Candy box technique with sutures and Nice knot

Abstract

Aims

Our study explores the candy box (CB) technique with sutures and Nice knot as a novel treatment for inferior pole patellar fractures, potentially superior to traditional wire fixation.

Methods

CT data from five adult knee joints were extracted to create finite element models for inferior pole patellar fractures and four internal fixation models. These included CB technique combined with high-strength sutures and Nice knot (CB-H), CB technique combined with tendon sutures and Nice knot (CB-T), CB technique combined with steel wires (CB-S), and tension-band wiring combined with cerclage wiring (TBWC). Displacement and stress distribution during knee flexion and extension were compared. Physical models were created using 3D printing technology. These models were then subjected to static tensile test and dynamic fatigue test. Data from 21 patients treated with CB-H and 22 patients treated with TBWC were analyzed to assess the effectiveness.

Results

Finite element analysis (FEA) indicated that displacements for CB-H and CB-T were below the failure threshold in all knee joint states. Stresses on the patella and internal fixation were lower in the CB-H and CB-T groups compared with the CB-S and TBWC groups. Both static and dynamic biomechanical experiments confirmed that displacements of CB-H and CB-T also remained below the failure threshold. In clinical research, the CB-H group outperformed the TBWC group in operating time, intraoperative blood loss, length of incision and time to clinical union, and complication control.

Conclusion

The CB technique combined with sutures and Nice knot can provide sufficient fixation strength for inferior pole patellar fractures. This method enables early functional exercise and avoids the need for secondary surgery. It could be a promising alternative to traditional TBWC surgery.

Cite this article: Bone Joint Res 2025;14(8):721–734.

Article focus

• Comparing candy box (CB) technique combined with sutures and the Nice knot to traditional internal fixation techniques, demonstrating that this could potentially be an emerging and effective treatment for inferior pole patellar fractures.

Key messages

• In finite element analysis (FEA), the suture-based fixation demonstrated lower bone stress and implant stress compared to traditional methods, while maintaining adequate internal fixation strength.

• Dynamic and static biomechanical tests showed that the CB technique with sutures and the Nice knot provided adequate internal fixation strength.

• Clinical results showed that the CB technique with sutures and the Nice knot achieved satisfactory outcomes, enabling early functional exercise and eliminating the need for secondary surgeries to remove internal fixation.

Strengths and limitations

• For FEA, we used models extracted from five volunteers instead of a single model.

• Our research integrates FEA and mechanical experiments, and culminates in the reporting of clinical data.

• The finite element model still requires further refinement and more comprehensive validation.

Introduction

Inferior pole patellar fractures account for approximately 9.3% to 22.4% of all patellar fractures and are considered extra-articular fractures.^1,2 Due to the anatomical location and traumatic characteristics of this fracture, it presents major challenges to surgical treatment.³ During fracture, the inferior pole of the patella often experiences concentrated stress, leading to comminuted fragments due to both direct external forces and the indirect tension of the patellar ligament.^4,5 As the attachment point of the patellar ligament, the inferior pole is crucial for enhancing the quadriceps force arm and knee extension.^6,7 Furthermore, the mechanical integrity of the patella is directly related to normal knee function. Loss of this integrity not only impairs knee extension but can also disrupt the anatomical and biomechanical properties of the patellofemoral joint, potentially causing severe damage to overall knee joint function.⁸ Therefore, accurate treatment of inferior pole patellar fractures is essential.⁹

However, standardized treatment for inferior pole patellar fractures has not yet been established.^10,11 The unique position and function of the patella in the knee joint require careful consideration of factors such as biomechanics, soft-tissue injury, and rehabilitation needs when selecting treatment methods.¹² While metal implants such as steel wires and Kirschner wires are commonly used to ensure bone stability and aid healing in patellar fractures, challenges and controversies persist.¹³ Steel wires, as rigid materials, are prone to friction and collision with surrounding soft-tissues during knee movement, potentially causing knee pain, bone abrasion, and surgical site infections.^14-17 For instance, Lazaro et al,¹⁸ who followed up on 30 patients with patellar fractures who had metal internal fixation implants, found that 11 of these patients requested removal of the internal fixation due to metal irritation, and four patients experienced breakage of the implanted material. Moreover, some patients may request further surgery to remove the implant after the fracture has healed.^16,19 However, after a second surgery to remove the implant, patients may still face challenges in rehabilitation and require an extended recovery period to achieve optimal results. This not only increases the number of surgical procedures and burden on patients, but may also have a substantial negative impact on their mental health.²⁰

Recently, suture anchoring techniques have gained much attention for their effectiveness in treating rotator cuff injuries, ankle ligament injuries, and certain fractures.^21,22 This technique offers an easy operation and minimal soft-tissue irritation, and eliminates the need for a second surgery to remove it. Moreover, the sutures attached to the anchors are exceptionally strong, allowing patients to engage in early-stage functional exercises.^23,24

Despite advancements in suture anchoring technique, challenges remain. The high cost has hindered its widespread adoption in clinical settings. Moreover, elderly patients with osteoporosis may face issues with the anchor’s holding force, potentially increasing the risk of retraction and fixation failure.^25,26 In this context, high-strength sutures present a promising alternative, offering comparable strength to steel wire for internal fixation while being more affordable than suture anchors, and this affordability facilitates their widespread promotion and use in clinical practice.^27,28

The Nice knot is a suture fixation technique that offers wide applicability, strong fixation, ease of use, and positive outcomes.²⁹ The technique combines double stitching with a sliding knot design to provide dual fixation strength.³⁰ Uniquely, it allows surgeons to make adjustments according to surgical requirements, enabling tailored operations for individual patients.³¹ Collin et al³² mechanically tested Nice knots with high-strength sutures, and found a mean failure load of 527.01 N (SD 50.03), a mean stiffness of 77.38 N/mm (21.20), and a mean dynamic stiffness of 225.36 N/mm (SD 49.12). These results demonstrate the excellent mechanical properties of Nice knot fixation with high-strength sutures.

Therefore, it is essential to investigate an internal fixation treatment that reduces irritating symptoms, supports early functional exercise, eliminates the need for secondary surgery, and is suitable for widespread clinical use in treating inferior pole patellar fractures.

Candy box (CB) fixation with stainless steel wires for treating inferior pole patellar fractures has previously been developed (Figure 1). Fan et al³³ demonstrated the biomechanical advantages of this technique by FEA and biomechanical tests. In the present study, we replaced the fixation material from steel wires with sutures and used the Nice knot technique for fixation. Through FEA and biomechanical tests, we characterized the advantages of CB fixation with high-strength sutures and Nice knotting. The technique was applied on 21 patients, whose laboratory and clinical results we report here.

Fig. 1.

Schematic diagram of Candy box (CB) technique. A) Front view of CB technique. B) Side view of CB technique. a) Three separate vertical steel wires. b) Upper 1/3 steel wires of the patellar tunnel. c) Middle 1/3 steel wires of the patellar tunnel.

Methods

FEA: patellar model

Five healthy adult volunteers participated in the study after providing written informed consent (Table I). The knee was maintained in a neutral position, and scanned from the lower femur to the upper tibia by spiral CT (CT, Siemens 128, Germany; slice thickness: 0.6 mm, slice spacing: 0.6 mm, resolution: 512 × 512 pixels).³⁴ Bony structures were differentiated by threshold segmentation and region growing (Mimics Research 21; Materialise, Belgium) to create a 3D model of the patella. The model was processed by meshing, wrapping, and smoothing (Geomagic 2021; Geomagic, USA). Although local density variations may exist within the cortical and trabecular bone of the patella, we used a uniform Young’s modulus of 10,000 MPa for cortical bone and 840 MPa for cancellous bone to focus on comparing the mechanical performance of different internal fixation techniques and to ensure experimental consistency.³⁵ All materials were assumed to be homogeneous, linearly elastic, and isotropic. Finally, the inferior pole fracture model (AO/OTA 34-A1) was created with SolidWorks 2021 (Dassault Systèmes, France).

Table I.

Information of volunteers for finite element analysis models.

Volunteer	Sex	Age, yrs	Side	Height, cm	Weight, kg	BMI, kg/m2
Volunteer 1	Male	30	Left	175	65	21.22
Volunteer 2	Male	60	Left	158	56	22.43
Volunteer 3	Female	24	Right	160	49	19.14
Volunteer 4	Male	18	Right	167	60	21.51
Volunteer 5	Female	50	Right	155	58	24.14

FEA: internal fixation model

The fracture model was imported into ANSYS Workbench 2021 (Swanson Analysis, USA). Mechanical parameters configured for the bones, wires, and sutures are shown in Table II.^35-39 Frictional contacts were configured for bone-bone (coefficient of friction: μ = 0.45), bone-implant (μ = 0.3), and implant-implant interfaces (μ = 0.2).^{33,36,38,40-42} Four sets of internal fixation were established with SolidWorks 2021: CB technique combined with high-strength sutures and Nice knot (CB-H), CB technique combined with tendon sutures and Nice knot (CB-T), CB technique combined with steel wires (CB-S), and tension-band wiring combined with cerclage wiring (TBWC) (Figure 2). The models were meshed using quadratic tetrahedral elements (C3D10). A convergence analysis was performed to ensure that the difference caused by further grid refinement did not exceed 5%. Considering stress singularities and computational efficiency, we ultimately selected a mesh size of 0.75 mm for the patella and metal implants, and 0.5 mm for the sutures. Further details of the mesh convergence analysis are provided in the Supplementary Material. Given that the inferior pole of the patella is primarily constrained by the patellar ligament and has a limited range of motion (ROM), and to ensure computational stability while focusing on the comparison of the mechanical performance of different internal fixation techniques, a fixed boundary condition was applied to the inferior pole of the patella, while loads were applied to the upper pole. Compression Only Support plugin (an internal component in ANSYS Workbench 2021; Ansys) was used behind the patella to simulate the medial and lateral condyles of the femur. To simulate knee joint extension and flexion states, a 500 N load was applied at both 0° and 45° to the long axis of the patella in each model.⁴³ For knee joint extension (0°), the plugin supported the lower one-third to one-half of the posterior patellar articular surface, and at 45° flexion, the entire posterior surface was supported (Figure 3).^43-46 Finally, the von Mises stress and displacement distribution of the models were calculated.

Table II.

Model material parameters.

Material	Elastic modulus, MPa	Poisson ratio
Cortical bone35	10,000	0.30
Cancellous bone35	840	0.29
Steel wire35	100,000	0.29
Kirschner wire36,37	200,000	0.30
High-strength suture38	3,000	0.40
Tendon suture39	2,757	0.40

Fig. 2.

Schematic diagram of different internal fixation techniques. a) Candy box (CB) technique combined with high-strength sutures and Nice knot (CB-H). b) CB technique combined with tendon sutures and Nice knot (CB-T). c) CB technique combined with steel wires (CB-S). d) Tension-band wiring combined with cerclage wiring (TBWC).

Fig. 3.

Schematic diagram of patella loading. a) Boundary and loading conditions of simulated knee extension (0° axial load). b) Boundary and loading conditions of simulated knee flexion (45° bending load).

Biomechanical experiments

In our biomechanical experiments, we conducted both static tensile and dynamic fatigue tests. The models were created using acrylonitrile butadiene styrene (ABS) resin with the aid of a 3D printing lithography machine.^33,47 Four groups of internal fixations were applied to the models by an experienced surgeon (YY) (Figures 4a to 4d): CB-H, CB-T, CB-S, and TBWC (n = 5/group).

Fig. 4.

Schematic diagram of biomechanical experiments and results. a) 3D printing model of candy box (CB) technique combined with high-strength sutures and Nice knot (CB-H). b) 3D printing model of CB technique combined with tendon sutures and Nice knot (CB-T). c) 3D printing model of CB technique combined with steel wires (CB-S). d) 3D printing model of tension-band wiring combined with cerclage wiring (TBWC). e) Statistical diagram of static tensile biomechanical experiments. f) Schematic diagram of low-cycle fatigue test results. g) Schematic diagram of high-cycle fatigue test results. One-way analysis of variance was used for statistical analyses (***p < 0.001; ns, non-significant).

For the static tensile test, the models were fixed and pre-tensioned on an electronic biomechanical testing machine (Instron ElectroPuls E3000). Constraints were applied at the inferior pole of the patella, and load was applied to its upper pole at a rate of 1 mm/min until reaching 500 N tension. Then, we calculated the mean displacement of each type of internal fixation at 500 N.

In dynamic fatigue experiments, models were fabricated, fixed, and pre-tensioned using the same methodology. Constraints were applied to the inferior pole of the patella, while loads were applied to the upper pole for dynamic fatigue testing at 200 ± 120 N for 10,000 cycles with a frequency of 4 Hz. The results of the low cycle dynamic fatigue test were based on data from the first 1,000 cycles, while data from cycles 1,000 to 10,000 determined the results of the high cycle dynamic fatigue test.

Clinical research: retrospective case control study

All patients signed informed consent forms. The clinical and follow-up data of 43 patients with inferior pole patellar fractures admitted to our hospital between January 2020 and May 2023 were retrospectively analyzed. The CB and TBWC groups included 21 and 22 patients, respectively. The inclusion criteria were as follows: 1) diagnosis of inferior pole patellar fractures, 2) age > 18 years, 3) absence of life-threatening complications before operation, and 4) informed consent and complete clinical data. The exclusion criteria were as follows: 1) presence of concurrent fractures such as tibial plateau or distal femoral fractures, 2) concomitant tumours or serious internal diseases, 3) lost to follow-up or with follow-up time of less than 12 months, and 4) refusal to sign the informed consent form.

Clinical research: surgical procedure

After disinfecting the surgical area, a longitudinal incision was made at the centre of the knee joint to expose the fractured end of the patella. Three longitudinal bone tunnels and two transverse bone tunnels were created within the patella. Then, three No. 2 Ultrabraid high-strength sutures with double strands were passed through the longitudinal tunnels, and two single strands were passed through the transverse tunnels. Subsequently, three sets of double vertical strands and two single horizontal strands were passed beneath the patellar ligament to stabilize the inferior pole fracture of the patella. The Nice knot was used to secure the double sutures. Finally, passive flexion and extension of the knee joint were performed to assess the stability of the fracture fixation and knee joint movement (Figure 5).

Fig. 5.

Schematic diagram of the surgical procedure. a) Adequate exposure of the fracture ends. b) Three longitudinal bone tunnels and two transverse bone tunnels. c) and d) Three No. 2 Ultrabraid high-strength double strands pass through the longitudinal tunnels, and two single strands pass through the transverse tunnels. e) Fracture reduction under direct vision, Nice knot compression.

Clinical research: follow-up data collection

For all patients enrolled, we collected preoperative data such as sex, age, BMI, cause of injury, complications, and preoperative visual analogue scale (VAS) score for pain assessment (0 to 10 scale, with 0 indicating no pain and 10 indicating the most severe pain).^48,49 In addition, we collected intraoperative data such as operating time, intraoperative blood loss, and incision length. Finally, we collected postoperative data such as time to clinical union, Bostman score,⁵⁰ ROM of affected knee joints, and VAS score as well as postoperative complications.

Statistical analysis

The data were analyzed using Prism 9.0 statistical software (GraphPad Software, USA) and RStudio 4.4.1 (Posit, USA). Continuous variables were assessed for normal distribution with the Shapiro-Wilk test. Data following a normal distribution were reported as mean (SD) ($\overline{x}\pm s$), while non-normally distributed data were presented as median and IQR. For categorical data, we employed the chi-squared test or Fisher’s exact test as appropriate. An independent-samples t-test was used to analyze the two independent groups. One-way analysis of variance (ANOVA) compared displacement values among internal fixation groups under the same load, with pairwise comparisons between groups conducted using least significant difference (LSD) t-tests. The significance level was set at p < 0.05.

Results

FEA

We analyzed the displacement distribution, stress distribution, maximum displacement values, and maximum stress values of the models in each group under 0° (500 N) and 45° (500 N) loading conditions. The results showed that the maximum displacement in all groups was significantly lower than the failure threshold of 3 mm for inferior pole patellar fractures (Figure 6 and Table III). Additionally, we calculated the maximum stress values for each group under the same loading conditions. The results indicated that the maximum stress on the internal fixations and patella in the CB-H and CB-T groups was significantly lower than that in the CB-S and TBWC groups (Figure 7 and Figure 8, Table III).

Fig. 6.

Displacement schematic diagram of different internal fixation groups. a) Displacement nephogram of model 1 under 500 N force at 0° angle. b) Displacement nephogram of model 1 under 500 N force at 45° angle. c) Maximum displacement statistical diagram for different models under 500 N force at 0° angle. d) Maximum displacement statistical diagram for different models under 500 N force at 45° angle. One-way analysis of variance was used for statistical analyses (*p < 0.05; ns, non-significant). CB-H, candy box (CB) technique combined with high-strength sutures and Nice knot; CB-S, CB technique combined with steel wires; CB-T, CB technique combined with tendon sutures and Nice knot; TBWC, tension-band wiring combined with cerclage wiring.

Table III.

Results of finite element analysis and biomechanical experiments. All data are shown as mean (SD) unless otherwise stated.

Experimental methods	FEA (500 N）						Biomechanical experiments
Group	Max displacement 0° (mm)	Max displacement 45° (mm)	Max stress 0° (internal fixations, MPa)	Max stress 45° (internal fixations, MPa)	Max stress 0° (patellar, MPa)	Max stress 45° (patellar, MPa)	Static (500 N, mm)	Fatigue (200 ± 120 N, 1,000 cycles, mm)	Fatigue (200 ± 120 N, 10,000 cycles, mm)
CB-H	0.76 (0.17)	0.21 (0.07)	325.09 (72.31)	194.82 (80.64)	114.42 (47.87)	61.12 (23.17)	1.31 (0.06)	1.11 (0.14)	1.35 (0.16)
CB-T	0.81 (0.21)	0.24 (0.07)	318.81 (72.24)	204.37 (49.94)	133.61 (49.38)	72.54 (45.88)	1.88 (0.13)	1.64 (0.32)	1.73 (0.35)
CB-S	0.39 (0.14)	0.19 (0.06)	Median 1,098.00 (IQR 919.50 to 1107.00)	419.04 (107.22)	273.41 (64.53)	104.81 (66.63)	1.17 (0.08)	0.73 (0.08)	0.80 (0.08)
TBWC	0.71 (0.34)	0.15 (0.04)	1,193.47 (111.54)	631.63 (262.45)	208.03 (73.46)	96.12 (11.72)	2.09 (0.22)	1.71 (0.08)	1.78 (0.10)

CB-H, candy box technique combined with high-strength suture and Nice knot; CB-S, candy box technique combined with steel wires; CB-T, candy box technique combined with tendon sutures and Nice knot; FEA, finite element analysis; TBWC, tension-band wiring combined with cerclage wiring.

Fig. 7.

Schematic diagram of stress on internal fixations. a) Stress nephogram of internal fixations of Model 1 under 500 N force at 0° angle. b) Stress nephogram of internal fixations of Model 1 under 500 N force at 45° angle. c) Statistical diagram of maximum stress on internal fixations of different models under 500 N force at 0° angle. d) Statistical diagram of maximum stress on internal fixations of different models under 500 N force at 45° angle. One-way analysis of variance was used for statistical analyses (*p < 0.05; ***p < 0.001; ns, non-significant). CB-H, Candy box (CB) technique combined with high-strength sutures and Nice knot; CB-S, CB technique combined with steel wires; CB-T, CB technique combined with tendon sutures and Nice knot; ns, non-significant; TBWC, tension-band wiring combined with cerclage wiring.

Fig. 8.

Schematic diagram of stress of the patella under 500 N force. a) Stress nephogram of the patella of model 1 under 500 N force at 0° angle. b) Stress nephogram of the patella of model 1 under 500 N force at 45° angle. c) Statistical diagram of maximum stress on the patella in different models under 500 N force at 0° angle. d) Statistical diagram of maximum stress on the patella in different models under 500 N force at 45° angle. One-way analysis of variance was used for statistical analyses (*p < 0.05; ** p < 0.01; ns, non-significant). CB-H, candy box (CB) technique combined with high-strength sutures and Nice knot; CB-S, CB technique combined with steel wires; CB-T, CB technique combined with tendon sutures and Nice knot; TBWC, tension-band wiring combined with cerclage wiring.

Biomechanical experiments

In the biomechanical experiments, static tensile and dynamic fatigue tests were conducted to evaluate the biomechanical performance of each group. The static tensile results indicated that the CB-S and CB-H groups exhibited smaller displacements, while the CB-T and TBWC groups showed larger displacements. The results from both low-cycle (1,000 cycles) and high-cycle (10,000 cycles) fatigue tests were consistent, with the CB-S group demonstrating the smallest displacement and superior stability and durability. The CB-H group followed, while the CB-T and TBWC groups showed larger displacements.

Additionally, after static tensile testing at a 500 N load and 10,000 cycles of dynamic fatigue testing, no failure or fracture of the internal fixation occurred in any group, and the displacements did not exceed the failure threshold for inferior pole patellar fractures (Figure 4, Table III).

Clinical outcomes: comparison of clinical efficacy between CB-H and TBWC

There was no statistically significant difference (p > 0.05) between the CB-H group and TBWC group in terms of sex, age, BMI, injury side, cause of injury, and comorbidities. The two groups were comparable (Table IV). A typical case from the CB-H group is illustrated in Figure 9.

Table IV.

Baseline characteristics of the enrolled patients.

Characteristic	CB-H (n = 21）	TBWC (n = 22）	p-value
Sex, n (%)			0.223*
Male	14 (66.7)	10 (45.5)
Female	7 (33.3)	12 (54.5)
Mean age, yrs (SD)	51.33 (16.74)	55.45 (12.52)	0.364†
Mean BMI, kg/m2 (SD)	23.31 (2.62)	24.64 (2.82)	0.119†
Fracture side, n (%)			0.760*
Left	13 (61.9)	12 (54.5)
Right	8 (38.1)	10 (45.4)
Injury mechanism, n (%)			0.651*
Tumble	15 (71.4)	18 (81.8)
Car accident	4 (19.1)	3 (13.6)
High fall injury	2 (9.5)	1 (4.5)
Comorbidities, n (%)			0.951*
Hypertension	4 (19.0)	4 (18.2)
Diabetes	3 (14.2)	2 (9.1)
Heart disease	1 (4.8)	0 (0)
Osteoporosis	3 (14.3)	4 (18.2)
None	10 (47.6)	12 (54.5)
Median follow-up, mths (IQR)	15.00 (13.00 to 20.50）	16.50 (14.00 to 19.25）	0.217‡

^*Fisher's exact test.

^†Independent-samples t-test.

^‡Mann–Whitney U test.

CB-H, candy box technique combined with high-strength suture and Nice knot; TBWC, tension-band wiring combined with cerclage wiring.

Fig. 9.

Typical case and clinical outcomes. a) to h) Typical case of CB-H, a 33-year-old female with inferior pole patellar fracture due to a fall. a) Preoperative anteroposterior radiograph. b) Preoperative lateral radiograph. c) Lateral radiograph at one month postoperatively. d) Lateral radiograph at one year postoperatively. e) to h) Functional photos at one year postoperatively. i) to o) Schematic diagram of clinical outcomes. i) Length of incision. j) Intraoperative blood loss. k) Time to clinical union. l) Operation duration. m) Bostman score. n) Visual analogue scale (VAS) score. o) Range of motion (ROM). CB-H, candy box technique combined with high-strength suture and Nice knot; TBWC, tension-band wiring combined with cerclage wiring. An independent-samples t-test and one-way analysis of variance were used for statistical analyses (**p ＜ 0.01, ***p ＜ 0.001; ns, non-significant).

The median incision length for the CB-H group was 6.00 cm (IQR 5.00 to 7.00) compared to 9.00 cm (8.00 to 10.00) for the TBWC group; mean intraoperative blood loss was 55.21 ml (SD 9.28) for CB-H and 89.55 ml (SD 14.55) for TBWC; mean operation duration was 59.76 minutes (SD 5.80) for CB-H and 69.55 minutes (SD 6.71) for TBWC; mean time to clinical union was 7.23 weeks (SD 1.57) for CB-H and 10.41 weeks (SD 1.47) for TBWC. During the first six months of follow-up, Bostman scores and ROM were higher in the CB-H group than in the TBWC group, but no significant differences were observed after one year of follow-up. Additionally, VAS scores were significantly lower in the CB-H group than in the TBWC group from five days to six months post-surgery, with no significant difference observed after one year of follow-up (Figures 9i to 9o).

A few patients in the CB-H group experienced soft-tissue irritation symptoms within the first six months of follow-up, which gradually resolved with increased functional exercise. The TBWC group had a higher number of patients with soft-tissue irritation symptoms at each follow-up period compared to the CB-H group. Furthermore, two patients in the TBWC group experienced loosening of internal fixation, and one required revision surgery due to fixation failure caused by reseparation of the fracture ends (Table V).

Table V.

Complications of enrolled patients.

Complication	CB-H, n (%) (n = 21）	TBWC, n (%) (n = 22）	p-value*
Irritation of the soft-tissues (3 mths follow-up)	5 (23.8）	15 (68.1）	0.006
Irritation of the soft-tissues (6 mths follow-up)	2 (9.5）	9 (40.9）	0.033
Irritation of the soft-tissues (12 mths follow-up)	-	5 (22.7）	-
Loosening of internal fixation (12 mths follow-up）	-	2 (9.1）	-
Internal fixation failure/secondary surgery (during the entire follow-up)	-	1 (4.5）	-

^*Fisher’s exact test.

CB-H, candy box technique combined with high-strength suture and Nice knot; TBWC, tension-band wiring combined with cerclage wiring.

Discussion

To address the challenges in treating inferior pole patellar fractures, this study, building upon previous research, replaced the steel wires in the CB technique with different types of suture materials. Through FEA, biomechanical experiments, and clinical studies, we showed the benefits and feasibility of combining CB technique with sutures and Nice knot to treat inferior pole patellar fractures. In addition, compared to high-strength sutures, tendon sutures are more cost-effective. Therefore, we speculate that if tendon sutures can provide sufficient internal fixation strength as internal fixation materials, this may further reduce treatment costs. As a consequence, in the section of FEA and biomechanical experiments, we established the CB-T group in addition to the TBWC, CB-H, and CB-S groups.

Previous medical research and clinical data indicate that when the knee joint is extended, the maximum average tension exerted by the quadriceps tendon on the patella is 316 N.⁵¹ To improve the simulation of the knee joint motion and loading conditions of patients in real life, we set the conditions for dynamic fatigue experiments at 200 ± 120 N and evaluated the durability and stability of internal fixation materials over 10,000 cycles. Additionally, to verify the initial stability of internal fixation, we applied a force of 500 N in FEA and static tensile tests. Furthermore, based on biomechanical data and clinical experience, we define a 3 mm displacement at the fracture site as the threshold for internal fixation failure.^52-54

Studies have shown that there are variations in knee joints among individuals. Therefore, to reduce heterogeneity and enhance the generalizability of the results, we selected CT data from five volunteers for modelling.⁵⁵ In the FEA, we calculated the distribution of stress and displacement of the patella during both flexion and extension states. The results indicated that the displacements of all groups did not exceed the failure threshold (3 mm) in either state, demonstrating the stability and reliability of the various internal fixation methods used in this study. Both the CB-H and CB-T groups, which used sutures for internal fixation, exhibited lower internal fixation stress and patellar stress. This could be attributed to the suture material’s characteristics – its lower rigidity may reduce stress shielding effects during application, maintain biological activity at fracture sites, minimize bone cutting risks, and enhance fracture healing processes. Thus, using sutures for internal fixation may not only help reduce complications associated with steel wire materials, but also potentially improve treatment outcomes and accelerate patient recovery processes.

To explore whether the combination of CB technique and sutures can provide sufficient internal fixation strength and support early functional rehabilitation in the treatment of inferior pole patellar fractures, we also conducted biomechanical experiments. The static tensile test results indicate that none of the groups met the failure criteria, demonstrating adequate initial strength in the internal fixation methods. The CB-H and CB-S groups exhibited minimal displacement, suggesting superior initial stability. Although the CB-T and TBWC groups had slightly larger displacements, they still did not approach the failure criteria. In both low-cycle and high-cycle dynamic fatigue tests, the CB-S group exhibited the smallest displacement after varying numbers of cycles, indicating excellent fatigue resistance. The CB-H, CB-T, and TBWC groups followed with slightly larger displacements. Notably, even after 10,000 cycles, all groups were still far from reaching the critical threshold for fixation failure. While the CB-S group showed minimal displacement at the fracture site in fatigue tests, the mean maximum displacements for the CB-H and CB-T groups after 10,000 cycles were as low as 1.35 mm (SD 0.16) and 1.73 mm (SD 0.35), respectively — well within safe limits.

Based on our FEA and biomechanical experimental results, we tentatively conclude that the combination of CB technique with sutures and Nice knot technique may provide sufficient strength for fixation of inferior pole patellar fractures, allowing early functional exercise for patients. The biomechanical experimental results also suggest that high-strength sutures exhibit superior characteristics in terms of stability and durability compared to tendon sutures. Consequently, in our clinical research, we reviewed cases where high-strength sutures were used as internal fixation materials and compared this approach with the classic TBWC technique to evaluate its effectiveness in treating inferior pole patellar fractures.

In the section of clinical research, as shown in Figure 9, the CB-H group outperformed the TBWC group in various indicators, including operating time, intraoperative blood loss, incision length, and time to clinical union. Although there were no significant differences in ROM, Bostman score, and VAS score between the two groups at the final follow-up, patients in the CB-H group demonstrated better early postoperative functional recovery and pain management.

When observing complications, as shown in Table V, we noted a higher incidence of soft-tissue irritation symptoms in the TBWC group compared to the CB-H group throughout all follow-up stages. This difference may be attributed to the use of sutures for internal fixation in the CB-H technique, which could effectively reduce soft-tissue irritation and pain levels, leading to improved early functional recovery and better clinical outcomes for patients.

During the early follow-up period for the CB-H group, we noticed some patients experiencing soft-tissue irritation symptoms (Table V). We speculate that this may be related to the suture knot’s inappropriate placement during surgery. Additionally, we observed that over time, the suture material’s characteristics may help resolve these symptoms naturally. In subsequent surgeries, we optimized the position of the suture knot and placed it on the upper pole or side of the patella, thereby effectively avoiding complications such as soft-tissue irritation.

In addition, as shown in Table V, in the TBWC control group, we observed two cases of loose internal fixation and one case requiring a second surgery due to internal fixation failure. These issues not only increase the demand on healthcare resources but also cause additional physical and psychological distress for patients. In contrast, no internal fixation-related problems were found in the CB-H group throughout the follow-up period, highlighting the potential safety advantages and superior complication management capabilities of this technique.

In the treatment of inferior pole patellar fractures, we have adopted an innovative surgical approach combining CB technique with sutures and the Nice knot. This method offers notable advantages over traditional steel wires or Kirschner wire techniques. Firstly, the surgical area does not involve the upper part of the patella, eliminating the need to detach the quadriceps tendon during surgery. This simplifies surgical steps and results in a statistically significant reduction in operating time and incision size, easing postoperative pain for patients. These conditions promote early functional exercise and speed up recovery. Secondly, the success of this surgical approach lies not only in using sutures as internal fixation materials but, more importantly, in integrating the advantages of Nice knot, biomechanical properties of high-strength sutures, and bone tunnel advantages established by the CB technique. This comprehensive treatment method ensures sufficient internal fixation strength and provides favourable conditions for healing inferior pole patellar fractures. Additionally, compared to traditional wire fixation methods, using sutures as internal fixation materials avoids stimulation to soft-tissues from wires and potential complications, which further enhances treatment safety and patient comfort.⁴³ Finally, since this type of internal fixation material does not require a second surgery for removal, patients can avoid additional surgical risks and costs while recovering to normal life and work activities in a shorter period of time.

This study still has several limitations. In the finite element model, although different elastic moduli were assigned to cortical and cancellous bone, the internal variations within these bone types were not further refined. Additionally, the fixed boundary condition at the distal patella may not fully reflect the actual in vivo biomechanical environment. Regarding failure criteria, our study primarily referenced displacement values but has not explored potential failure mechanisms caused by localized stress concentrations. Although biomechanical experiments were conducted, validation of the finite element model remains incomplete. Moreover, the clinical aspect of this study is limited by a small sample size and a relatively short follow-up period. Future studies will incorporate more comprehensive analytical methods, such as maximum/minimum principal stress or strain, to further investigate localized failure mechanisms. Efforts will also be made to refine the bone material model and boundary conditions, incorporating non-linear materials and anisotropic properties, along with more extensive validation, to improve the accuracy and reliability of FEA. Furthermore, a multicentre, prospective study design may be employed to enhance the reliability and applicability of our findings.

In conclusion, the CB technique, in combination with sutures and Nice knot, not only provides sufficient fixation strength for the treatment of inferior pole patellar fractures but also meets the needs of early functional rehabilitation for patients, effectively reducing the level of pain. This innovative surgical approach has achieved satisfactory therapeutic outcomes, eliminating the need for secondary surgeries, and presents a potential alternative to traditional TBWC surgery. Through further clinical research and practical applications, we hope to provide a safer and more effective treatment option for patients with inferior pole patellar fractures.

Data Availability

The data that support the findings for this study are available to other researchers from the corresponding author upon reasonable request