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Abstract

Background

Symptoms of knee stiffness after open wedge high tibial osteotomy (OW-HTO) can significantly affect surgical effectiveness, but no studies have reported risk factors for knee stiffness after OW-HTO.

Methods

Patients treated with OW-HTO for the first time between 2018 and 2021 were included. Data were collected on patient demographics, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Short Form (SF) 12 scores, hip-knee-ankle angle (HKA) and patient satisfaction before and after surgery. Patients with worse WOMAC stiffness scores at 1 year were defined as the 'increased stiffness' group and the other cohort as the 'non-stiffness' group. The primary outcome of the study was to compare postoperative knee function scores (WOMAC and SF-12), HKA and patient satisfaction rate between the two groups. The secondary outcome was the use of logistic regression to analyze independent predictors of increased postoperative stiffness symptoms.

Results

At 1 year postoperatively, 95 (11.3%) patients had a significant increase in stiffness. Patients had significantly (p < .001) less improvement in pain, function, and total WOMAC scores, and SF-12 score than those in the non-stiffness group (n = 745). However, the differences in WOMAC and SF-12 scores in increased stiffness group at 1 year post-operatively were statistically significant (p < .001) compared to the non-stiffness group. There was no statistically significant difference in HKA in the increased stiffness group (172.9° ± 2.3°) compared to non-stiffness group (173.4° ± 2.6°) at 1 year postoperatively (p = .068). Patient satisfaction was significantly lower in the increased stiffness group (p < .001). Logistic regression analysis showed that diabetes (odds ratio (OR) 1.809, p = .034) and preoperative WOMAC stiffness score of 44 or less (OR 4.255 p < .001) were predictors of increased stiffness.

Conclusions

Patients with increased stiffness after OW-HTO had worse functional outcomes and lower patient satisfaction rates and patients at risk of being in this group should be informed pre-operatively.

Keywords

knee osteoarthritis high tibial osteotomy absorbable spacer knee stiffness

Introduction

Osteoarthritis is the main cause of knee pain and physical disability in middle-aged and elder people,^1,2 affecting over 500 million people worldwide.³ Open wedge high tibial osteotomy (OW-HTO) is a knee-preserving procedure currently used for the treatment of knee osteoarthritis.⁴ At present, the surgical technique is mature and widely used, with a high patient satisfaction rate of 88.6%.⁵ However, some studies have found that knee stiffness after OW-HTO is not uncommon, which is predominantly subjectively reported by patients, with an incidence of 1.2%-17.7%,^6,7 significantly affecting patients' daily lives and potentially leading to lifelong disability.⁸ The incidence of knee stiffness is reported to be widely variable, with risk factors including individual patient differences, surgical technique, osteoarthritis grading, patient expectations, and rehabilitation process.⁹ In addition, we believe that the size of the implants may also have a relationship with knee stiffness, because the larger the implants, the larger the incision required, the more extensive the stripping of the surrounding soft tissues, and the longer the surgical time, which may lead to an increase in the chances of tissue ischemia and hypoxia, and prolonged postoperative rehabilitation, which in turn leads to an increase in the chances of knee stiffness. Physicians can reduce the incidence of knee stiffness by modifying surgical techniques or developing rehabilitation protocols for patients. Our team first proposed in 2014 that the uneven subsidence of the tibial plateaus based on osteoporosis is the determining factor for knee varus, and subsequently in 2016 and 2018, advocate the use of resorbable spacers or double triangles locking compression plates to fix the osteotomy gap, and with reasonably good clinical outcomes.¹⁰ Theoretically, these two implants are smaller, the surgical incision is more minimal at 4 cm, and they can fix the osteotomy gap securely, overcoming the disadvantages of large plates,¹¹ but no study has yet confirmed this.

The main purpose of this study was to compare the clinical outcome (Western Ontario and McMaster Universities Osteoarthritis Index and Short Form-12 score), hip-knee-ankle angle (HKA) and patient satisfaction rate with increased stiffness symptoms at 1 year after OW-HTO with patients without increased stiffness. The secondary purpose was to identify independent predictors of stiffness increase 1 year after OW-HTO. The authors hypothesized that patients with worsening stiffness symptoms will have worse clinical outcomes and lower satisfaction rate with surgery, and identification of independent predictors of worsening stiffness will facilitate targeted interventions to improve prognosis.

Materials and methods

Patient data

This is a retrospective study of patients who underwent OW-HTO surgery between 2018 and 2021. The patients included in this study were identified from the high tibial osteotomy database at the study centre. A total of 1067 patients who underwent unilateral OW-HTO were asked to complete a questionnaire before the surgery.

Inclusion criteria were as follows: (1) age between 40 and 70 years; (2) the lesion is confined to the medial compartment of the knee and the lateral compartment is normal; (3) osteoarthritis grade 3 according to the Kellgren-Lawrence classification¹²; (4) medial compartment knee osteoarthritis with varus deformity ranging from 5° to 14°; (5) the knee range of motion (ROM)≥120° and knee flexion contracture<10°; (6) patients without cognitive impairment.

The exclusion criteria were as follows: (1) patients with other arthritis, such as progressive patellofemoral osteoarthritis, traumatic, or rheumatoid arthritis, etc.; (2) patients with a history of knee trauma, including fracture, lateral meniscus injury, and ligaments injury; (3) patients diagnosed with severe cartilage damage or bone defects in the medial compartment based on imaging data; (4) patients who had undergone other surgical treatments; (5) patients with limited knee joint ROM; (6) patients with severe knee instability.

A total of 840 patients who underwent OW-HTO surgery during the study period and had complete preoperative and postoperative data met the inclusion criteria (Figure 1). There were 293 male patients and 547 female patients, with a mean age of 58.3 (standard deviation (SD) 7.7) years. All patients included in the study were followed up preoperatively, 1 month, 6 months and 12 months postoperatively. Patients were asked to complete a questionnaire during the follow-up and were contacted by telephone if they did not respond within 3 weeks. Basic preoperative patient information was collected, including age, gender, BMI, and disease. Lung diseases included, pneumonia, pulmonary bullae, and interstitial lung disease. Kidney diseases included, chronic nephritis, renal hypertension, kidney stones, diabetic nephropathy, and renal cysts. Liver diseases include, fatty liver, viral hepatitis, liver cysts, cirrhosis. anaemia was defined as Hb < 120 g/L in adult males and Hb < 110 g/L in adult females (non-pregnant).

Figure 1.

Flow chart of data processing.

Surgical procedure

Preoperative full-length radiographs of both lower limbs in a standing position, frontal and lateral radiographs of the knee joints, and CT image of the knee joint were obtained to determine the grade of knee osteoarthritis and the corrective angle of the osteotomy.¹³ The position of the hinge for osteotomy is identified on full-length radiographs of both lower limbs and connects the midpoint of the femoral head with the lateral condyle of the tibial plateau as the line of osteotomy; the hinge is used as the centre of rotation and the distance from the centre of the ankle joint to the hinge is used as the radius of rotation until it crosses the line of osteotomy, the angle of rotation being the angle to be corrected. Intraoperatively, multiple test models of different thicknesses were inserted in the osteotomy gap according to the varus angle and the osteotomy gap was fixed with different implants. At the end of the operation, a line along the anterior superior iliac spine to the midpoint of the ankle joint was used to determine the degree of correction of the lower limb force line, and the angle of internal rotation of the affected limb was measured by re-taking X-rays after the operation. All procedures were performed by the same surgical team, and anesthesia was a general laryngeal mask combined with femoral nerve block. A balloon tourniquet was used intraoperatively on the hip. Arthroscopy was performed routinely and treated as necessary. Then, an osteotomy was performed using a medial longitudinal incision of the proximal tibia, which preserves the pes anserinus tendon during exposure and completely releases the superficial layer of the medial collateral ligament. The horizontal osteotomy plane is above the hamstring tendon stop and parallel to the posterior tilt of the tibial plateau, preserving at least 1.5 cm of lateral cortex. After the osteotomy, the osteotomy gap was propped open with multiple test models of varying thicknesses, and the desired angle of correction was applied to create a slight valgus angle (mechanical valgus angle in the range of 6°-8°), which means laterally shifting the weight-bearing line of the lower limb to the Fujisawa point¹⁴ (maximum position of 62.5 % of the medial to the lateral width of the tibia). Then, the osteotomy gap is fixed using different implants (Figures 2 and 3).

Figure 2.

Double triangles locking compression plate and knee radiographs after surgery (a) Front view (b) Lateral view.

Figure 3.

Frontal and lateral radiographs 1 year after TPOASI (a) Front view (b) Lateral view.

We used three different implants to fix the osteotomy gap during the surgery, Tomofix plate (Dabo Medical Technology Co., Ltd), double triangular six-point support plate (Dabo Medical Technology Co., Ltd), and absorbable mesh spacer (Weigao Medical Technology Co., Ltd), all of which made a longitudinal incision on the medial tibial side from the level of the joint line to the stopping point of the pes anserinus. The length of the Tomofix plate incision was approximately 6 cm, and the length of the incision for the double triangular six-point support plate and absorbable spacer was approximately 4-5 cm. The choice between the three types of internal fixation depends on the patient’s preference. In addition, the patient’s physical condition, such as general condition, severity of osteoarthritis of the knee, and condition of the surrounding soft tissues, may also influence the choice of internal fixation. Finally, alignment correction was confirmed on intraoperative projection, and the stability of the proximal tibia was evaluated by intraoperative flexion.

All patients received the same postoperative rehabilitation exercises. From the second postoperative day, after ensuring that there were no significant adverse effects (abnormal vital signs, dizziness, nausea, etc.), patients were allowed to perform knee range of motion exercises. The patient lies flat on the bed, holds the knee with both hands, and performs knee flexion and extension exercises. The range of motion exercises seek to improve the angle of movement each time. At the same time, the quadriceps muscle isometric contraction exercises can be carried out, and can be partially weight-bearing walking on crutches. One month after surgery, patients can walk with full weight bearing. Ultrasound of the deep veins of the lower limbs was performed before and after surgery to check for deep vein thrombosis (DVT), and all patients received subcutaneous injections of low-molecular-weight heparin (LWH) once a day from the second day after surgery to prevent deep vein thrombosis of the lower limbs. For patients with post-operative DVT in the lower limbs, the subcutaneous injection of LMH was changed to twice daily until the thrombus disappeared.

Clinical outcome

The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) is commonly used to assess the severity of knee osteoarthritis and the effectiveness of treatment and is divided into three components: pain, stiffness, and function,¹⁵ of which the stiffness component can be used to assess the patient’s stiffness symptoms. The WOMAC score used in this study was consisting of 24 questions that were answered on a five-point Likert scale (none, mild, moderate, severe, and extreme). The WOMAC pain subscale had five questions scored from 0 to 4, and it had a range of 0 (no pain) to 20 (maximal pain). The WOMAC function subscale has 17 questions scored 0–4, had a range of 0 (best function) to 68 (worst function). The WOMAC stiffness subscale had two questions, had a range from 0 (no stiffness) to 8 (maximal stiffness). We have converted the score to a percentage where 0 is the best and 100 is the worst.¹⁶

The Short Form 12 (SF-12) is a measure of patient health that is divided into two parts: the Physical Component Summary (PCS) and the Mental Component Summary (MCS).¹⁷ The SF-12 scale ranges from 0 (worst level of functioning) to 100 (best level of functioning). HKA was measured on full-length radiographs of the lower limbs before and after surgery and was used to determine the degree of knee inversion. Patient satisfaction was measured by asking "Are you satisfied with OW-HTO surgery?" at 1 year after surgery. Patients who answered very satisfied or somewhat satisfied were classified as satisfied and those who were somewhat dissatisfied or very dissatisfied were classified as dissatisfied. The increased stiffness group was defined as patients who had a worsening or negative change in the WOMAC stiffness score. This group with increased stiffness symptoms was compared with those who had no change or improvement in symptoms at 1 year.

Statistical analysis

SPSS (version 17.0) was used for statistical analysis. The two independent samples t test or Mann-Whitney test was chosen to compare the differences between the two groups based on whether the continuous variables conformed to a normal distribution. Continuous variables are expressed as mean ± standard deviation. Categorical variables were statistically analyzed using Fisher exact test or chi-square test. The total postoperative WOMAC score and each subscore were included in the Receiver operating characteristic (ROC) curve. ROC curve analysis was used to identify thresholds in linear variables that were significantly different between the two groups. The area under the ROC curve ranges from 0.5, indicating a test with no accuracy, to 1.0 where the test is perfectly accurate. The threshold corresponds to the point of highest sensitivity and specificity for patient satisfaction.¹⁸ Binary or multivariate logistic regression analyses were used to determine the independent predictors of worse stiffness 1 year after surgery. Variables with p < .1 in one-way analyses were included in logistic regression. The threshold was used to divide WOMAC scores into dichotomous variables as predictors in regression models. Differences were considered to be statistically significant at p < .05.

A post hoc power calculation was performed using the WOMAC score as the primary outcome. Using the defined minimal clinically important difference in the WOMAC of 16 points,¹⁹ a standard deviation (SD) of 20, an alpha 0.05 with 95 in the increased stiffness group and 745 in the control group this offered a power of 100%.

Result

A total of 95 patients (11.3%) had worsening stiffness 1 year after OW-HTO, including 58 with Tomofix plates, 16 with absorbable spacers, and 21 with double triangles locking compression plates. Univariate analysis identified preoperative factors that predicted increased stiffness at 1 year postoperatively (Table 1). Among these, lung disease, diabetes, liver disease, all components of WOMAC and total scores, different implants were statistically different between the two groups preoperatively (p < .05). In the univariate analysis, p < .1 was also found for body mass index (BMI) and kidney disease, and therefore we included them in the subsequent logistic regression analyses.

Table 1.

Preoperative demographic characteristics and clinical functional scores between the two groups.

Demographic	Descriptive	Increased Stiffness^*		Odds ratio/difference	95%CI of difference		p value
Demographic	Descriptive	Yes (n = 95)	No (n = 745)		Lower	Upper
Gender	Male	30(31.6)	263(35.3)	0.8	0.535	1.337	0.473
	Female	65(68.4)	482(64.7)
Age		57.7 ± 7.4	58.3 ± 7.7	0.6	−2.325	0.591	0.411
BMI		27.3 ± 3.8	26.7 ± 3.4	0.6	−0.111	1.353	0.096
Comorbidity	Heart disease	5(5.3)	65(8.7)	0.6	0.228	1.482	0.250
	Hypertension	40(42.1)	279(37.4)	1.2	0.787	1.874	0.379
	Lung disease	18(18.9)	84(11.3)	1.8	1.050	3.224	0.031
	Cancer	1(1.1)	3(0.4)	2.6	0.271	25.553	0.382
	Neurological disease	2(2.1)	3094.0)	0.5	0.121	2.180	0.568
	Diabetes mellitus	16(16.8)	75(10.1)	1.8	1.005	3.257	0.045
	Digestive diseases	11(11.6)	63(8.5)	1.4	0.719	2.796	0.312
	Kidney disease	5(5.3)	16(2.1)	2.5	0.906	7.074	0.078
	Liver disease	14(14.7)	49(6.6)	2.5	1.298	4.642	0.004
	Anemic	10(10.5)	74(9.9)	1.1	0.531	2.144	0.856
WOMAC	Total	55.8 ± 13.3	61.8 ± 10.9	−6.0	−8.379	−3.575	<0.001
	Pain	57.3 ± 17.0	63.1 ± 14.5	−5.8	−9.428	−2.194	0.002
	Stiffness	48.9 ± 19.5	63.3 ± 22.1	−14.4	−19.042	−9.673	<0.001
	Function	56.2 ± 12.5	61.3 ± 11.9	−5.1	−7.593	−2.487	<0.001
SF-12	PCS	28.1 ± 6.6	27.5 ± 7.9	0.5	−1.110	2.195	0.520
	MCS	45.2 ± 10.2	44.2 ± 10.9	1.0	−1.293	3.320	0.398
HKA		184.7° ± 2.7°	184.1° ± 2.6°	0.6	−0.088	1.089	0.095
Internal fixation	Absorbable spacer	16(16.8)	183(24.6)				0.038
	Double triangles locking compression plate	21(22.1)	211(28.3)
	Tomofix	58(61.1)	351(47.1)

BMI body mass index (kg/m²); WOMAC western ontario and mcmaster universities osteoarthritis index; SF-12 Short Form (SF) 12 scores; PCS physical component summary; MCS Mental component summary; HKA hip-knee-ankle angle. p Value less than 0.05 was taken as statistically significant.^*Data are presented as mean ± standard deviation or number (proportion).

Patients with increased stiffness had a mean decrease (worse) of 16.5 [95% confidence interval (CI) 20.3 to 12.6] points in the WOMAC stiffness score relative to their preoperatively. Compared to the preoperative period, both groups had a significant improvement in the other WOMAC components and total scores and the SF-12, PCS, and MCS, except for the increased stiffness group, which had a worsening of the WOMAC stiffness score (Table 2). However, the improvement in WOMAC scores was greater in the non-stiffness group compared to the increased stiffness group. The non-stiffness group showed at least 27.2% improvement in all components of the WOMAC and the total score, whereas the increased stiffness group showed the greatest improvement in the pain component, which was 13.2% (Figure 4). HKA at 1 year postoperatively was significantly improved in both groups compared to preoperative (p < .001), but there was no statistically significant difference (p = .068) in HKA in the increased stiffness group (172.9° ± 2.3°) compared to the non-stiffness group (173.4° ± 2.6°). The proportion of patients in the increased stiffness group who were satisfied was significantly lower (n = 61, 64.2%, p < .001) compared to the control group (n = 707, 94.9%).

Table 2.

1 year post-operative clinical functional scores, imaging indices and satisfaction between the two groups.

		Increased stiffness		Difference	P value
WOMAC		Yes (n = 95)	No (n = 745)	Difference	P value
Total	1 year postoperatively*	46.6 ± 12.1	30.1 ± 8.9	16.5	<0.001
	Change (95%CI)	−8.6(-7.0 to −10.2)	−31.7(-30.8 to −32.6)	23.1	<0.001
	p value	<0.001	<0.001
Pain	1 year postoperatively*	44.1±15.8	18.1±9.1	26.0	<0.001
	Change (95%CI)	−13.2(-9.9 to −16.5)	−45.0(-43.9 to −46.1)	31.8	<0.001
	p value	<0.001	<0.001
Stiffness	1 year postoperatively*	57.6±18.5	26.6±10.1	31.0	<0.001
	Change (95%CI)	16.5(20.3 to 12.6)	−36.7(-35.2 to −38.2)	53.2	<0.001
	p value	<0.001	<0.001
Function	1 year postoperatively*	46.1±13.1	34.1±10.7	28.1	<0.001
	Change (95%CI)	−10.2(-8.5 to −11.8)	−27.2(-26.1 to −28.3)	17.0	<0.001
	p value	<0.001	<0.001
SF-12
PCS	1 year postoperatively*	31.9±7.8	39.5±8.7	7.6	<0.001
	Change (95%CI)	3.9(3.2 to 4.6)	12.0(11.5 to 12.4)	8.1	<0.001
	p value	<0.001	<0.001
MCS	1 year postoperatively*	49.3±11.0	52.3±12.1	3.0	0.024
	Change (95%CI)	4.1(1.7 to 6.5)	8.1(7.1 to 9.1)	4.0	<0.001
	p value	<0.001	<0.001
HKA	1 year postoperatively*	172.9°±2.3°	173.4°±2.6°	0.5	0.068
	Change (95%CI)	−11.8(-12.4 to −10.9)	−10.7(-10.9 to −10.4)
	p value	<0.001	<0.001
Satisfaction**		61 (64.2)	707 (94.9)		<0.001

WOMAC western ontario and mcmaster universities osteoarthritis index; SF-12 Short Form (SF) 12 scores; PCS physical component summary; MCS Mental component summary. HKA hip-knee-ankle angle. p Value less than 0.05 was taken as statistically significant.

^aData are presented as mean ± standard deviation.

^bData are presented as the number (percentage).

Figure 4.

Mean change in increased stiffness group and control group for WOMAC pain (circle), stiffness (triangle), function (square) and total (pentagon) Score. Error bars represent 95% confidence intervals.

ROC curve analysis was used to identify threshold values in the linear variables that were demonstrated to be significantly different between the two groups (Figure 5). The threshold for the postoperative WOMAC total score, pain, and function component was 57, and the threshold for the postoperative WOMAC stiffness component was 44. The most reliable predictor of increased stiffness at 1 year was the preoperative WOMAC stiffness score (odds ratio (OR) 4.255, AUC of 0.752), whereas pain, function, and total WOMAC score were poorer predictors with AUC of less than 0.7 (Table 3). The threshold was used to divide the WOMAC score into a dichotomous variable to be used as a predictor in the regression model. Logistic regression analysis showed that diabetes (p = .034), and preoperative WOMAC stiffness score≤44 (p < .001) was predictive of increased stiffness at 1 year postoperatively. The use of absorbable spacers and double triangles locking compression plates may be associated with a lower incidence of postoperative stiffness (Table 4).

Figure 5.

ROC curve for predicting increased stiffness 1 year after surgery using the pre-operative components and total WOMAC score.

Table 3.

ROC curve analysis identifying the threshold value for the WOMAC scores that predict increased stiffness.

	Threshold value	Sensitivity	Specificity	AUC	95%CI of difference
	Threshold value	Sensitivity	Specificity	AUC	Lower	Upper
Pain	57	64.3	49.5	0.601	0.538	0.665
Stiffness	44	78.0	40.0	0.752	0.698	0.806
Function	57	65.5	47.4	0.612	0.552	0.672
Total	57	66.6	44.2	0.630	0.567	0.693

Table 4.

Independent predictors of increased stiffness after HTO using logistic regression analysis.

Demographic	Descriptive		OR	95% CI of difference		P value
Demographic	Descriptive		OR	Lower	Upper	P value
BMI			1.057	0.993	1.125	0.082
Comorbidity	Not present		Reference
	Lung disease		0.815	0.137	4.853	0.838
	Diabetes		1.809	1.005	3.257	0.034
	Kidney disease		0.692	0.180	2.655	0.591
	Liver disease		1.429	0.182	11.192	0.734
Functional measure
WOMAC	Total	>57	Reference
		≤57	1.910	1.470	3.949	0.065
	Pain	>57	Reference
		≤57	1.256	0.675	2.335	0.429
	Stiffness	>44	Reference
		≤44	4.255	2.641	6.856	<0.001
	Function	>57	Reference
		≤57	1.148	0.490	2.690	0.685
Internal fixation	Tomofix		Reference
	Double triangles locking compression plate		0.607	0.358	1.029	0.064
	Absorbable spacer		0.531	0.297	0.949	0.033

BMI body mass index (kg/m²); WOMAC western ontario and mcmaster universities osteoarthritis index; p value less than 0.05 was taken as statistically significant.

Discussion

The main findings of the study were that patients with subjectively increased stiffness symptoms after OW-HTO have worse clinical scores and quality of life, as well as a lower rate of postoperative satisfaction and that diabetes, preoperative WOMAC stiffness score of 44 or less were predictors of increased postoperative stiffness. The use of absorbable spacers and double triangles locking compression plates may be associated with a lower incidence of postoperative stiffness compared to Tomofix plates.

Knee stiffness is an important factor affecting patients' postoperative quality of life and satisfaction and could be regarded as an independent part of postoperative efficacy which was indicated by F Wolfe et al.²⁰ Various studies have reported that the cause of postoperative knee stiffness may be related to knee arthrofibrosis, which refers to the abnormal proliferation of fibrous tissue in and around the knee joint and may lead to limited knee extension and flexion function.²¹ Postoperative knee arthrofibrosis may be associated with reduced postoperative knee motion, severity of knee injury, ligament damage, or other secondary factors.^22,23 We also obtained an interesting finding that preoperative one-way analyses showed that all preoperative WOMAC scores were lower (better) in the stiffness-increased group compared to the non-stiffness group. At 1 year post-operatively, all components except the stiffness component of the WOMAC were significantly better than preoperatively. However, the stiffness increased group had worse postoperative knee function than the non-stiffness group. We believe the likely reason for this is that patients with lower preoperative knee stiffness are more likely to have increased knee stiffness postoperatively, which affects the patient’s overall knee function. In contrast, patients with poor preoperative knee stiffness were less likely to have increased knee stiffness after surgery; instead, surgery could significantly improve the patient’s knee stiffness, leading to better postoperative knee function. Therefore, we conclude that OW-HTO surgery has good results in both groups of patients, but the results are better in patients with worse preoperative knee function.

This study also found that satisfaction with surgery was lower in the group with increased stiffness than in the group without increased stiffness. We believe that patient satisfaction with surgery is related to higher preoperative expectations, as our preoperative questionnaire showed that most patients who underwent OW-HTO surgery expected to be able to perform some daily activities such as walking up and down stairs, standing, and sitting or lying down 1 year after surgery. Therefore, in patients with increased knee stiffness, if these expectations are not met postoperatively because of increased symptoms of joint stiffness, patients may be dissatisfied with the surgery. However, this study also found that the HKA at 1 year post-operatively was significantly improved in the stiffness increase group and the non-increase group compared to the preoperative period (p < .001), but there was no statistically significant difference between the two groups (p = .068). It shows that the lower limb force lines were well corrected after OW-HTO surgery in both groups. Therefore, we believe that the increased symptoms of knee stiffness after surgery may not be related to the lower limb force line.

We also found that the use of absorbable spacers to fill the osteotomy gap resulted in the lowest rate (16.8%) of postoperative stiffness among the three types of internal fixation, but patients with Tomofix plates had the highest rate (61.1%) of increased postoperative stiffness, and that absorbable spacers were an independent predictor of decreased postoperative stiffness (OR 0.531 p = .033). David A Parker et al. found that larger incisions were associated with postoperative pain of incision and soft tissue scarring at the incision site, which leads to postoperative knee arthrofibrosis and increased postoperative stiffness.²⁴ Due to the low amount of soft tissue on the anteromedial aspect of the tibia, the use of the Tomofix plate will compress the subcutaneous soft tissue in this area, which will further aggravate the incisional pain and discomfort and lead to increased knee stiffness. In line with our belief that the use of smaller implants can significantly reduce post-operative stiffness, fixing the osteotomy gap with the double triangles locking compression plate or absorbable spacer can theoretically achieve the same fixation strength as the Tomofix plate, with the added benefit of a smaller incision. The incision for Tomofix plates is traditionally around 6 cm, whereas with absorbable spacers and double triangles locking compression plates the surgical incision is only around 4 cm, which can significantly reduce the patient’s post-operative stiffness.

In addition, we found that diabetes, WOMAC stiffness score of 44 or less were independent predictors of increased postoperative stiffness after OW-HTO. In recent years, several studies have demonstrated that diabetes is a recognized complication associated with inflammation²⁵ and exhibits abnormalities in maintaining a normal inflammatory response. Hyperglycemic conditions can lead to increased inflammatory responses, and the cytokines and inflammatory mediators released during the inflammatory process can promote the proliferation and deposition of fibrous tissue.^26,27 This supports the findings of our study, suggesting that there is a correlation between diabetes and postoperative knee joint stiffness after OW-HTO. Furthermore, diabetes can also affect the blood supply and nerve function around the joints. Patients with diabetes often have vascular and neurological disorders, which can affect the normal metabolism and repair capacity of the joints, thereby promoting the development of fibrosis.²⁸

A preoperative WOMAC stiffness score of 44 or less was a significant predictor of increased stiffness symptoms 1 year after HTO (AUC 0.75, p<.001). WOMAC stiffness score can serve as a screening tool to assess the high-risk population for knee joint stiffness. The identified high-risk population may benefit from perioperative interventions, enabling patients to be aware of the likelihood of developing knee joint stiffness before surgery. Interventions can be used in these patients to prevent postoperative stiffness, reduce pre-operative expectations and improve patient satisfaction with surgery. With further understanding of knee arthrofibrosis, there may be new inhibitors developed to impede the progression of postoperative stiffness. Based on this study, it is crucial for surgeons to be aware that a certain proportion of patients may experience increased symptoms of stiffness after OW-HTO surgery, which can be accompanied by poorer knee joint function and lower postoperative satisfaction rates.

Therefore, our study identifies risk factors for knee stiffness, which allows for preoperative screening of patients with potentially increased knee stiffness and postoperative rehabilitation instructions, which may lower patients' preoperative expectations and have a positive effect on improving surgical satisfaction. The main limitations of this study are, firstly the study is retrospective in nature. Secondly, knee range of motion (ROM) was not collected for all patients at 1 year postoperatively; therefore, knee stiffness was not assessed in terms of ROM. There is no consensus in the literature on the definition of knee "stiffness", which is usually defined as a limited range of motion but may not be used by patients as a criterion for symptoms of stiffness. Third, there may be potential selection bias in the choice of surgical endoprostheses, as the surgical procedure is influenced by patient preference and physical condition, which may affect the outcome in other ways. Fourthly, a more in-depth subgroup analysis of patients with increased stiffness symptoms was not performed using a prospective study design. Subgroup analysis could have evaluated knee joint stiffness using objective measures such as knee joint range of motion, rather than the patient-reported outcome measures used in the present study. This would have provided us with a better understanding of why those patients with increased stiffness symptoms were dissatisfied with their OW-HTO procedures.

Conclusion

Patients with increased stiffness after OW-HTO had worse functional outcomes and lower patient satisfaction rates and patients at risk of being in this group should be informed pre-operatively.

Footnotes

Author contributions

J-hY writes the manuscript. J-hY, B-xS, and Y-bZ acquired the data for the work. All authors contributed to the analysis and the interpretation of data. JW and Y-zZ design and revise the paper. All authors have critically revised the manuscript and have agreed to the final version.

Declaration of conflicting interests

The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by several grants from the National Natural Science Foundation of China (91949203), Hebei Department of Science and Technology High-Level Talent Team Construction Project (225A7703D) and Hebei Provincial Key Research Projects (192777113D).

Ethical statement

ORCID iD

Jiahao Yu .

Data Availability Statement

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

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Increased symptoms of stiffness after opening-wedge high tibial osteotomy are associated with worse postoperative knee function outcomes and lower patient satisfaction rate