marked femoral bowing directly, but the results of this study showed a 0% rate of outliers for this group. Therefore, the results suggest that portable navigation systems are effective even in marked femoral deformity.

The TKA with CAS showed accu­rate prosthesis settings in previous stud­ies. The rates of outliers for the femo­ral coronal component were 3.1%32 to 0.83%.33 The previous reports showed good postoperative alignment after TKA using CAS for patients with severe bow­ing femur.31 Another report showed that there was no outlier of femoral coronal component alignment even in the severe femoral bowed cases.5 These results sug­gest that the common CAS and portable navigation should help surgeons to per­form TKA precisely in cases with strong curved limbs as well.

Previous reports presented outliers associated with prosthesis alignments in conventional TKA. Sparman et al33 re­ported that the rate of outliers for the fem­oral coronal component was 28.3% and that of the tibial coronal component was 10%. Importantly, many reports indicate that the outlier of implant alignment of navigated TKA is significantly less than non-navigated TKA in severe deformity cases.11,20 In marked femoral bowing cas­es, the risk of malalignment increases.24 In previous reports, approximately 30% of outliers were for the femoral compo­nent in bowed femurs that underwent the conventional bone resection technique.3 Even without marked femoral bowing, the rate of outliers for conventional TKA was 12.4% from the mechanical axis in a meta-analysis.34 The utilities of CAS and an accelerometer based portable naviga­tion system were certain.


Computer-assisted navigation surgery has some disadvantages, including the ad­ditional time needed to set up the devices, which prolongs the operation time.12,13 Portable navigation may be the solution for these problems due to its small size and easy handling. This device includes an ac­celerometer, and it does not require the use of other devices, such as a large monitor. The portable navigation system certainly is more convenient and easier to use than are most standard CASs.26 The current authors found that the total operation time was not significantly different between the 2 groups. In addition, there are no reports of pin tract fracture related to accelerometer-based portable navigation systems, as with the current study. Some previous reports have reported CAS pin tract fracture: 1.64% in the femur35 and 1.36% in the tibia.36

Total perioperative estimated blood loss associated with intramedullary rod use has been discussed in previous re­ports.37 In general, the amount of post­operative blood loss is larger with con­ventional TKA than with CAS TKA.37,38 Postoperative hemorrhage was larger for conventional TKA than for CAS,37 but the postoperative decrease in hemoglobin was smaller.38 However, this study did not show significant differences between the navigation group and the conventional group. The authors always injected 1 g of tranexamic acid into the joint intraop­eratively to reduce blood loss; therefore, drilling for the intramedullary guide did not cause significant blood loss periopera­tively.39

The limitation of the current study is that TKA was performed only by expert surgeons. The rates of outliers of tibial components in conventional TKA were lower than those in previous reports,32,33 which may have been a result of the sur­geons’ experience.40 Therefore, the actual differences between accelerometer-based portable navigation systems and conven­tional methods may be larger.


This study showed that accelerometer-based portable navigations improved fem­oral coronal alignment, even for Asian pa­tients, and did not increase the operation time and blood loss. Surgeons should con­sider using accelerometer-based portable navigation system, especially for patients with a marked femoral bow.


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