Keywords
total knee arthroplasty - postoperative knee stability - posterior-stabilized - medial
preserving gap technique - measured resection technique - soft tissue balance
Measured resection technique (MRT) and gap balancing technique are two common surgical
methods for total knee arthroplasty (TKA).[1]
[2] The gap balancing technique is more effective for achieving an equalized rectangular
gap, which is an equal soft tissue balance, at extension and flexion than MRT.[3] However, the risk of medial instability arises in many cases when aiming for the
perfect ligament balance at extension because of excessive medial release. The medial
instability after TKA causes postoperative knee pain[4] and nonphysiologic movement of the knee.[5]
To overcome the concerns of the gap balancing technique, a novel medial preserving
gap technique (MPGT) has been developed and recently performed. MPGT focuses on medial
knee stability, essentially making not equal rectangular gaps but equal trapezoidal
gaps in extension and flexion.[6] It also provides quantitative gap balancing technique using tensor measurements
independent of surgeons' feels and experiences. Posterior-stabilized (PS) TKA with
MPGT provides more consistent intraoperative soft tissue balance during knee flexion
than MRT.[7] However, postoperative knee stability especially in medial compartment, which was
reported as an important factor associated with postoperative ambulation function
and patient satisfaction,[8] has not been investigated in the knees after PS-TKA with MPGT.
The present study aimed to determine whether MPGT could provide the higher postoperative
medial knee stability, which was essential for clinical outcome, compared with MRT
as long as 3 years after PS-TKA. It was hypothesized that postoperative medial knee
stability both at extension and flexion with MPGT was better than that with MRT after
PS-TKA.
Materials and Methods
Patients
This retrospective study included 130 patients with varus knee osteoarthritis (OA)
who met the criteria and underwent primary PS-TKA (NexGen LPS Flex; Zimmer, Warsaw,
IN) between 2008 and 2012. Varus knee OA was defined as hip-knee-ankle (HKA) ≥ 2 degrees
in varus from full-limb radiograph.[9] Inclusion criteria were substantial pain, loss of function due to knee OA, and ineffectiveness
of conservative treatment including rehabilitation, medication, and intra-articular
injection of hyaluronic acid or steroids. Exclusion criteria were valgus knee deformity,
severe bony defects a requiring bone graft or augmentation, previous knee surgery,
revision TKA, and active knee joint infection. Patients undergoing cruciate-retaining
TKA were also excluded; thus, 135 patients were excluded. PS-TKA using MPGT was performed
in 65 patients (MPGT group) between 2010 and 2012, whereas the control group comprised
65 patients who underwent PS-TKA using MRT (MRT group) between 2008 and 2010. The
mean HKAs of the MPGT and MRT groups were 13.1 degrees in varus (range: 2.4–24.5 degrees)
and 11.4 degrees in varus (range: 4.2–25.6 degrees), respectively, and there was no
significant difference between the two groups. Other patient demographic data are
shown in [Table 1]. Each surgery was performed by the same surgeon, the senior author (H.M.). The hospital
ethics committee approved the study protocol, and the patients provided written informed
consent for their participation.
Table 1
Patient demographic data
|
Medial preserving gap technique
|
Measured resection technique
|
p-Value
|
|
Cases
|
65
|
65
|
|
|
Age at surgery (y)
|
74.3 ± 5.7
|
73.4 ± 6.4
|
n.s.
|
|
Sex (male/female)
|
9 / 56
|
7 / 58
|
n.s.
|
|
Weight (kg)
|
58.5 ± 10.8
|
57.8 ± 8.7
|
n.s.
|
|
BMI (kg/m2)
|
25.6 ± 3.9
|
25.4 ± 3.6
|
n.s.
|
|
Active flexion angle (degree)
Active extension angle (degree)
|
107.4 ± 17.2
−7.8 ± 9.0
|
107.5 ± 20.3
−5.7 ± 10.9
|
n.s.
n.s.
|
Abbreviations: BMI, body mass index; n.s., not significant.
Data are shown as mean ± standard deviation.
Offset Repo-Tensor
The Offset Repo-Tensor (OFR tensor) (Zimmer, Warsaw, IN) consists of three parts:
an upper seesaw plate, a lower platform plate with a spike, and an extraarticular
main body, as previously described.[10]
[11]
[12]
[13] This device facilitates the measurement of joint ligament balance and joint center
gap, both before and after femoral trial component placement, while applying a constant
joint distraction force. The PS type seesaw plate has a post at the proximal center
to fit the intercondylar space and the cam of the femoral trial component in PS-TKA.
This post-cam mechanism controls the tibiofemoral position in both the coronal and
sagittal planes, reproducing the joint constraint and alignment after the components
are implanted. Joint distraction forces ranging from 20 lbs (9.1 kg) to 60 lbs (27.2 kg)
can be exerted between the seesaw and platform plates using a specially made torque
driver that can change the maximum torque value within an error for joint distraction
within ± 3%. This torque driver is placed on a rack that contains a rack-and-pinion
mechanism along the extraarticular main body. The appropriate torque is applied to
generate the required distraction force, and two scales are evaluated according to
the tensor measurement: the angle (degree, positive value in varus ligament balance)
between the seesaw and the platform plates, and the distance (mm, joint center gap)
between the center midpoints of the upper seesaw plate and the proximal tibial cut.
By measuring these angular deviations and distances under a constant joint distraction
force, both the angle and joint center gap could be measured. The smallest scales
of these measurements are 1 degree and 1 mm, respectively.
Surgical Procedures
MPGT
After inflating the tourniquet, the surgeries were performed with a medial parapatellar
approach. First, both the anterior cruciate ligament and posterior cruciate ligament
were resected. Then, a distal femoral osteotomy was performed perpendicular to the
mechanical axis of the femur in the coronal plane using an intramedullary resection
guide, according to preoperative long-leg radiographs. A proximal tibial osteotomy
was then performed with a 10-mm bone resection from lateral tibial plateau and perpendicular
to the mechanical axis in the coronal plane and with 7 degrees of posterior inclination
according to the manufacture's instruction along the sagittal plane using an extramedullary
guide. No bony defects were observed along the eroded medial tibial plateau. The medial
soft tissue release was limited by femoral and tibial osteophyte removal and deep
medial collateral ligament (MCL) release within 1cm from medial tibial plateau until
a 10-mm spacer block could be inserted. Lateral physiological laxity was allowed to
avoid excessive medial releases. Neutral alignment was confirmed with each cut of
the distal femur and proximal tibia by ensuring that the alignment rod was connected
to a spacer block located at the center of the femoral head and ankle joint.
The OFR tensor was placed with the lower platform plate located at the center of the
proximal tibia. Thereafter, knee gap was measured between the osteotomized femoral
and tibial surfaces at extension, and between posterior femoral condyles and the osteotomized
tibial surface at flexion. A distraction force of 40 lbs was loaded, and the varus
ligament balance and joint center gap were measured both at extension and flexion.
The joint distraction force was loaded several times until the joint center gap remained
constant to reduce the error that could result from creep elongation of the surrounding
soft tissue. We confirmed that the bone was not compressed or deformed during distraction
force application. During each measurement, the thigh was held and the knee was aligned
along the sagittal plane to eliminate the external load on the knee both at extension
and flexion.
The main feature of MPGT consists of planning the posterior femoral condylar osteotomy.
According to a previous report, the differences in varus ligament balance between
extension and flexion were consistent across different joint distraction forces.[13] These findings suggested that the differences in varus ligament valance between
extension and flexion could be an index for the external rotation angle of the posterior
femoral condyle osteotomy. Thus, the femoral external rotation angle was determined
based on the differences in varus ligament balance between extension and flexion ([Fig. 1A]). The difference in the joint center gap between extension and flexion was used
to determine both the femoral component size and the corresponding amount of posterior
femoral condyle resection. The original femoral anteroposterior size was measured
using the anterior reference technique beforehand, while the gap difference between
extension and flexion was compared with the distal thickness of the femoral component;
thereafter, a femoral component of appropriate size was selected to reduce the gap
difference between extension and flexion ([Fig. 1B]). After each bony resection, the tensor and femoral trial components were fixed
and the patellofemoral joint was reduced by temporarily suturing the medial parapatellar
arthrotomy. The final soft tissue balance, including the joint center gap and varus
ligament balance, was then measured with the knee from extension to flexion. The thickness
of the polyethylene insert was determined based on the joint center gap with a femoral
trial component in place at extension, with 40 lbs of joint distraction forces.[6] Finally, a polyethylene insert was implanted, as well as a NexGen component with
cement.
Fig. 1 Determination method of femoral external rotation angle and thickness of posterior
femoral condylar resection. (a) The external rotation angle of the posterior femoral condyle is determined within
the range of 0 to 7 degrees with reference to the difference in varus ligament balance
between extension and flexion. This angle is independent from joint distraction forces.
(e.g., varus ligament balances at extension and flexion were 3 degrees (=α) and 7
degrees (=β), respectively. External rotation angle = 7–3 degrees = 4 degrees). (b) The thickness of posterior femoral condylar resection is defined as the difference
in joint center gap between extension and flexion. The original femoral anteroposterior
(AP) size is measured using the anterior reference technique beforehand, while the
gap difference between extension and flexion (A–B) is compared with the distal thickness of the femoral component (C); thereafter, a femoral component of appropriate size is selected. Index for determination
of femoral component size was described as follows; A–B < C: Femoral component with
larger AP size should be selected compared with original femoral AP size. (e.g., 23–16mm < 9mm:
Femoral component with larger AP size than original femoral AP size by 2mm should
be selected.) A–B = C: Femoral component with the original femoral AP size should
be selected. A–B > C: Femoral component with smaller AP size should be selected.
MRT
Distal femoral and proximal tibial osteotomies were performed in the same manner as
previously mentioned in MPGT. No bony defects were noted along the eroded medial tibial
plateau in any of the cases. At this point, the osteophytes were removed. Following
confirmation of neutral alignment of the knee with the spacer block and alignment
rod, a posterior femoral cut was made using the anterior referencing technique. Femoral
external rotation was usually set at 3 or 5 degrees relative to the posterior condylar
axis,[14] with reference to the Whiteside's line[15]
[16] and the epicondylar axis,[17] as well as the condylar twist angle, measured using the preoperative computed tomography
images.[18] Thereafter, we appropriately released the soft tissues (deep layer MCL, medial,
and posteromedial capsule) along the medial knee structures. The soft tissue balance
was finally adjusted in coronal plane by additional soft tissue release (e.g., superficial
layer of MCL) to obtain equal extension flexion gap. The thickness of the polyethylene
insert was determined based on the joint center gap with a femoral trial component
in place at extension, with 40 lbs of joint distraction forces.[6] Subsequently, a NexGen prosthesis was implanted using cement, as well as a polyethylene
insert.
Postoperative Radiographic Assessment of Components
Postoperative anteroposterior radiographs of the lower limb were obtained for evaluation
of the coronal femoral component angle (FCA) and tibial component angle (TCA). Lateral
radiographs of the lower limb were obtained for the evaluation of the sagittal TCA.
FCA was defined as the angle between the femoral mechanical axis and the joint surface
line of the femoral implant. TCA was defined as the angle between the tibial mechanical
axis and the base plate of the tibial implant.
Postoperative Knee Stability Measurements
Postoperative knee stabilities at extension and flexion were assessed using stress
radiographies at 1 month, 6 months, 1 year, and 3 years after TKA. Postoperative knee
stability at extension was assessed using varus–valgus stress X-ray with a Telos SE
arthrometer (FaTelos; Medizinisch-Technische, Griesheim, Germany) following a previously
reported method.[19] Valgus or varus force of 10 kg was applied immediately above the joint on the lateral
or medial femoral condyle, whereas the proximal thigh and middle leg were held by
the counter support at 10 degrees of flexion. Joint separation distance (mm) between
the lowest point of femoral prosthesis and the line in contact with the lower surface
of tibial prosthesis at medial and lateral compartments was measured. The distance
was adjusted using magnification based on the keel width of the tibial prosthesis.
The distance between the lowest point of femoral prosthesis and the upper surface
of the polyethylene insert was calculated and defined as medial joint opening (MJO)
and lateral joint opening (LJO). The joint opening (mm) was calculated as follows:
joint opening = joint separation distance – insert thickness. On varus–valgus stress
radiographies, the angles between the line in contact with the medial and lateral
lowest points of femoral prosthesis and the line in contact with the lower surface
of the tibial prosthesis were measured. These varus and valgus angles indicated the
degree of lateral and medial laxities, respectively. “Varus ligament balance at extension”
was defined as “varus angle at extension – valgus angle at extension,” which means
lateral laxity (positive value in varus), following a previously reported method[20] ([Fig. 2A]).
Fig. 2 Postoperative knee stability measurements using stress radiographies. (A) Extension knee stability at lateral and medial sides was assessed using varus–valgus
stress radiographies. a, lateral joint opening; b, lateral joint separation distance;
c, insert thickness; d, medial joint opening; e, medial joint separation distance;
f, varus angle at extension; g, valgus angle at extension; varus ligament balance
at extension = (f) – (g). (B) Flexion knee stability was assessed using stress epicondylar view radiograph. H,
lateral joint opening; i, lateral joint separation distance; j, insert thickness;
k, medial joint opening; l, medial joint separation distance; m, varus angle at flexion = varus
ligament balance at flexion.
Postoperative knee stability at flexion was assessed using the stress epicondylar
view with 1.5 kg weight at the ankle,[21]
[22] which enabled to visualize the posterior condylar axis and the tibia articular line.
MJO and LJO were measured according to the same method at extension. The angle between
the line in contact with the medial and lateral lowest points of the femoral prosthesis
and the line in contact with the lower surface of the tibial prosthesis was measured;
varus angle was defined as “varus ligament balance at flexion” (positive value in
varus) ([Fig. 2B]). The intrarater interclass correlation coefficient (ICC) for the MJOs and LJOs
at extension was 0.931 (95% confidence interval [CI]: 0.824–0.973) and 0.952 (95%
CI: 0.879–0.981), respectively. The intrarater ICC for the MJOs and LJOs at flexion
was 0.947 (95% CI: 0.866–0.979) and 0.944 (95% CI: 0.857–0.978), respectively. These
results indicate excellent reliability of the measurements.
Statistical Analysis
All values are presented as mean ± standard error. Data analyses were performed using
IBM SPSS statistics software (version 21; IBM Corp., Armonk, NY). The Shapiro–Wilk
test was used to analyze normally distributed data using IBM SPSS statistics software.
The unpaired t-test was performed to evaluate the differences in demographic data, femoral external
rotation angle, and postoperative active range of motion (ROM) measured by lateral
knee radiograph between the two techniques. Coronal FCA, TCA, and sagittal TCA were
also evaluated using unpaired t-test between the two groups. The differences between MJO and LJO in each group were
compared using a paired t-test. Each MJO and LJO was compared between the two techniques using one-way analysis
of variance. Spearman's rank correlation analysis was conducted to assess the correlation
between varus ligament balance at extension and flexion. A statistical priori power
analysis was performed to determine the sample size based on the difference in MJO
at 3 years postoperatively between the two techniques. In this analysis, G*Power software
(version 3.1.9.2; Heinrich Heine Universität Düsseldorf, DE) was used with a prespecified
significance level of α < 0.05, a power level of 95%, and an effect size based on
the results of the pilot study with 15 cases (effect size d = 0.63). The estimated sample size was 55 patients. The post hoc power analysis for
MJO at 3 years postoperatively further confirmed that the power was 0.95. A p-value of <0.05 was set as the level of significance.
Results
Femoral External Rotation Angle and Postoperative ROM
A significant difference was found in the femoral external rotation angle between
MPGT (4.2 ± 0.2 degrees) and MRT (3.6 ± 0.1 degrees, p < 0.01). The mean active ROMs in the MPGT group were −2.5 ± 0.5–101.7 ± 1.6 degrees
at 1 month, −1.8 ± 0.6–109.5 ± 1.7 degrees at 6 months, −0.2 ± 0.7–111.2 ± 1.5 degrees
at 1 year, and 0.5 ± 0.6–112.0 ± 1.7 degrees at 3 years postoperatively. The mean
active ROMs in the MRT group were −1.9 ± 0.7–104.4 ± 1.8 degrees at 1 month, −0.7 ± 1.0–111.7 ± 1.9
degrees at 6 months, 0.8 ± 0.8–111.5 ± 1.8 degrees at 1 year, and 0.6 ± 0.9–112.9 ± 1.9
degrees at 3 years postoperatively. No significant difference was found in the postoperative
active ROMs between the two techniques at any time point.
Postoperative Radiographic Assessment of Components
Postoperative coronal FCA, TCA, and sagittal TCA in the MPGT group were 7.0 ± 0.2
degrees in valgus, 1.3 ± 0.2 degrees in varus, and 6.7 ± 0.2 degrees with posterior
inclination. Postoperative coronal FCA, TCA, and sagittal TCA in the MRT group were
6.6 ± 0.5 degrees in valgus, 1.2 ± 0.5 degrees in varus, and 6.9 ± 0.2 degrees with
posterior inclination. No significant difference was found in the postoperative coronal
FCA, TCA, and sagittal TCA between the two techniques.
Medial and Lateral Joint Opening
A smaller joint opening indicates a higher postoperative stability; the mean MJOs
and LJOs at extension and flexion using each technique are listed in [Tables 2] and [3]. MJOs both at extension and flexion using each technique were significantly smaller
than the LJOs at all time points ([Table 2]). MJOs and LJOs at extension in the MPGT group were significantly smaller than those
in the MRT group, suggesting that medial and lateral postoperative knee stability
at extension was greater following MPGT, at 1 and 3 years postoperatively ([Table 3], [Fig. 3]). Furthermore, MJOs at flexion in the MPGT group were significantly smaller than
those in the MRT group, suggesting that medial postoperative knee stability at flexion
was greater following MPGT, at 6 months, 1 year, and 3 years postoperatively ([Table 3], [Fig. 4]). No significant difference in LJO at flexion was found between the two surgical
techniques.
Fig. 3 Comparison of medial and lateral joint opening at extension between the two surgical
techniques. Medial joint opening (MJO) and lateral joint opening (LJO) at extension
in the medial preserving gap technique (MPGT) group were significantly smaller than
those in the measured resection technique (MRT) group at 1 and 3 years postoperatively.†
p < 0.05 MPGT group versus MRT group in LJO. *p < 0.05 MPGT group versus MRT group in MJO.
Fig. 4 Comparison of medial and lateral joint opening at flexion between the two surgical
techniques. Medial joint opening (MJO) at flexion in the medial preserving gap technique
(MPGT) group was significantly smaller than those in the measured resection technique
(MRT) group at 6 months, 1 year, and 3 years postoperatively. No significant difference
in lateral joint opening (LJO) at flexion was found between the two surgical techniques.
*p < 0.05 MPGT group versus MRT group in MJO.
Table 2
MJO and LJO at extension and flexion in each surgical technique
|
At extension
|
|
At flexion
|
|
|
MPGT group
|
MJO
|
LJO
|
p-Value
|
MJO
|
LJO
|
p-Value
|
|
1 mo
|
2.94 ± 0.16
|
3.58 ± 0.22
|
0.01[a]
|
0.77 ± 0.10
|
1.85 ± 0.22
|
<0.01[a]
|
|
6 mo
|
2.63 ± 0.14
|
3.52 ± 0.22
|
<0.01[a]
|
0.88 ± 0.09
|
2.14 ± 0.22
|
<0.01[a]
|
|
1 y
|
2.68 ± 0.17
|
3.72 ± 0.20
|
<0.01[a]
|
0.98 ± 0.09
|
2.16 ± 0.22
|
<0.01[a]
|
|
3 y
|
2.76 ± 0.18
|
3.64 ± 0.18
|
<0.01[a]
|
0.69 ± 0.07
|
2.04 ± 0.27
|
<0.01[a]
|
|
MRT group
|
MJO
|
LJO
|
p
-Value
|
MJO
|
LJO
|
p
-Value
|
|
1 mo
|
3.17 ± 0.17
|
4.21 ± 0.23
|
<0.01[a]
|
1.09 ± 0.13
|
2.17 ± 0.25
|
<0.01[a]
|
|
6 mo
|
3.05 ± 0.19
|
4.02 ± 0.23
|
<0.01[a]
|
1.38 ± 0.16
|
2.67 ± 0.27
|
<0.01[a]
|
|
1 y
|
3.23 ± 0.21
|
4.38 ± 0.24
|
<0.01[a]
|
1.42 ± 0.16
|
2.72 ± 0.30
|
<0.01[a]
|
|
3 y
|
3.40 ± 0.24
|
4.45 ± 0.26
|
<0.01[a]
|
1.15 ± 0.16
|
2.26 ± 0.37
|
<0.01[a]
|
Abbreviations: LJO, lateral joint opening; MJO, medial joint opening; MPGT, medial
preserving gap technique; MRT, measured resection technique.
a Statistically significant MJO versus LJO.
Table 3
Comparison of MJO and LJO at extension and flexion between two surgical techniques
|
At extension
|
|
At flexion
|
|
|
MJO
|
MPGT group
|
MRT group
|
p-Value
|
MPGT group
|
MRT group
|
p-Value
|
|
1 mo
|
2.94 ± 0.16
|
3.17 ± 0.17
|
0.31
|
0.77 ± 0.10
|
1.09 ± 0.13
|
0.06
|
|
6 mo
|
2.63 ± 0.14
|
3.05 ± 0.19
|
0.08
|
0.88 ± 0.09
|
1.38 ± 0.16
|
<0.01[a]
|
|
1 y
|
2.68 ± 0.17
|
3.23 ± 0.21
|
0.03[a]
|
0.98 ± 0.09
|
1.42 ± 0.16
|
0.02[a]
|
|
3 y
|
2.76 ± 0.18
|
3.40 ± 0.24
|
0.03[a]
|
0.69 ± 0.07
|
1.15 ± 0.16
|
<0.01[a]
|
|
LJO
|
MPGT group
|
MRT group
|
p
-Value
|
MPGT group
|
MRT group
|
p
-Value
|
|
1 mo
|
3.58 ± 0.22
|
4.21 ± 0.23
|
0.05
|
1.85 ± 0.22
|
2.17 ± 0.25
|
0.34
|
|
6 mo
|
3.52 ± 0.22
|
4.02 ± 0.23
|
0.13
|
2.14 ± 0.22
|
2.67 ± 0.27
|
0.14
|
|
1 y
|
3.72 ± 0.20
|
4.38 ± 0.24
|
0.04[a]
|
2.16 ± 0.22
|
2.72 ± 0.30
|
0.13
|
|
3 y
|
3.64 ± 0.18
|
4.45 ± 0.26
|
0.01[a]
|
2.04 ± 0.27
|
2.26 ± 0.37
|
0.63
|
Abbreviations: LJO, lateral joint opening; MJO, medial joint opening; MPGT, medial
preserving gap technique; MRT, measured resection technique.
a Statistically significant MPGT versus MRT.
Varus Ligament Balance
The mean varus ligament balances at extension and flexion are listed in [Table 4]. No significant correlation was found between varus ligament balance at extension
and flexion in the MRT group at any time point; however, the varus ligament balance
at extension was significantly correlated to the varus ligament balance at flexion
in the MPGT group at all time points. This suggests that compared with the MRT group,
the MPGT group was able to achieve the equal trapezoid gaps at extension and flexion.
Table 4
Correlation with varus ligament balance at extension and flexion in each surgical
technique
|
Medial preserving gap technique group
|
Measured resection technique group
|
|
1 mo
|
6 mo
|
1 y
|
3 y
|
1 mo
|
6 mo
|
1 y
|
3 y
|
|
Varus angle at extension (degree)
|
3.12 ± 0.07
|
3.09 ± 0.07
|
3.27 ± 0.06
|
3.19 ± 0.06
|
3.62 ± 0.07
|
3.43 ± 0.07
|
3.76 ± 0.08
|
3.86 ± 0.08
|
|
Valgus angle at extension (degree)
|
2.58 ± 0.05
|
2.32 ± 0.05
|
2.36 ± 0.05
|
2.44 ± 0.06
|
2.73 ± 0.05
|
2.61 ± 0.06
|
2.78 ± 0.07
|
2.95 ± 0.07
|
|
Varus ligament balance at extension (degree)
|
0.53 ± 0.21
|
0.78 ± 0.18
|
0.86 ± 0.18
|
0.69 ± 0.15
|
0.79 ± 0.23
|
0.73 ± 0.18
|
0.87 ± 0.18
|
0.79 ± 0.15
|
|
Varus ligament balance at flexion (degree)
|
0.97 ± 0.20
|
1.11 ± 0.19
|
1.04 ± 0.18
|
1.12 ± 0.20
|
0.78 ± 0.14
|
0.89 ± 0.14
|
0.91 ± 0.17
|
0.53 ± 0.11
|
|
Coefficient of correlation
|
0.33
|
0.49
|
0.37
|
0.30
|
-0.03
|
0.15
|
0.02
|
0.10
|
|
p-Value
|
<0.01[a]
|
<0.01[a]
|
<0.01[a]
|
<0.03[a]
|
0.84
|
0.27
|
0.91
|
0.42
|
a Correlations are statistically significant.
Discussion
The most important finding of the present study was that compared with MRT, MPGT was
better at preserving postoperative medial knee stability both at extension and flexion
after PS-TKA, supporting our hypothesis. According to a recent development, MPGT focuses
on medial stability and essentially involves making unequal rectangular gaps using
the modified gap technique, but equal trapezoidal gaps at extension and flexion.[6] Nagai et al reported that the varus ligament balance and joint center gaps, both
at extension and flexion, increased with increased joint distraction force; however,
the differences in varus ligament balance and joint center gap at extension and flexion
were consistent among different joint distraction forces from 20 to 60 lbs.[13] This novel index was considered useful to determine the femoral rotational angle
and resection thickness of posterior femoral condyles in the modified gap technique;
therefore, a novel surgical technique for varus type knee OA based on the modified
gap technique and tensor measurement was devised, namely MPGT. The main concepts of
MPGT are as follows: first, the procedure preserves medial knee stability both at
extension and flexion but allows for lateral physiological laxity. This means that
equal trapezoidal osteotomy gaps, both at extension and flexion, are aimed with varus
ligament balance between the cutting surfaces of the femur and tibia. Second, this
procedure provides a safe and quantitative surgical technique via the use of a tensor
device and is therefore independent of the surgeon's experience. The femoral rotational
angle and resection thickness of the posterior femoral condyles are based on the differences
in the varus ligament balance and joint center gap between extension and flexion before
posterior femoral condylar resections ([Fig. 1]).
The rotational alignment of the femoral component can be determined in various ways;
in the gap-balancing technique, the surgeon performs ligamentous releases to balance
the knee at extension after the femoral and tibial cuts are performed. This is followed
by resection of the posterior femoral condyle parallel to the prepared cut tibial
surface by applying equal loads to the medial and lateral compartments.[1]
[23] In MRT, a surface-derived reference axis of the femur is used as a guide to determine
the position of the femoral component in the axial plane[2]; several different reference axes derived from the bony landmarks have been introduced,
of which the posterior condylar axis,[14] Whiteside's line,[16] and epicondylar axis[17] are the most popular. Luyckx et al reported a tendency toward more femoral external
rotation in the gap-balancing technique than in MRT.[24] Lee et al also reported that the gap-balancing technique provided a more accurate
and precise method for obtaining an adequate FCA than MRT during postoperative evaluations
of the FCA.[25] Additionally, the FCA obtained using MRT was significantly more internally rotated
than that when using the gap-balancing technique.[25] In this study, a small but significant difference was found in the femoral external
rotation angle between the MPGT (4.2 ± 0.2degrees) and MRT (3.6 ± 0.1 degrees) groups.
This result supports the concept of MPGT, which is based on the modified gap technique;
femoral rotational angle is based on the differences in the varus ligament balance
between extension and flexion in MPGT.
Previous studies reported that medial and lateral ligamentous laxities were not balanced
in normal knees; lateral ligamentous laxity was observed compared with medial ligamentous
laxity.[22]
[26] Medial stability with relative lateral laxity was also reported to be associated
with good functional outcomes after well-aligned TKA.[27] Furthermore, several reports indicated that medial instability after TKA may cause
postoperative knee pain,[4] more abnormal kinematics including anteroposterior instability,[5]
[28] and poor functional outcomes.[27] MPGT aims to preserve the medial stability of the knee both at extension and flexion
while allowing for lateral physiological laxity. In this study, MJOs in the MPGT group,
both at extension and flexion, were significantly smaller than the LJOs at all time
points, supporting the aim of MPGT.
MJOs in the MPGT group, both at extension and flexion, were significantly smaller
than those in the MRT group at 1 and 3 years postoperatively, suggesting that medial
postoperative knee stability both at extension and flexion was greater than that when
using MPGT. The medial soft tissue release was different between MPGT and MRT in this
study. Using MPGT, the medial soft tissue release was conservatively performed via
femoral and tibial osteophyte removal and deep MCL release, within 1cm from the medial
tibial plateau until a 10-mm spacer block could be inserted, allowing physiological
lateral laxity. On the other hand, using MRT, medial soft tissue balance was performed
via femoral and tibial osteophyte removal, as well as deep layer MCL, medial, and
posteromedial capsule release along the medial knee structures. Furthermore, additional
soft tissue release (e.g., superficial layer of MCL) was performed to adjust the soft
tissue balance in the coronal plane while evaluating balance using a 10-mm spacer
block. Yagishita et al reported that release of the medial and posteromedial capsule
increases both medial and lateral joint gaps in extension, and particularly, in flexion.[29] Mullaji et al also reported that although deep MCL release has a negligible effect,
superficial MCL release increases the medial joint gap in extension, and particularly,
in flexion.[30] In this study, although there was a chronological change of soft tissue tension,
medial soft tissue release during MPGT was more limited than that during MRT, as described
above; as a result, MJOs in the MPGT group at extension—and especially at flexion—were
significantly smaller than those in the MRT group at 1 and 3 years postoperatively,
which also supports the concept of MPGT.
The modified gap technique is advocated to obtain equal mediolateral soft-tissue balance
and rectangular parallel gaps both in extension and flexion.[31] However, the risk of medial instability arises in many cases when aiming for the
perfect parallel ligament balance of the knee at extension. MPGT focuses on medial
stability, aiming for equal trapezoidal gaps in extension and flexion and allowing
for lateral physiological laxity. In this study, no significant correlation was found
between varus ligament balance at extension and that at flexion in the MRT group at
any time points. However, the varus ligament balance at extension was significantly
correlated to the varus ligament balance at flexion in the MPGT group at all time
points. This was because equal trapezoidal gaps in extension and flexion were accomplished
in MPGT according to the preserving medial stability while allowing for lateral physiological
laxity.
The present study has some limitations. First, the study population was almost composed
of women and limited to patients with varus type OA and excluded those with severe
osteoarthritic knees with large bony defects. Conducting research in a group with
equal proportions of each sex and valgus and severe osteoarthritic knees and comparing
with the populations in this series are necessary for future investigations. Second,
an implant with dual radius curvature was used in this study, which might have influenced
the results. Different prosthetic types should be examined in the future. Third, this
was a retrospective study. Furthermore, no clinical instruments were reported and
the relationships between postoperative knee stabilities and functional outcomes were
not evaluated. In future studies, patient-reported subjective outcomes, functional
outcomes, radiographic parameters, and complication rates should be investigated.
Forth, femoral component alignment in the sagittal and axial planes and tibial component
alignment in the axial plane were not investigated in this study. Finally, the two
techniques were not randomized and were performed during different periods. However,
each surgery was performed by an experienced single surgeon, and the number of subjects
was larger based on the power analysis; hence, the bias would be minimal.
Conclusions
This study investigated the differences in postoperative knee stability after PS-TKA
with MPGT and MRT. MPGT achieved better postoperative medial knee stabilities both
at extension and flexion than MRT even at 3 years after PS-TKA. MPGT is a more feasible
method for preserving postoperative medial knee stability than MRT after PS-TKA, which
would help avoid postoperative knee pain, abnormal kinematics including anteroposterior
instability, and poor functional outcome; caused by postoperative medial instability
after TKA.
