Effects of obturator and femoral nerve blocks on pain and kinematic gait parameters in patients undergoing unicompartmental knee arthroplasty: a retrospective cohort study

Abstract

Background

Postoperative data indicate that 15–30% of patients undergoing unicompartmental knee arthroplasty (UKA) experience moderate-to-severe pain during the early recovery period, impeding rehabilitation. Due to the complex innervation in the knee, continuous femoral nerve block (FNB) is often administered but provides incomplete analgesia. Although the analgesic effects of nerve blocks are well studied, less is understood about their effects on postoperative rehabilitation and gait kinematics. Thus, in this study, we aimed to investigate the impact of ultrasound-guided obturator nerve block (ONB) combined with FNB on gait kinematics in patients undergoing UKA. This is the first study to quantify the biomechanical benefits of ONB combined with FNB in patients undergoing UKA by gait analysis.

Methods

Patients undergoing UKA and admitted to the Department of Orthopedics, People’s Hospital of Ningxia Hui Autonomous Region between March 2024 and December 2024 were retrospectively enrolled. The patients were allocated based on their postoperative nerve block procedure into the FNB or FNB + ONB group, with 30 cases in each group. The FNB group underwent ultrasound-guided, single-shot FNB with catheterization, whereas the FNB + ONB group underwent additional ipsilateral ONB. Patient demographics, preoperative and postoperative visual analog scale (VAS) scores (at rest and during 30° knee flexion), adverse events, kinematic gait parameters, Knee Society Score, and range of motion were recorded.

Results

No intergroup differences were observed in the preoperative VAS scores (P > 0.05). Both groups showed improved postoperative VAS scores (P < 0.05) with comparable resting VAS scores (P > 0.05). The FNB + ONB group demonstrated significantly lower activity-associated VAS scores than did the FNB group (P < 0.05). Analgesic rescue needs and adverse event rates showed no intergroup differences (P > 0.05). The FNB + ONB group exhibited superior postoperative Knee Society Scores, greater range of motion, and reduced kinematic gait abnormalities compared with the FNB group (all, P < 0.05).

Conclusion

ONB combined with FNB provides superior postoperative analgesia compared with FNB alone in patients undergoing UKA, particularly during activity, thereby facilitating early rehabilitation and mitigating postoperative gait disturbances.

Background

McKeever first proposed the concept of unicompartmental knee arthroplasty (UKA) in the 1950s; this procedure has evolved over six decades of clinical application [1, 2]. Postoperative data indicate that 15–30% of patients undergoing UKA experience moderate-to-severe pain during the early recovery period, which significantly impedes rehabilitation progress. Anatomical studies have revealed rich neural innervation in the knee region, which explains why a continuous femoral nerve block (FNB) alone often provides incomplete analgesia [3–5]. The obturator nerve block (ONB), previously employed for chronic knee pain management, has demonstrated efficacy in postoperative pain relief [6]. Although the current research has predominantly focused on the analgesic effects of nerve blocks, few studies have investigated their effects on postoperative rehabilitation and gait kinematics. Prior analgesia-focused studies have not investigated the link between ONB combined with FNB with kinetic chain restoration. Therefore, in this study, we aimed (1) to evaluate the effects of combined ONB and FNB on kinematic gait parameters and (2) provide theoretical support for understanding the dual role of peripheral nerve blocks in pain management and biomechanical recovery.

Methods

Patients and study design

Between March and December 2024, 60 patients who underwent UKA in the Orthopedic Department of Ningxia Hui Autonomous Region People’s Hospital were enrolled in the study. The patients were allocated into two groups based on their postoperative nerve block procedure: 30 and 30 cases in the FNB and FNB + ONB groups, respectively. The inclusion criteria were: (1) those who met the “Chinese Guidelines for the Diagnosis and Treatment of Osteoarthritis (2021 Edition)” [7] knee osteoarthritis diagnostic criteria; (2) those who had no history of surgery or trauma to both lower limbs; (3) those who had not taken any analgesic drugs in the week prior to admission; (4) those who had no contraindications for FNB or ONB; and (5) patients and their families provided informed consent for participation in this study and signed an informed consent form. The exclusion criteria included: (1) patients with congenital developmental malformations of either lower limb or hip joint; (2) those with severe knee joint malformations [8]; (3) those with serious internal diseases; and (4) those with incomplete clinical data. This study was approved by the Ethics Committee of Ningxia Hui Autonomous Region People’s Hospital (approval number: [2024]-KJCG-001).

Anesthesia and monitoring

After patients entered the operating room, non-invasive blood pressure, electrocardiogram, and blood oxygen saturation were routinely monitored. The surgeries for both groups used fine needles and were performed under heavy lumbar anesthesia. The L3–4 gap was selected for subarachnoid blockage. The anesthetic formula was 0.75% bupivacaine (2 ml plus 0.05 mg fentanyl citrate for a 3 ml total injection). After subarachnoid blockade, the patient’s anesthesia plane was measured and recorded.

Surgical methods

All patients who underwent UKA at our hospital used a SLED fixed-bearing unicompartmental prosthesis (LINK, Germany). The operation was routinely performed by the same group of surgeons [9].

Nerve block method: Postoperative nerve blocks were performed by the same anesthesiologist under ultrasound guidance. All procedures were performed after the completion of surgical incision closure and dressing. Group allocation was randomized using a live draw.

In the FNB group: An ultrasound of the FNB procedure is shown in Fig. 1. The patients were placed in a supine position, wherein the puncture site was disinfected and draped. The ultrasonic high-frequency linear array probe was placed laterally on the inguinal ligament, femoral nerve located on the outside of the femoral artery, and 10 ml of 0.375% ropicaine was injected through a 22G short inclined puncture needle above and below the outside of the femoral nerve, so that the femoral nerve was fully surrounded by local anesthetic solution. Finally, an arterial puncture needle was inserted under the femoral nerve and connected to the blocking pump for continuous FNB.

Fig. 1.

FNB diagram. The numbers indicate the various anatomical areas: (1) femoral nerve, (2) fascia lata, (3) fascia iliaca, and (4) iliopsoas muscles. The red arrow represents the location of the needles

In the FNB + ONB group: An ultrasound of the ONB procedure is shown in Fig. 2. After inserting the FNB juxtaposition tube in accordance with the method described above in the FNB group, ONB was guided by ultrasound. The patients were placed in a supine position, their thigh mildly extended and rotated, and a high-frequency linear array probe was placed. The ultrasonic probe was placed at the fold of the groin, perpendicular to the skin, such that the pelvic floor, long adductor, short adductor, and large adductor muscles could be observed. Under the guidance of in-plane ultrasound, local anesthetics were injected into the fascia between the pubic bone and short adductor muscles or between the long and short adductor muscles to block the anterior branch, and local anesthetics were injected into the fascia between the short and adductor muscles to block the posterior branch.

Fig. 2.

ONB diagram. The numbers indicate the various anatomical areas and needles: (1) sartorius muscle; (2) needles, (3) adductor longus muscle, (4) adductor brevis muscle, and (5) adductor magnus muscle. The red arrow indicates the anterior branch of obturator nerve and the blue arrow indicates the posterior branch of obturator nerve

Postoperative treatment and functional exercise

A standardized analgesic protocol, excluding nerve blocks, was used for managing all patients during their hospitalization. The patients were provided electronic analgesic pumps for use postoperatively, which administered 4 mg of hydromorphone and 225 mg of ropivacaine diluted with physiological sodium chloride (0.9%) to 150 ml infused at 3 ml/h with a locking time of 5 min. Oral celecoxib (0.2 g twice/d) was administered from the first day after surgery until discharge. If the patients’ resting VAS score was ≥ 6 points after surgery or when their pain was unbearable, other analgesic drugs were administered orally at home.

Postoperative functional exercises included ankle pump exercises and quadriceps training starting the first day after the operation. Leg compression exercises began on the second day after the operation, thrice daily for 3–5 min. Straight leg elevation exercise started on the third day postoperatively (3–5 sets daily, 5–10 repetitions per set) as well as leg bending exercises (thrice daily, the duration was subject to tolerance) and active knee flexion and extension activities (thrice daily, 3–5 min each). The patients were discharged when they had no signs of severe anemia, poor wound healing, or other postoperative complications; when their knee pain was tolerable; and when their knee flexion reached 90°.

Observation indicators

Research data on all phases were collected and recorded under the supervision of two physicians who were blinded to group allocation and were not involved in the surgical procedures. The analgesic effect was measured using the VAS scores in the two groups of patients before and after surgery at 6, 12, 24, and 48 h, resting and active (bending the knee at 30°). Adverse reactions were recorded if the patients experienced nausea, vomiting, urinary retention, or dizziness or required additional analgesics after the operation.

In addition, the kinematic gait parameter indicators were assessed. All patients were examined using a Qualisys three-dimensional motion capture system to collect and process gait data. Data collection was synchronized with the clinical assessments, conducted on the day of admission and postoperative day 5. The Qualisys system used in this study comprised eight high-performance infrared cameras, two force plates, multiple marker reflective spheres, a set of calibration bars, and a data processing server. The infrared cameras were mounted on the walls of the laboratory, and the force plates were set on the floor. The “Consortium for Advanced Science and Technology (CAST)” model was selected as the gait analysis protocol. According to the marking points of the “CAST” model, the marked reflective balls bilaterally attached to the following anatomical landmarks: anterior and posterior superior iliac spines, medial and lateral condyles of the knee joint, the medial and lateral ankles of the ankle joint, the first and fifth metatarsalphalangeal joints, second metatarsal head, and heel of the foot bilaterally (at the same height as the second metatarsal head markers). The patients walked along a 10-meter path at a self-selected speed; gait data were averaged over five valid trials after performing two practice trials and eliminating outliers [10]. The collected exercise parameters included the maximum flexion angle of the hip and knee joints and the dorsiflexion angle of the ankle joints. Finally, the Knee Society Score (KSS) and range of motion (ROM) were determined as clinical scores.

Statistical methods

SPSS 24.0 software was used for data analysis. The measurement data were examined using the Shapiro–Wilk normality test. The normal and approximately normal distributions are represented as mean and standard deviation (x ± s). The non-normally distributed data are represented as the median and quartile spacing [M(P25, P75)]. Comparisons between the groups were assessed using an independent t-test or a non-parametric test. Counting data were expressed as a ratio (%). The chi-square test was used for comparing between the two groups; a difference of P < 0.05 was considered statistically significant.

Results

Patient characteristics

The sex, age, body mass index (BMI), operation time, and length of hospital stay were not significantly different between the FNB and FNB + ONB groups (P > 0.05; Table 1).

Table 1.

Comparison of patient characteristics between the two groups

Characteristic		FNB Group	FNB + ONB Group	t/χ2/Z	P
Sex	Male	5(16.67%)	9(30.00%)	1.491	0.222
	Female	25(83.33%)	21(70.00%)
Age (years)		61.00(58.00,67.00)	65.30 ± 5.62	-1.655	0.098
BMI (kg/ m2)		25.60 ± 2.59	25.35 ± 3.11	0.341	0.735
Operation time (min)		67.57 ± 4.21	67.53 ± 3.97	0.032	0.975
Hospital stay (days)		8.00(7.00,9.00)	8.00(7.00,9.00)	-0.671	0.502

Note: Data are presented as x ± s or M(P25, P75) based on the Shapiro-Wilk normality test (α = 0.05); Z-values from Mann-Whitney U tests reflect standardized rank differences

Analgesic effects and adverse reactions

There was no significant difference in the preoperative VAS scores between the two groups of patients (P > 0.05). In both groups, the postoperative VAS scores improved compared with the preoperative VAS scores (P < 0.05). There was no significant difference in the resting VAS scores between the postoperative groups (P > 0.05). The VAS scores of the FNB + ONB group during postoperative activities were lower than those of the FNB group (P < 0.05); there was no significant difference in the incidence of analgesic drug administration or adverse reactions after the operation (P > 0.05; Table 2).

Table 2.

Comparisons of VAS scores and adverse reactions between the two groups

			FNB Group		FNB + ONB Group		χ2/Z	P
VAS	Resting state	Preoperative	6.00(5.00,7.00)		6.00(4.75,7.00)		-0.085	0.932
		6 h	3.00(2.00,3.00)*		2.00(1.00,3.00)*		-1.777	0.076
		12 h	2.50(2.00,4.00)*		3.00(2.00,3.00)*		-0.229	0.819
		24 h	3.50(2.00,4.00)*		3.00(3.00,4.00)*		-0.550	0.583
		48 h	4.00(3.00,4.00)*		4.00(3.00,4.25)*		-0.307	0.759
	Active status	Preoperative	6.00(5.00,7.00)		7.00(6.00,7.00)		-1.112	0.266
		6 h	5.00(4.00,5.00)*		3.00(2.00,4.00)*		-4.460	< 0.001
		12 h	4.00(3.00,5.30)*		4.00(3.00,4.00)*		-2.231	0.026
		24 h	5.00(3.75,5.00)*		3.00(3.00,4.00)*		-3.444	0.001
		48 h	4.50(4.00,5.00)*		4.00(3.00,4.00)*		-2.687	0.007
Additional Analgesic Drugs			5(16.67%)		3(10.00%)		0.577	0.448
Adverse Reactions	Vertigo		1	2(6.67%)	1	4(13.33%)	0.741	0.389
	Nausea, Vomiting		1		1
	Urinary Retention		0		2
	Fall		0		0

Note: Note: Data are presented as x ± s or M(P25, P75) based on the Shapiro-Wilk normality test (α = 0.05); Z-values from Mann-Whitney U tests reflect standardized rank differences; Comparison with Preoperative, *P < 0.05

Kinematic gait parameters

There was no significant difference in the preoperative kinematics or gait parameters between the two groups (P > 0.05). The patients in the FNB + ONB group had the largest hip flexion, largest knee flexion, and largest ankle dorsiflexion angle, which were all higher than those in the FNB group (P < 0.05; Table 3).

Table 3.

Comparison of kinematic gait parameters in two groups (°)

	Maximum hip flexion		Maximum knee flexion		Maximum ankle dorsiflexion
	Preoperative	Postoperative	Preoperative	Postoperative	Preoperative	Postoperative
FNB Group	17.50(15.75,23.25)	22.43 ± 5.22	43.70 ± 5.50	39.33 ± 4.69	14.23 ± 3.68	11.90 ± 2.28
FNB + ONB Group	17.50(15.75,21.50)	25.20 ± 3.08	43.50 ± 5.73	44.00(40.00,46.50)	15.00(11.00,16.50)	14.00(11.00,17.00)
t/Z	-0.402	-2.499	0.138	-2.942	-1.130	-2.457
P	0.688	0.016	0.891	0.003	0.258	0.014

Note: Data are presented as x ± s or M(P25, P75) based on the Shapiro-Wilk normality test (α = 0.05)

Clinical score

There was no significant difference between the KSS and ROM in the two groups (P > 0.05). Patients in the FNB + ONB group had a higher postoperative KSS and ROM than did those in the FNB group (P < 0.05; Table 4).

Table 4.

Comparison of the clinical scores between the two groups

	KSS		ROM/°
	Preoperative	Postoperative	Preoperative	Postoperative
FNB Group	109.73 ± 35.28	116.97 ± 19.32	114.17 ± 15.26	90.00(90.00,95.00)
FNB + ONB Group	100.93 ± 23.23	126.50(115.30,137.00)	120.00(110.00,121.25)	95.00(90.00,100.00)
t/Z	1.141	-2.441	-0.622	-2.211
P	0.259	0.015	0.534	0.027

Discussion

Most patients undergoing UKA surgery in this study had knee osteoarthritis with narrowing of the medial interventricular gap of the knee joint and destruction of the articular cartilage. Thus, the UKA surgery was considered to be traumatic. Severe postoperative pain limits the movement of the patient’s knee joint, affects rehabilitation, and induces a series of stress reactions, which in turn cause a variety of complications [11, 12]. Therefore, the administration of effective analgesics is of great significance for reducing postoperative pain and stress responses and promoting joint function recovery.

Traditional intravenous self-controlled analgesia is commonly used for artificial joint replacement. This method of analgesia exerts its key analgesic effects through the intravenous injection of opiate analgesics. The most commonly used analgesic drug is the opiate sufentanyl, which is 5–10 times potent than fentanyl. It is highly lipophilic and easily passes through the blood-brain barrier, significantly reducing pain [5, 13]. However, the efficacy of intravenous self-controlled analgesia is influenced by drug dose. If adequate analgesia cannot be achieved with low doses, higher doses are administered. However, excessive dosing can lead to adverse effects such as nausea and vomiting, urinary retention, dizziness, and other serious complications, which can limit its clinical application [14]. In recent years, with the continuous improvements in anesthesia and analgesic technology, FNB technology has gradually been applied to artificial joint replacement surgery. Under ultrasound guidance, anesthetic drugs are injected around the local nerve stem to block impulse conduction; continuous FNB analgesics can relax the quadriceps femoris and effectively relieve pain in the front of the knee joint. However, the analgesic effect of FNB on pain in the medial and popliteal sockets of the knee joint is suboptimal [15–18], making FNB alone insufficient for the analgesic effect after knee surgery [19]. The distribution of nerves around the knee joint is very rich. The obturator nerve innervates the sensory area of the knee joint. Thus, lower extremity ONB immediately after the operation can avoid the extensive use of anesthetic drugs, which only improves the blocking effect, but also reduces adverse drug reactions [20, 21]. In this study, a multi-modal analgesic regimen led by nerve block was used. Ultrasound-guided FNB posterior tube was placed before starting the operation, whereas ONB and oral celecoxib were administered after the operation. If the patients still experienced severe pain, an intramuscular injection of parecoxib was administered to remedy the pain. This multi-modal analgesic method effectively relieved severe pain during the perioperative period, whereas there was no significant difference in the dosage of the postoperative remedial analgesic drug parecoxib. In this study, there was no significant difference in the resting VAS scores between the two groups after surgery, demonstrating that both methods can meet the analgesic requirements for UKA’s resting state after surgery. The VAS score in patients in the FNB combined with ONB group was significantly reduced during postoperative activities, showing that ONB blocks the sensory area that dominates the rear of the knee joint; it is more effective in relieving pain and allowing postoperative exercise of the knee.

The pain caused by knee osteoarthritis and the restriction of some motor functions often lead to significant gait changes, including pain-reducing and escape gaits. The pain-reducing gait occurs mainly due to knee pain, which is manifested in the patient as reduced standing time of the affected limb and a shortened step length. The escape gait occurs due to the decrease in the angle of movement of the knee joint, especially the decrease in the maximum flexion angle during the ground phase of the foot strike [22]. The preoperative gait parameters of 60 patients with knee osteoarthritis in this study were in line with prior research. To reduce pain and increase knee joint stability, patients continuously reduce the flexion angle of their knee joint during walking, increase the right angle of extension of their knee joint, and transfer the carrying load to the support side. In this study, the maximum hip flexion angle, VAS score, and KSS of postoperative patients after UKA improved compared with their preoperative values. However, the maximum knee flexion angle, ankle dorsiflexion angle, and ROM after surgery decreased compared with preoperative measurements. The reason for these changes may be that the UKA procedure itself is a type of trauma in older patients; it takes a long time for rehabilitation exercises after surgery. Kleijn et al. [23] demonstrated that the functional recovery in patients after UKA occurred more than 6 months to 2 years after surgery. Moreover, the knee flexion range reached its maximum at 1 year postoperatively. Although the hip joint is not directly innervated by the obturator nerve, improvements in hip flexion may reflect compensation within the kinetic chain. Alleviating knee pain during the stance phase can normalize weight transfer, reduce compensatory trunk tilt, and promote greater hip flexion through activation of the iliopsoas muscles. This observation aligns with findings associating knee pain with abnormal hip kinematics [24]. However, compared with the FNB group, the improvement in VAS and clinical scores in patients who received FNB combined with ONB was clearer. Additionally, the degree of abnormality in gait parameters was lower than that in the FNB group. The universal minimum clinically important difference (MCID) for kinematic gait parameters during the early rehabilitation period following UKA surgery has not yet been established in the literature. However, we could elucidate the significance of the observed improvements through functional benchmarks. The FNB + ONB group achieved an additional increase of approximately 5° in maximum knee flexion angle postoperatively, which facilitates heel-off clearance during the stance and swing phases of gait, reduces compensatory hip elevation, and possibly improves overall gait efficiency. Pain may be an important cause of abnormal gait in patients with knee osteoarthritis. Although ONB provides short-acting analgesia, its synergy with continuous FNB enables critical early activity. The immediate postoperative period is the peak of central sensitization [25]; effective dynamic pain control during this window enables patients to actively participate in rehabilitation. Early, pain-free activity disrupts the cycle of pain avoidance [26], promotes neuroplastic adaptation and prevents joint muscle inhibition. Consequently, normalized proprioceptive feedback accelerates neuromuscular re-education, leading to sustained functional improvement.

This study has some limitations. First, the evaluation indicators used in this study were not comprehensive and did not include other gait parameters. Second, the follow-up time in this study was short, and further observation of the long-term effects is needed, examining whether early biomechanical improvements could translate into long-term quality of life gains. Third, although we excluded patients with severe systemic diseases, common comorbidities (e.g., mild lumbar stenosis, early-stage diabetes) and pre-existing gait abnormalities were not systematically assessed or stratified. Such conditions might confound postoperative gait recovery by altering movement patterns or pain perception. Future studies should incorporate comorbidity indices and baseline gait assessments to control for these potential confounding factors. Finally, we acknowledge that our single-center and retrospective study design with relatively homogeneous patient population might limit the immediate extrapolation of our findings to all clinical settings. However, we also highlight that the ultrasound-guided nerve block techniques and gait analysis protocols we employed are standardized and reproducible, which could facilitate future validation in more diverse populations and multi-center settings. Smaller sample sizes might limit the ability to detect secondary endpoints (e.g., adverse reactions); results should be interpreted with caution. In the future, conducting large-scale multicenter studies to improve the robustness and generalizability of the results is necessary. Future large-scale, multicenter studies should incorporate patient-reported outcome measures, such as the Western Ontario and McMaster Universities Osteoarthritis Index and Short Form-36 scores, alongside biomechanical gait analysis. This integrated approach would provide a more comprehensive understanding of the influence of the intervention on both functional performance and patient-perceived quality of life. The MCID for kinematic indicators in this population also needs to be further clarified. This would help interpret the extent to which patients benefit from the combined use of ONB and FNB.

Conclusions

In summary, ONB combined with FNB under the guidance of ultrasound had a good effect on the postoperative active analgesic effect after UKA, promoted early rehabilitation exercise in postoperative patients, and reduced the degree of abnormal gait in patients after surgery, which was in line with the concept of rapid rehabilitation.

Abbreviations

BMI

Body mass index

FNB

Femoral nerve block

KSS

Knee Society Score

ONB

Obturator nerve block

ROM

Range of motion

UKA

Unicompartmental knee arthroplasty

VAS

Visual analog scale

MCID

Minimum clinically important difference

Author contributions

TY and YH were responsible for case data compilation and manuscript preparation. XL and JL were responsible for statistical analysis and literature review. HS and TM were responsible for case data collection. DC supervised the manuscript preparation.

Funding

This study was supported by the National Natural Science Foundation of China (82060408), the 2024 Key Scientific and Technological Achievement Transformation Project of the Autonomous Region (2024CJE09046), the Ningxia Clinical Medicine First-class Discipline Open Subject (NXYLXK2017A05), the Key Research and Development Program of Ningxia Hui Autonomous Region (2021BEG03049), the Innovation and Entrepreneurship Project of Ningxia Overseas Students (NRS2021-5), and the Open Research Fund of Ningxia Clinical Research Institute, People’s Hospital of Ningxia Hui Autonomous Region (2023KFZD01). The financial support for the project did not influence the views stated in the article or the analysis of the results of the study and their reporting.

Data availability

The datasets used during the current study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

This study was approved by the Ethics Committee of Ningxia Hui Autonomous Region People’s Hospital (approval number: [2024]-KJCG-001, date: February 19, 2024). We confirm that this study adhered to the Declaration of Helsinki and all methods were performed in accordance with the relevant guidelines and regulations. All patients and their families consented to the study.

Consent for publication

All patients and their families consented to the study’s publication.

Competing interests

The authors declare no competing interests.