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Optimal intraoperative medial joint gap in extension to prevent flexion contracture following total knee arthroplasty using modified gap balancing technique
Primary aim to identify the ideal medial joint gap in extension needed to prevent post-operative flexion contracture following total knee arthroplasty (TKA) in varus osteoarthritic knees by the modified gap balancing technique. A secondary aim was to analyze multiple factors that influence knee extension in TKA by modified gap balancing.
Methods
This is a prospective cohort study of 150 patients diagnosed with osteoarthritic varus knee who underwent TKA using the modified gap balancing technique. Operative and clinical records were assessed to determine factors including age, BMI (Body mass index), pre-operative extension angle and the medial extension laxity. Patients were followed for 6-months post-operatively and reviewed for knee extension angle.
Results
Six months final follow-up information was available for 148 patients with an average age of 75.5 years. Pre-operative knee extension and BMI were significantly associated with post-operative knee extension. Post-operative flexion contracture of 100 was not seen in any of 34 patients with a medial extension laxity 0 mm who had no pre-existing flexion contracture, and in 1/9 (11.1%) patients with a medial extension laxity 1 mm who had pre-existing flexion contracture.
Conclusions
Pre-operative extension angle and BMI significantly influence the post-operative knee extension angle in TKA using the modified gap balancing technique. A medial extension laxity of at least 1 mm is ideally needed to prevent post-operative flexion contracture in patients with a pre-existing contracture 100.
The ability to extend the knee after total knee arthroplasty (TKA) is an important determinant of patient satisfaction. Post-operative knee stiffness was identified as one of the common causes of readmission and revision.
Quadriceps of the ipsilateral side are exerted more in order to generate sufficient force to stabilize the flexed knee during weight-bearing. As a consequence, this increases the contralateral adductors and extensor moments, leading to altered gait biomechanics and resulting in anterior knee pain
. Hence, achieving full extension of the knee post TKA is one of the primary goals of the arthroplasty surgeon.
The modified Gap balancing (GB) and Measured resection (MR) methods both aim to mechanically align the knee. The major difference between GB and MR techniques is the flexion gap creation, with both having similar steps for extension gap creation.
Comparison of gap balancing vs measured resection technique in patients undergoing simultaneous bilateral total knee arthroplasty : one technique per knee.
Several studies have evaluated the factors affecting post-operative extension in TKA performed by the MR technique, and it could be argued the results also apply to the GB technique.
Sugama et al. and Minoda et al. reported that posterior femoral condyle bone resection can also alter terminal extension, hence we believe the preceding statement does not hold true.
The tightness of the posterior capsule of knee joint increases with increasing size of the posterior femoral condyle offset and thus influencing the extension gap.
Therefore, there is still a lack of knowledge regarding the factors that affect post-operative extension in TKA by the GB technique.
Many potential factors could affect post-operative extension following TKA by the GB technique in varus knees. Of these, medial extension gap laxity (MEL) is thought to be important since it is the only factor that is subject to the surgeon's control during surgery. The aim of this study was therefore to determine the ideal MEL in extension by the GB technique in order to avoid post-operative flexion contracture.
2. Material and methods
2.1 Patients
One hundred and fifty patients with osteoarthritic varus deformity knee were included in the study who underwent primary TKA at our institute between August 2015 and May 2017. All patients suffered pain because of osteoarthritis that affected their activities of daily living. None had a history of knee injury or of any lower extremity surgery prior to TKA. Excluded from the study were patients with fixed flexion deformity 300, valgus knee deformity, severe bony defects needing bone graft and augmentation, active knee joint infection, or those undergoing revision. Post-operatively, all patients were assessed for the beta-angle of Knee Society Score in an Antero-posterior radiograph. Two cases were excluded from the study due to beta-angle over 3° of varus/valgus osteotomy. The final study group at 6-months follow-up was therefore comprised of 125 females and 23 males with a mean age of 75.5 years (range 59–87 years). The average BMI of all patients was 25.59 ± 3.58, as shown in Table 1.
Table 1Demographic characteristics of the study group.
All patients were preoperatively evaluated with X-rays of standard views (anteroposterior and lateral), Rosenberg view, long leg standing X-rays and computed tomography (CT) of the knee. The Zedknee system (LEXI, Tokyo, Japan) was used to perform CT-based 3D pre-operative planning. The hospital ethics committee approved the study protocol and patients provided informed consent for their participation in the study.
2.2 Surgical procedure
A single experienced surgeon performed posterior stabilized TKA with Persona knee prosthesis (Zimmer, Warsaw, USA), the GB technique and using Fuzion™. The device is used to measure the center joint gap and the tilting angle between the seesaw plate and the platform plate by applying a constant joint distraction force with patella in the reduced position. The device has 3 parts: an upper seesaw plate, a lower platform plate, and an extra-articular body that shows the two measurements i.e., centre gap and tilting angle (Fig. 1). After inflating an air tourniquet attached to the proximal thigh at the start of the procedure, arthrotomy was performed using the conventional medial parapatellar approach. Anterior and posterior cruciate ligaments, osteophytes of medial femur and tibia were resected with minimal medial soft tissue release. Distal femur osteotomy was performed based on pre-operative templating perpendicular to the mechanical axis using an intramedullary guide. Tibial osteotomy was performed perpendicular to the mechanical axis in the coronal plane with a 50 posterior inclination in the sagittal plane using an extramedullary jig. None of the cases had large bony defect comprising more than 20% of the resected tibial plateau. Akagi's line determined the rotational axis of the tibia and was marked before tibial osteotomy as a reference
The center gap and tilting angle between the osteotomized surfaces of tibia and femur were measured with a distraction force of 40 lbs (176.4 N) using the Fuzion™ system with the knee in a fully extended position. Similarly, measurements were made between the osteotomized surface of the tibia and posterior femoral condyles with the knee in a 900 flexed position and the patella reduced.
The amount of bone resection in the posterior femoral condyles was calculated to create the shape of flexion gap to be the same as the extension gap. This was done using a formula based on the difference between the joint center gap and the tilting angle in full extension and 900 flexion (Fig. 2).
Fig. 2(A) Diagrammatic illustration of the tilting angle in Extension(X0), Flexion(Y0) between the osteotomized surfaces and the difference(Y-X0) determined as the femoral external rotation angle. (B) Illustration for determining the amount of resection of posterior femoral condyle (A-Bmm), the difference of central gap in Extension (A mm), Flexion (B mm) to create an equal gap.
The line of resection of the posterior femoral condyles was drawn on the cut surface of the femur. A posterior referencing sizer adjusted to the desired femoral external rotation was placed on the femoral osteotomized surface. Two drill holes were made for the 4 in 1 femoral cutting guide for femoral bone cut. The appropriate femoral cutting guide in which the cutting slit for the posterior femoral condyle was closest to the line drawn was used on the femoral osteotomized surface (Fig. 3).
Fig. 3(A) Posterior femoral condyle to be resected and marked intra-operatively. (B) Femoral cutting block matching the desired amount of resection.
2.3 Gap measurement and medial extension gap laxity (MEL)
Following bony resection of the posterior femoral, chamfer and post-cam bony resection, the femoral trial component was positioned. Fuzion™ with post-cam mechanism was used to measure the center gap and the tilting angle between the femoral trial component and the cut surface of the proximal tibia at 00 and 900 of flexion with a measured distraction force of 40 lbs.
The medial compartment gap was calculated using the formula described in Fig. 4. An appropriately sized Persona prosthesis was implanted with cement. The polyethylene insert was selected based on the calculated medial compartment gap.
The medial compartment gap (MCG) = Center gap–Bearing space length × sin (tilting angle)/2, while the lateral compartment gap (LCG) = Center gap + Bearing space length × sin (tilting angle)/2.
Fig. 4Diagrammatic representation of the mathematical equation for calculation of the medial and lateral gap.
MEL was calculated in order to quantify the intra-operative soft tissue tension. This can be defined as the distance between the tibial component including the polyethylene liner from the medial compartment gap (Fig. 5).
Fig. 5Diagrammatic illustration of Medial gap laxity (MEL) and Lateral gap laxity (LEL).
The passive knee range of motion (ROM) in the supine position was assessed with a 30 cm standard goniometer by a single investigator. This was done pre-operatively, immediately after surgery under general anesthesia, and post-operatively at 6-months follow-up.
2.5 Flexion contracture
In this study, the definition of flexion contracture is an extension angle of 10°.
This allows the separation of patients into two groups: those with or without flexion contracture. Thus, patients were divided into Group FC [−] (Flexion Contracture absent) and Group FC [+] (Flexion Contracture present).
2.6 Statistical analysis
Multiple linear regression analysis was performed to investigate the multiple risk factors associated with post-operative extension at 6-months. The variables analyzed were age, BMI, pre-operative extension angle, medial extension gap laxity (MEL). Differences in incidence between groups were tested using Fischer's test for two samples for variance. All values are expressed as mean ± standard deviation (SD). The level of significance was deemed as p-value 0.05. Statistical analysis was performed using SPSS software version 22.0 (IBM Software group, Chicago, IL, USA).
3. Results
The mean extension angle of the knee in full extension was −8.8 ± 6.3°preoperatively, −0.39 ± 2.1 just after surgery, and −3.5 ± 5.0 at 6 months after surgery. There was a significant improvement in the extension angle from before surgery to 6 months after surgery (p < 0.05, Student's t-test). The mean MEL for all 148 patients was −0.31 ± 0.98. Average operation time was 97 ± 12 min, and the tourniquet time was usually in the range of 60–75 min.
To evaluate the factors affecting postoperative flexion contracture, multiple regression analysis was performed on the following parameters: age, BMI, MEL, pre-operative knee extension angle as explanatory variables, and post-operative knee extension angle as a target variable. Correlations between the independent parameters and post-operative knee extension angle are shown in Table 2. Preoperative knee extension angle was positively correlated with the post-operative knee extension angle (R = 0.29, p-value < 0.0001). In contrast, BMI was negatively correlated with post-operative knee extension (R = −0.25, p-value <0.05).
Table 2Multivariate analysis of risk factors associated with post-operative knee extension after total knee arthroplasty.
Regression coefficient
f- value
p- value
95% lower limit
95% upper limit
Age
0.06
1.3
0.22
−0.04
0.18
BMI
−0.25
5.4
0.022
−0.46
−0.03
MEL
0.60
2.4
0.12
−0.17
1.3
Pre-op extension ROM
0.29
23.1
0.000
0.17
0.41
∗BMI, Body mass index; MEL, Medial extension gap laxity; ROM, Range of movement.
Pre-operatively, Group FC [−] consisted of 76 patients and Group FC [+] of 72 patients. Demographic details of patients in Group FC [−] and Group FC [+] are shown in Table 3.
At 6-months follow-up the number of patients with flexion contracture of the knee decreased to 22. Amongst these patients, 18 (81.8%) had flexion contracture pre-operatively. The incidence of postoperative flexion contracture of the knee was significantly higher in Group FC [+] than in Group FC [−] (Fischer exact value = 0.005).
The relationship between MEL and the knee extension angle at 6-months postoperatively was investigated for each group. Scatter diagrams are helpful for correlating MEL with the postoperative knee extension angle in Group FC [−] (Fig. 6) and Group FC [+] (Fig. 7). For determining the ideal MEL to avoid postoperative flexion contracture, when the cut-off value was set at MEL = 0 mm, none of the patients in Group FC [−] with MEL ≧ 0 mm had postoperative flexion contracture (Fig. 6).
Fig. 6Group FC [−]: Scatter diagram illustrating the relation between Medial extension gap laxity (MEL) and knee extension post-operatively at 6-months.
Fig. 7Group FC [+]: Scatter diagram illustrating the relation between Medial extension gap laxity (MEL) and knee extension post-operatively at 6-months.
When the same cut-off value was set in Group FC [+], 5 of 72 patients (6.94%) had postoperative flexion contracture of the knee (Fig. 7). If the cut-off value was raised to MEL≧1 mm (dotted line in Fig. 7), the incidence of patients with postoperative flexion contracture in this group decreased to one in 72 (1.38%).
4. Discussion
Previous studies have reported on various factors that affect postoperative knee flexion contracture. In most of these, preoperative knee flexion contracture was found to be the most influential factor.
This was confirmed in the present study, where preoperative knee flexion contracture had the strongest association with postoperative flexion contracture amongst the variables studied. Therefore, it is reasonable to consider the presence or absence of preoperative knee flexion contracture when determining the optimal MEL required to prevent postoperative knee flexion contracture. In the absence of preoperative knee flexion contracture and when the cut-off value was set to MEL ≥0 mm, the incidence of knee flexion contracture 6 months postoperatively was 0% (0/78). Therefore, we conclude the optimal MEL should be ≥ 0 mm in order to prevent postoperative knee flexion contracture in this group of patients. On the other hand, in patients with preoperative flexion contracture, the incidence of postoperative knee flexion contracture 6 months postoperatively was 21% (15/72). If the cutoff value was MEL ≥0 mm, postoperative knee flexion contracture was still found in 5 cases (7%). If the cut-off value was raised to MEL ≥1 mm, the incidence of flexion contracture 6 months after surgery was reduced to only 1 case (1.2%). These results highlight the importance of optimal MEL for preventing postoperative knee flexion contracture in patients with pre-existing contracture.
Since the medial extension gap is the tightest gap in the knee, increasing the gap in extension by 1 mm would also consequently increase the flexion gap. Winemaker et al. and Yagishita et al. noted that a gap difference of > 3 mm is unacceptable between extension and at 90° flexion, thus we believe our gap difference is well within the acceptable limits.
Comparison of soft tissue balancing, femoral component rotation, and joint line change between the gap balancing and measured resection techniques in primary total knee arthroplasty: a meta-analysis.
Therefore, we believe that 1 mm looseness or laxity would have negligible impact on the functioning of the knee in patients operated by the modified gap balancing technique as opposed to the measured resection technique.
Another important finding of the present study was that BMI significantly affected postoperative flexion contracture using GB in the 148 patients analyzed. Some earlier studies reported no effect of BMI on postoperative knee flexion contracture, whereas others did report a significant effect.
We propose that a significant correlation indicates the relationship of BMI to the physiological state and laxity of soft tissue around the knee which influences the component gap, and more so in modified gap balancing technique TKA.
In the present study, factors that are easily measurable, reproducible and applicable in a practical clinical setting were evaluated and analyzed. Okamoto et al. and Nagai et al. measured the post-operative knee extension angle using lateral radiographs, in contrast to the present study in which knee extension was measured with a standard goniometer.
Intra-and intertester reliability and criterion validity of the parallelogram and universal goniometers for measuring maximum active knee flexion and extension of patients with knee restrictions.
Although there is reported variability between goniometric and radiographic evaluation, this is more commonly with the flexion angle being underestimated by goniometry.
The extension angle measured by lateral X-ray may not correspond to the commonly measured clinical extension angle of the knee. The standard of ≧5° defined as flexion contracture by Okamoto et al. on a lateral radiograph may be equal to or more than the standard flexion contracture used in clinical assessment.
In fact, Okamoto et al. reported that nearly half of the preoperative knee extension angles were either neutral or hyper-extended, with the average extension angle being −1.2° flexion contracture.
In the present study, the average extension angle of the knee was −8.8° before surgery and −3.5° at 6 months after surgery, consistent with values encountered in normal clinical practice. We defined knee flexion contracture as ≧10°. This has been employed as the standard in previous studies using the Goniometer.
There are several limitations to the present study. We considered only varus deformity osteoarthritic knees with flexion contracture < 300, hence the results cannot be generalized to valgus knees and to patients with flexion contracture deformity >30°. The study was performed with Fuzion™ and a 40lbs distraction force, but the results may vary with different tensor systems which is an important limitation of our study. Another limitation is that the cement mantle thickness was not accounted for as the gap measurements were taken prior to cementing.
5. Conclusions
The present study is the first report of optimal MEL for preventing flexion contracture of the knee following surgery for PS TKA performed using the modified gap balancing technique. A minimum of 1 mm MEL is ideal for the prevention of post-operative flexion contracture in patients with a pre-existing contracture ≧100. This optimal MEL has different reference values, depending on the presence or absence of knee flexion contracture prior to surgery. Finally, pre-operative extension angle and BMI both significantly influence the post-operative knee extension angle in TKA using the modified gap balancing technique.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Ethical approval
This study was approved by the Hospital ethics committee.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References
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Restrepo C.
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Unplanned readmission after total joint arthroplasty: rates, reasons, and risk factors.
Comparison of gap balancing vs measured resection technique in patients undergoing simultaneous bilateral total knee arthroplasty : one technique per knee.
Comparison of soft tissue balancing, femoral component rotation, and joint line change between the gap balancing and measured resection techniques in primary total knee arthroplasty: a meta-analysis.
Intra-and intertester reliability and criterion validity of the parallelogram and universal goniometers for measuring maximum active knee flexion and extension of patients with knee restrictions.
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