Effect of Medial Closing Wedge Distal Femoral Osteotomy on Tibial Tuberosity–Trochlear Groove Distance in Patients With Genu Valgum According to Diagnosis (Lateral Osteoarthritis or Patellar Dislocation)

Abstract

Background:

Medial closing wedge distal femoral osteotomy (CWDFO) is used to correct valgus deformities and manage lateral osteoarthritis (OA) and patellar dislocation (PD). However, its effect on the tibial tuberosity–trochlear groove (TT-TG) distance remains debated, with limited studies evaluating contributing factors.

Purpose:

To determine whether TT-TG distance changes after CWDFO differ by the underlying diagnosis prompting the procedure and to identify preoperative factors influencing the extent of TT-TG distance changes.

Study Design:

Case series; Level of evidence, 4.

Methods:

This study analyzed 43 knees of 34 patients with valgus deformity who underwent CWDFO, categorized into the lateral OA (n = 14) and PD (n = 29) groups. Radiographic parameters, including the TT-TG distance, were assessed preoperatively and at least 1 year postoperatively. Regression analyses were conducted to identify the factors influencing TT-TG distance changes. A predictive model with 95% confidence intervals determined the thresholds for deviations from the expected values.

Results:

The TT-TG distance significantly decreased from 14.7 ± 5.1 to 11.5 ± 4.7 mm after CWDFO (P < .001). On average, the TT-TG distance decreased by 21.8% after CWDFO. Changes in the TT-TG distance did not significantly differ by diagnosis (lateral OA: −3.0 ± 2.3 mm; PD: −3.3 ± 4.5 mm; P = .771). The preoperative TT-TG distance and osteotomy type significantly influenced TT-TG distance changes. A higher preoperative TT-TG distance was correlated with a greater reduction (odds ratio, −0.375; P = .001), and biplanar osteotomy showed a more pronounced effect than uniplanar osteotomy (odds ratio, −3.083; P = .013). Patients with preoperative TT-TG distances >23.5 mm demonstrated greater variability in TT-TG distance changes in the predictive model.

Conclusion:

CWDFO effectively reduced the TT-TG distance regardless of the diagnosis. Preoperative TT-TG distance and osteotomy type were key determinants of this reduction. In cases with preoperative TT-TG distances >23.5 mm, TT-TG distance changes demonstrated greater variability, suggesting that radiographic outcomes may deviate from expected values. These findings highlight the need for careful postoperative assessments and consideration of additional procedures in select cases.

Keywords

tibial tuberosity–trochlear groove medial closing wedge distal femoral osteotomy lateral osteoarthritis patellar dislocation

Medial closing wedge distal femoral osteotomy (CWDFO) is a well-established surgical technique designed to correct valgus deformities, particularly in young patients, by addressing biomechanical and anatomic abnormalities in the knee joint.^8,19,21 In patients with lateral osteoarthritis (OA), CWDFO alleviates pain and slows OA progression by reducing excessive loading on the lateral compartment.^{8,13,19,20,34} Conversely, in those with patellar dislocation (PD), CWDFO mitigates the lateralizing force vector on the patella, improving stability and reducing the risk of recurrent dislocation.^{6,10,12,16,17,21,23} These distinct therapeutic objectives highlight the versatility of CWDFO in managing different abnormalities related to knee alignment and patellofemoral mechanics.^8,19,21

Despite its widespread application, the effect of CWDFO on the tibial tuberosity–trochlear groove (TT-TG) distance remains debated.^6,12,16,23 The TT-TG distance is a critical parameter in evaluating patellar tracking and stability, with a reported mean range of 10 to 13 mm.²⁹ A TT-TG distance ≥15 mm is associated with a higher risk of patellar instability, whereas values ≥20 mm indicate significant lateral displacement of the tibial tuberosity, often necessitating corrective tibial tubercle osteotomy.^7,12,26,28 Although studies have suggested that CWDFO decreases the TT-TG distance by altering alignment and reducing lateral patellar forces,^12,23 others have found no significant changes in the TT-TG distance after the procedure.¹⁶ This inconsistency highlights the need for further investigation to elucidate the effect of CWDFO on the TT-TG distance.

Although CWDFO is generally performed to achieve slight varus alignment postoperatively, the degree of correction does not significantly differ based on whether the indication is lateral OA or PD.^23,30 However, patients with PD have been reported to present significantly greater preoperative TT-TG distances than controls.^4,31 TT-TG distance changes are influenced by a biomechanical leverage mechanism in which varus correction induces a rotational effect on the TT-TG distance.³³ Nevertheless, it remains unclear whether the extent of TT-TG distance reduction varies based on the underlying diagnosis. No studies have examined whether the underlying diagnosis prompting CWDFO affects these changes.

The purpose of this study was to evaluate whether the TT-TG distance changes after CWDFO and, if so, whether the extent of these changes differs according to the underlying diagnosis prompting the procedure. We also aimed to identify preoperative factors influencing the degree of TT-TG distance changes after CWDFO. We hypothesized that the TT-TG distance would decrease after CWDFO and that the underlying diagnosis would influence the extent of this reduction. Additionally, we hypothesized that the degree of TT-TG distance changes after CWDFO could be predicted based on preoperative factors.

Methods

Study Design and Population

This retrospective study evaluated patients with valgus deformities who underwent CWDFO between August 2012 and December 2023 at 2 institutions, regardless of whether they had lateral OA or PD. Of the initial 70 knees, 27 knees were excluded for the following reasons: patients without preoperative or postoperative computed tomography (CT) scans (n = 24) and those who underwent additional procedures during CWDFO that could influence the TT-TG distance, including high tibial osteotomy and tibial tubercle osteotomy (n = 3). Among the excluded cases, 19 knees had lateral OA, and 8 knees had PD. Notably, most exclusions for missing preoperative CT scans originated from one institution, where CT was not consistently performed during the early study period (2012-2014). Finally, 43 knees (34 patients) were included (Figure 1). The mean age was 30.2 ± 15.5 years, the mean body mass index was 26.0 ± 5.1 kg/m², and the mean follow-up period was 26.3 ± 6.7 months. The included patients were categorized into 2 groups according to their diagnosis: lateral OA (n = 14) and PD (n = 29). Patients in the PD group were significantly younger (21.1 ± 5.6 vs 49.0 ± 12.0 years, respectively; P < .001), had a higher body mass index (27.3 ± 5.2 vs 23.3 ± 3.7 kg/m², respectively; P = .014), and had a longer tibial length (356.9 ± 31.9 vs 336.7 ± 22.4 mm, respectively; P = .039) than those in the lateral OA group (Table 1). Our institutional review board approved this study before the retrospective review of the data.

Table 1

Baseline Characteristics of Patients by Underlying Diagnosis ^a

	Lateral OA (n = 14)	PD (n = 29)	P Value
Age, y	49.0 ± 12.0	21.1 ± 5.6	<.001
Female sex	11 (78.6)	14 (48.3)	.099
Height, cm	157.7 ± 10.6	164.3 ± 10.7	.062
Weight, kg	58.4 ± 13.5	74.6 ± 19.1	.007
Body mass index, kg/m²	23.3 ± 3.7	27.3 ± 5.2	.014
Right laterality	4 (28.6)	14 (48.3)	.325
Length, mm
Femur	427.6 ± 24.1	457.8 ± 104.9	.296
Tibia	336.7 ± 22.4	356.9 ± 31.9	.039
Width, mm
Femur	80.0 ± 9.3	78.3 ± 8.1	.541
Tibia	76.1 ± 6.8	72.9 ± 7.3	.168
Closing gap, mm	7.4 ± 2.8	6.2 ± 1.5	.078
Biplanar osteotomy	5 (35.7)	5 (17.2)	.179
Lateral hinge fracture	5 (35.7)	14 (48.3)	.437
Type I	4 (80.0)	13 (92.9)
Type II	00 (0.0)	00 (0.0)
Type III	1 (20.0)	1 (7.1)
Follow-up, mo	27.4 ± 5.7	25.8 ± 7.1	.468

Data are presented as mean ± SD or n (%). Bolded P values indicate statistical significance (P < .05) OA, osteoarthritis; PD, patellar dislocation.

Radiographic Evaluation

All patients underwent standing knee radiography in the anteroposterior and lateral views as well as lower extremity radiography preoperatively and at least 1 year after surgery. Additionally, CT scans were evaluated both preoperatively and postoperatively to find changes in the TT-TG distance and the osteotomy site. A total of 12 radiographic parameters were assessed independently before and after surgery by 2 orthopaedic surgeons, including the hip-knee-ankle (HKA) angle, weightbearing line ratio, mechanical lateral distal femoral angle, length and width of the femur and tibia, TT-TG distance, and presence of lateral hinge fractures. These parameters were selected to evaluate the potential factors influencing TT-TG distance changes after CWDFO. Previous biomechanical models have suggested that TT-TG distance changes occur because of a biomechanical leverage mechanism associated with varus correction.³³ To systematically assess this effect, we included parameters reflecting the degree of, and anatomic factors related to, rotational mechanics.

The HKA angle was defined as the angle between the mechanical axis of the femur and tibia and was expressed as a positive value for valgus alignment, indicated by an HKA angle <0° (Figure 2A).^2,14,22,25 The weightbearing line ratio was determined by drawing a line from the center of the femoral head to the center of the superior surface of the talus (Figure 2B).¹⁸ The intersection of this line with the tibial plateau was expressed as a percentage of the length of the tibial plateau (medial border: 0%; lateral border: 100%). The mechanical lateral distal femoral angle was defined as the angle between the femoral mechanical axis and a line connecting the distal femoral condyles (Figure 2A).^15,32 The lengths and widths of the femur and tibia were evaluated on standing anteroposterior radiographs of the knee (Figure 2C). The femoral length was defined as the distance between 2 specific points: (1) the midpoint of a line connecting the most distal points of the medial and lateral femoral condyles and (2) the most superior point of the femoral head, measured along a line parallel to the distal condylar line (Figure 2C). The tibial length was defined as the distance from the midpoint of the tibial plateau to the center of the superior surface of the talus (Figure 2C). The femoral width was measured as the distance between the medial and lateral epicondyles, and the tibial width was defined as the distance between the medial and lateral edges of the tibial plateau (Figure 2C).³³ The TT-TG distance was measured on axial CT as the distance between the deepest point of the trochlear groove and the center and most prominent point of the tibial tubercle (Figure 2D).^3,5 Lateral hinge fractures were also assessed using postoperative CT and categorized into 3 types according to the classification system proposed by Fujita et al.^11,24 The majority of the lateral hinge fractures identified were classified as type I, with only 2 cases of type III fractures (Table 1). When a lateral hinge fracture was detected, weightbearing was restricted for 6 weeks, followed by a gradual progression starting with partial weightbearing. In all cases, stability was confirmed during follow-up without the need for additional surgical interventions. The reliability of the measurements was assessed using the intraclass correlation coefficient, with all values >0.9.

Surgical Technique

CWDFO was performed by 2 senior surgeons at 2 institutions. Uniplanar osteotomy was performed until 2016 (n = 33), whereas biplanar osteotomy was performed after 2017 (n = 10). The degree of correction was determined preoperatively using standing lower extremity radiographs, with the alignment target set at 45% of the medial edge of the proximal tibial plateau.²³ The patient was positioned supine on the operating table, and a diagnostic arthroscopic examination was conducted to evaluate intra-articular abnormalities.^20,23 A 10-cm longitudinal incision was made along the anteromedial side of the distal thigh. After an incision of the muscle fascia, the vastus medialis muscle was stripped from the intermuscular septum and elevated using a Hohmann retractor. To protect neurovascular structures, a blunt Hohmann retractor was positioned on the posterior side of the distal femur. Osteotomy was performed using an oscillating saw. For transverse osteotomy, 2 K-wires were inserted at the planned osteotomy site, with 2 additional K-wires at a calculated distance based on the preoperative correction plan. The hinge point was established just proximal to the lateral epicondyle. In biplanar osteotomy, ascending osteotomy of approximately 3 to 5 cm in length was performed at an angle of 110° to the transverse osteotomy site. The medial bone wedge was removed, and the osteotomy site was carefully compressed under consistent pressure to prevent torsion. After lower extremity alignment was confirmed using fluoroscopy, the osteotomy site was stabilized with 1 of 2 locking plates (TomoFix MDF [DePuy Synthes] or OhtoFix plate [OhtoMedical]).²³ No cases involved intentional derotational correction, and no transverse osteotomy was performed to correct femoral anteversion. For patients with PD, if patellar maltracking persisted after the osteotomy site was fixed, medial reefing was additionally performed.²³ Patients were encouraged to perform range of motion exercises starting on postoperative day 2 or 3, with partial weightbearing ambulation allowed for 4 weeks after surgery.

Statistical Analysis

To assess the primary outcome, an independent-samples t test was used to compare TT-TG distance changes between the 2 groups. Additionally, a paired t test was performed within each group to evaluate preoperative to postoperative TT-TG distance changes. The chi-square test was conducted to compare categorical variables between the groups. Univariate analysis was conducted using preoperative measurements to identify factors associated with changes in the TT-TG distance. Subsequently, significant variables identified in univariate analysis were analyzed using multivariate analysis to assess their independent effect. If multiple variables were found to be significant, interaction terms were incorporated to evaluate whether the effect of one variable was influenced by another, and their statistical significance was assessed accordingly. A predictive model was developed using preoperative variables to estimate TT-TG distance changes, with predicted values calculated alongside their 95% confidence intervals (CIs). These CIs represent the range within which the true parameter value is expected to lie with a specified level of confidence, providing insight into the precision and reliability of the model’s predictions.¹ To determine the thresholds beyond which preoperative variables no longer reliably predicted TT-TG distance changes, we assessed points at which CI widths demonstrated a significant increase, reflecting reduced predictive precision. Based on a previous study, the threshold was defined as the preoperative value corresponding to the top 5% of the CI width distribution.³⁵ The statistical power for analyzing the primary outcome of our study, which involved comparing the TT-TG distance before and after CWDFO, was 0.99, with a type I error set at 0.05. Power analysis was conducted using G*Power (Version 3.1.9.7). All statistical analyses were performed using SPSS (Version 20; IBM), and statistical significance was defined as a P value <.05.

Results

Although the TT-TG distance decreased after CWDFO, the extent of this reduction did not differ by diagnosis. Across all patients, the mean TT-TG distance decreased from 14.7 ± 5.1 mm preoperatively to 11.5 ± 4.7 mm postoperatively (P < .001) (Table 2). When analyzed by diagnosis, both the lateral OA and PD groups demonstrated statistically significant reductions in the TT-TG distance from preoperatively to postoperatively (lateral OA group: from 11.3 ± 1.9 to 8.3 ± 2.9 mm [P < .001]; PD group: from 16.4 ± 5.3 to 13.1 ± 4.7 mm [P = .001]) (Table 2). Although the PD group showed higher preoperative and postoperative TT-TG distance values than the lateral OA group, the change in the TT-TG distance was not significantly different between the 2 groups. The mean change in the TT-TG distance was −3.0 ± 2.3 mm in the lateral OA group and −3.3 ± 4.5 mm in the PD group (P = .771) (Table 3). However, the PD group demonstrated a greater standard deviation in the TT-TG distance preoperatively and postoperatively (Table 3 and Figure 3).

Table 2

TT-TG Distance ^a

	Total	Lateral OA	PD
Preoperative, mm	14.7 ± 5.1	11.3 ± 1.9	16.4 ± 5.3
Postoperative, mm	11.5 ± 4.7	8.3 ± 2.9	13.1 ± 4.7
Difference, mm	−3.2 ± 3.9	−3.0 ± 2.3	−3.3 ± 4.5
P value	<.001	<.001	.001

Data are presented as mean ± SD. Bolded P values indicate statistical significance (P < .05). OA, osteoarthritis; PD, patellar dislocation; TT-TG, tibial tuberosity–trochlear groove.

Table 3

Radiographic Measurements ^a

	Lateral OA (n = 14)	PD (n = 29)	P Value
Preoperative
HKA angle, deg	7.3 ± 4.8	4.8 ± 2.5	.086
WBL ratio, %	77.8 ± 18.4	69.0 ± 11.0	.135
mLDFA, deg	83.0 ± 2.4	82.7 ± 3.1	.709
TT-TG distance, mm	11.3 ± 1.9	16.4 ± 5.3	<.001
Postoperative
HKA angle, deg	−2.0 ± 2.8	−2.5 ± 3.2	.657
WBL ratio, %	38.2 ± 13.7	36.1 ± 13.7	.648
mLDFA, deg	90.4 ± 3.1	90.1 ± 2.9	.271
TT-TG distance, mm	8.3 ± 2.9	13.1 ± 4.7	<.001
Difference
HKA angle, deg	−9.3 ± 5.2	−7.3 ± 2.9	.101
WBL ratio, %	−39.2 ± 20.0	−32.9 ± 14.1	.241
mLDFA, deg	7.4 ± 3.2	7.5 ± 3.1	.954
TT-TG distance, mm	−3.0 ± 2.3	−3.3 ± 4.5	.771

Data are presented as mean ± SD. Bolded P values indicate statistical significance (P < .05). HKA, hip-knee-ankle; mLDFA, mechanical lateral distal femoral angle; OA, osteoarthritis; PD, patellar dislocation; TT-TG, tibial tuberosity–trochlear groove; WBL, weightbearing line.

The preoperative TT-TG distance and type of osteotomy were identified as significant factors affecting changes in the TT-TG distance after CWDFO. A greater preoperative TT-TG distance was associated with a larger reduction in the TT-TG distance. However, the variability of these changes also increased as the preoperative TT-TG distance increased. No significant association was found between the underlying diagnosis and TT-TG distance changes (odds ratio [OR], −0.308 [95% CI, −2.920 to 2.304]; P = .813) (Table 4). Both preoperative TT-TG distance and type of osteotomy were inversely associated with TT-TG distance changes. The OR for preoperative TT-TG distance was −0.375 (95% CI, −0.575 to −0.174]; P = .001), and that for osteotomy type was −3.083 (95% CI, −5.475 to −0.692]; P = .013) (Table 4). No statistically significant interaction was found between the preoperative TT-TG distance and type of osteotomy (P = .313) (Figure 4A), indicating that these factors independently influenced TT-TG distance changes. Consequently, the predictive model for TT-TG distance changes after CWDFO was developed based solely on preoperative TT-TG distance values (Table 4 and Figure 4B). The model showed that as the preoperative TT-TG distance increased, the CI width also increased, reflecting greater variability in TT-TG distance changes (Figure 4B). The preoperative TT-TG distance threshold at which CI widths fell within the top 5% of the distribution was determined to be 23.5 mm (Figure 5).

Table 4

Factors Associated With Changes in TT-TG Distance Postoperatively ^a

	Univariate		Multivariate		VIF
	β Coefficient (95% CI)	P Value	β Coefficient (95% CI)	P Value	VIF
Age	0.029 (−0.051 to 0.108)	.473
Sex	−0.280 (−2.761 to 2.201)	.821
Body mass index	−0.029 (−0.272 to 0.215)	.814
Laterality	0.081 (−2.401 to 2.564)	.948
Preoperative diagnosis	−0.308 (−2.920 to 2.304)	.813
Type of osteotomy	−2.931 (−5.679 to −0.184)	.037	−3.083 (−5.475 to −0.692)	.013	1.001
Closing gap	−0.074 (−0.671 to 0.523)	.804
Lateral hinge fracture	−0.935 (−3.384 to 1.513)	.445
Femoral length	0.008 (−0.006 to 0.022)	.239
Tibial length	0.025 (−0.015 to 0.064)	.223
Femoral width	−0.020 (−0.166 to 0.126)	.779
Tibial width	−0.018 (−0.189 to 0.153)	.831
Preoperative HKA angle	−0.059 (−0.405 to 0.286)	.730
Preoperative WBL ratio	−0.028 (−0.115 to 0.059)	.526
Preoperative mLDFA	0.104 (−0.331 to 0.539)	.632
Preoperative TT-TG distance	−0.366 (−0.580 to −0.152)	.001	−0.375 (−0.575 to −0.174)	.001	1.001
Difference in HKA angle ^b	0.072 (−0.247 to 0.390)	.652
Difference in WBL ratio ^b	0.000 (−0.076 to 0.076)	.996
Difference in mLDFA ^b	0.039 (−0.360 to 0.437)	.846

The interaction between the preoperative TT-TG distance and osteotomy type was not statistically significant (P = .313). The change in the TT-TG distance can be predicted using the following equation: 6.112 + (−3.083 * type of osteotomy) + (−0.375 * preoperative TT-TG distance) (P < .001). This indicates that when biplanar osteotomy was performed, the change in the TT-TG distance decreased by 3.083 mm. For each 1 mm increase in preoperative TT-TG distance, the change in the TT-TG distance decreased by 0.375 mm. Bolded P values indicate statistical significance (P < .05). HKA, hip-knee-ankle; mLDFA, mechanical lateral distal femoral angle; TT-TG, tibial tuberosity–trochlear groove; VIF, variance inflation factor; WBL, weightbearing line.

Difference between postoperative and preoperative values.

Discussion

The principal finding of our study was that the TT-TG distance significantly decreased after CWDFO, regardless of the underlying diagnosis. However, the extent of this reduction was influenced by the preoperative TT-TG distance and the type of osteotomy. Furthermore, a predictive model for TT-TG distance changes after CWDFO was introduced based on preoperative variables. However, the predictive accuracy diminished when the preoperative TT-TG distance exceeded 23.5 mm, as indicated by broader CIs, resulting in less reliable outcomes in such cases.

Our results confirmed the hypothesis that the TT-TG distance decreased after CWDFO; however, the extent of reduction did not significantly differ between diagnostic groups. Considerable debate remains regarding changes in the TT-TG distance during CWDFO.^9,16,23 One study analyzed 25 patients with PD and valgus alignment who underwent CWDFO using a uniplanar osteotomy technique combined with medial patellofemoral ligament reconstruction.¹⁶ They reported no significant change in the TT-TG distance, with values changing from 17.2 ± 3.8 to 17.5 ± 3.2 mm (P = .739). This lack of change was suggested to result from the proximal location of the osteotomy site relative to the femoral trochlea. However, another study investigated 23 patients with PD and valgus alignment who underwent CWDFO using both uniplanar and biplanar osteotomy techniques.²³ Although their discussion did not provide a clear explanation for this reduction, they reported a significant reduction in the TT-TG distance from 20.4 ± 2.8 to 13.5 ± 4.7 mm (P < .001). Biomechanical studies utilizing 3-dimensional modeling have offered further insights into the mechanisms underlying these changes.^12,33 In one study, a lateral opening wedge DFO model was created using preoperative CT of 6 patients with valgus deformities, revealing a mean TT-TG distance reduction of 1.7 mm for every 2° of correction.¹² Another study evaluated medial CWDFO using CT from 14 patients with valgus deformities and 24 with neutral alignment, reporting a linear decrease of 1.05 mm per 1° of correction.³³ Although the magnitude of reduction differed between the 2 approaches, both studies consistently demonstrated that DFO reduced the TT-TG distance. In our study, the mean TT-TG distance decreased from 14.7 ± 5.1 to 11.5 ± 4.7 mm after CWDFO (P < .001), aligning with previous findings.^12,23,33 This reduction appeared to be driven by the biomechanical leverage effect (so-called pendulum effect) that has been suggested in biomechanical studies.^12,33 Given this effect, we initially hypothesized that the PD group, which had larger preoperative TT-TG distances, would demonstrate a greater reduction in the TT-TG distance than the lateral OA group. However, our findings did not support this hypothesis, as the degree of TT-TG distance reduction did not significantly differ between the 2 groups. Based on our regression analysis, the OR for preoperative TT-TG distance was −0.375, indicating that for every 1-mm increase in the preoperative TT-TG distance, the extent of TT-TG distance reduction increased by 0.375 mm. Given that the PD group had a preoperative TT-TG distance that was, on average, 5.1 mm greater than that of the lateral OA group, we expected approximately a 1.91-mm greater reduction in the TT-TG distance in the PD group. However, the actual difference between the 2 groups observed in our study was only 0.3 mm. This discrepancy may be attributed to substantial variability in TT-TG distance changes within each group (standard deviation, 2.3 mm in the lateral OA group and 4.5 mm in the PD group), which likely contributed to the lack of a statistically significant difference. These findings suggest that while preoperative TT-TG distance plays a role in determining the extent of TT-TG distance reduction, additional factors such as variations in surgical techniques, hinge point positioning, and unintended rotational effects may further influence the radiographic outcomes.⁹ Therefore, although CWDFO predictably reduced the TT-TG distance, orthopaedic surgeons should recognize that the degree of reduction may vary depending on preoperative TT-TG distance and intraoperative technical factors. Furthermore, although the mean change in the TT-TG distance did not differ based on diagnosis, the preoperative, postoperative, and change values varied more in patients with PD than those with lateral OA. This finding suggests a broader spectrum of TT-TG distance values within the PD population, highlighting the importance of preoperative and postoperative TT-TG distance assessments in these patients compared with those with lateral OA.

Our study confirmed that the degree of TT-TG distance changes after CWDFO could be predicted based on preoperative factors, with preoperative TT-TG distance and type of osteotomy identified as the primary determinants. Previous studies have reported factors influencing changes in the TT-TG distance.^9,33 One study analyzed 10 patients with lateral OA and valgus deformities who underwent CWDFO.⁹ Their findings indicated a linear reduction in the Q angle proportional to the degree of correction but no significant changes in the TT-TG distance. In contrast, another study conducted 3-dimensional simulations of CWDFO using preoperative CT and demonstrated a consistent reduction in the TT-TG distance corresponding to the degree of correction.³³ Additionally, their study identified tibial plateau width as a significant factor influencing TT-TG distance changes. In our study, preoperative TT-TG distance and osteotomy type were found to be significant factors influencing changes in the TT-TG distance during CWDFO. However, our predictive model could be constructed based solely on the preoperative TT-TG distance. This decision was made because no statistical interaction was observed between preoperative TT-TG distance and osteotomy type, indicating that their effects were independent. Additionally, given the limited number of cases (n = 10) undergoing biplanar osteotomy, including osteotomy type in the model could have led to overfitting and reduced generalizability. By focusing on preoperative TT-TG distance, we could develop a clinically applicable, simple model to assist in predicting TT-TG distance changes after CWDFO, regardless of the surgical technique. Our predictive model also revealed that a larger preoperative TT-TG distance was associated with greater linear reductions in the TT-TG distance after CWDFO. For example, in patients with a preoperative TT-TG distance of 20 mm, our regression model predicted a reduction of approximately 4.5 mm after biplanar osteotomy. Although a TT-TG distance ≥20 mm is often considered an indication for tibial tubercle osteotomy as part of a realignment procedure, our findings suggest that CWDFO alone may be sufficient in such cases without the need for mandatory tibial tubercle osteotomy.^26,28 However, as the preoperative TT-TG distance increased, the 95% CI of our regression model widened, indicating greater variability in TT-TG distance changes after CWDFO. Specifically, when the preoperative TT-TG distance exceeded 23.5 mm, postoperative changes deviated significantly from the expected values. In such cases, it is essential to reassess the TT-TG distance after CWDFO and consider whether additional surgical procedures such as tibial tubercle osteotomy are necessary. Additionally, the type of osteotomy was a key factor in the change in the TT-TG distance. In biplanar osteotomy, the ascending osteotomy site may occasionally deviate from the plane defined by the posterior cortex of the medial and lateral femoral condyles in the coronal plane, potentially resulting in minor displacement between the proximal and distal fragments. In addition, saw blade thickness and vibrations during osteotomy may create a minor gap at the ascending osteotomy site, which could partially close under compression. These factors may contribute to greater TT-TG distance reduction in biplanar osteotomy compared with uniplanar osteotomy.²⁷ Such variations likely explain some of the discrepancies in TT-TG distance changes observed across different studies. Our study identified lateral hinge fractures in up to 50% of cases in our study; however, they were not found to significantly influence changes in the TT-TG distance. According to the classification system proposed by Fujita et al,¹¹ lateral hinge fractures can be categorized into 3 types (I, II, III), with type I fractures considered stable. In our study, 17 of 19 cases (89.5%) were classified as type I based on CT findings, characterized predominantly by cortical breakage. The predominance of stable fractures likely explains why lateral hinge fractures did not substantially affect TT-TG distance changes in our study.

Our study has some limitations. First, although the sample size was sufficient to detect a significant difference in the TT-TG distance before and after CWDFO, it may have been relatively small to identify influencing factors, including different diagnostic groups. Given that the mean TT-TG distance changes were not significantly different between the 2 groups (lateral OA: −3.0 ± 2.3 mm; PD: −3.3 ± 4.5 mm), a substantially larger cohort would be necessary to ascertain whether the degree of TT-TG distance reduction differs based on the diagnosis. This limitation should be taken into account when interpreting subgroup comparisons in our findings. Second, although our study successfully predicted the degree of TT-TG distance changes based on the preoperative TT-TG distance and identified a threshold for deviations beyond the expected values, the interpretation of this threshold may vary depending on the clinician’s perspective. Furthermore, as the top 5% threshold in the CI width corresponded to only 2 or 3 cases in our study, the robustness of this threshold remains uncertain. While this approach is statistically grounded and aligns with previous methodology,³⁵ it should be regarded as a reference point rather than an absolute cutoff. Further validation with a larger cohort is necessary to confirm its clinical relevance. Third, multivariate analysis identified osteotomy type as a significant factor influencing TT-TG distance changes. However, we acknowledge that the surgical technique evolved over the study period, which may have influenced radiographic outcomes. This variability could potentially limit the generalizability of our findings. Therefore, future studies with larger cohorts and standardized surgical techniques are necessary to further validate these results across diverse patient populations. Fourth, we did not evaluate clinical outcomes according to the extent of TT-TG distance changes. In particular, this might be important for patients with PD who demonstrated greater variability, as understanding the clinical effect of TT-TG distance changes in this group remains an unmet need. Future studies addressing these aspects will provide a more comprehensive understanding of the topic. Despite these limitations, our findings contributed to resolving the ongoing debate surrounding changes in the TT-TG distance after CWDFO and offer valuable insights for clinical practice.

Conclusion

CWDFO effectively reduced the TT-TG distance, regardless of the diagnosis. Preoperative TT-TG distance and osteotomy type were key determinants of this reduction. In cases with preoperative TT-TG distances >23.5 mm, TT-TG distance changes demonstrated greater variability, suggesting that radiographic outcomes may deviate from expected values. These findings highlight the need for careful postoperative assessments and consideration of additional procedures in select cases.

Footnotes

Submitted January 2, 2025; accepted March 21, 2025.

The authors have declared that there are no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

ORCID iDs

Hyung Jun Park

Jae Gyoon Kim

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