Biomechanical Study on the Stress Distribution of the Knee Joint After Tibial Fracture Malunion with Residual Varus–Valgus Deformity

Abstract

Objective

To investigate the effect of residual varus and valgus deformity on the stress distribution of the knee joint after tibial fracture malunion.

Methods

Fourteen adult cadaver specimens were selected to establish the models of tibial fractures, which were fixed subsequently at neutral position (anatomical reduction) and malunion positions (at 5°, 10°, and 15° valgus positions, and 5°, 10°, and 15° varus positions). The stress distribution on the medial and lateral plateau of the tibia was quantitatively measured using ultra‐low‐pressure sensitive film technology. The changes in the stress distribution of the knee joint after tibial fracture malunion and the relationship between the stress values and the residual varus or valgus deformity were analyzed.

Results

Under 400 N vertical load, the stress values on the medial and lateral plateau of the tibia at the neutral position were 1.137 ± 0.139 MPa and 1.041 ± 0.117 MPa, respectively. When compared with the stress values measured at the neutral position, the stress on the medial plateau of the tibia was significantly higher at varus deformities and lower at valgus deformities, and the stress on the lateral plateau was significantly higher at valgus deformities and lower at varus deformities (all P < 0.05). The stress values on the medial plateau of the tibia were significantly higher than the corresponding data on the lateral plateau at neutral and 5°, 10°, and 15° varus deformities, respectively (all P < 0.05), and significantly lower than the corresponding data on the lateral plateau at 5°, 10°, and 15° valgus deformities, respectively (all P < 0.05).

Conclusion

Residual varus and valgus deformity after tibial fracture malunion can lead to obvious changes of the stress distribution of the knee joint. Therefore, tibial fractures should be reduced anatomically and fixed rigidly to avoid residual varus–valgus deformity and malalignment of lower limbs.

Introduction

The incidence of proximal tibial fractures due to high‐energy trauma has increased gradually, today accounting for 5%–11% of all tibial fractures1, 2, 3. Surgical treatment of these traumas remains a major clinical challenge4, 5, 6, and functional recovery is affected by various factors, including the injury mechanism, the severity of the initial injury, treatment options, and the quality of fracture reduction. Although continuous advances in internal fixation devices and surgical techniques have improved the therapeutic effect of tibial fractures, malunion of the proximal tibia fracture and malalignment of lower extremities still occur7, 8. Greenwood et al.9 compared 398 tibia fracture patients with 1573 age‐matched and gender‐matched populations and found that the incidence of knee pain and dysfunction in the fracture group was significantly higher than that in the control group. Malunion of fractures can disrupt normal joint movement, leading to non‐physiological stress and traumatic arthritis (TA) of the knee10, 11. Malunion and malalignment of lower limbs after fracture healing is an important factor for knee and ankle joints to develop TA. Studies show that an individual with tibial fractures that heal with residual deformity may have increased risk of pain and stiffness in knee and ankle joints12.

To identify the pathogenesis of traumatic arthritis and the effects of different interventions for traumatic fractures of lower extremities, some scholars have conducted research using techniques including imaging, biomechanics, and finite element analysis13, 14, 15. Kettelkamp et al.16 conducted a long‐term follow‐up study of patients with femoral shaft fractures, and found that knee TA with obvious local symptoms can occur after 32 years due to residual deformity and long‐term weight‐bearing. Patients with previous knee fractures who have severe arthritis and need total knee arthroplasty are not uncommon. Accordingly, improved biomechanical understanding of tibial varus or valgus malunions will help to formulate improved treatment algorithms.

Therefore, a biomechanical study was conducted using the cadaver models of middle and upper tibial fractures. The fractures were fixed at different varus and valgus deformities, and the stress distribution on the medial and lateral plateau of the tibia under weight‐bearing load were measured. The purpose of the present study was to: (i) investigate the changes of the stress on the medial and lateral plateaus after tibial fracture malunion through comparison with normal stress; (ii) identify the relationship between the stress on the plateaus and the residual varus or valgus deformity; and (iii) highlight to the surgeon the importance of reducing the risk of residual varus and valgus deformity during the treatment of tibial fractures.

Materials and Methods

The present study has been reviewed and approved by the Institutional Review Board (IRB) of the Third Hospital of Hebei Medical University. The experimental flowchart of this biomechanical study is presented in Fig. 1.

Figure 1.

The experimental flowchart of the biomechanical study illustrated the main procedures, including specimen preparation, establishment of tibial fracture and malunion models, assembly of specimens on the biomechanical testing machine, ultra‐low‐pressure sensitive film preparation, stress measurement and read, data collection, and statistical analysis.

Specimen Preparation

Fourteen formalin‐soaked specimens were all from adult male donated bodies. The body height of the specimens was 171 cm on average (range, from 163 to 181 cm). The age when donated was 55 years on average (range, from 42 to 65 years old). Samples from those who had rheumatism, tuberculosis, or tumors were excluded. The specimens were examined with digital radiography and excluded if they were identified to have osteoporosis, pathological or anatomical deformity, irregular joint surface, or other imaging abnormalities. All samples were required to have intact ligaments and tendons around the knee joint and an unbroken joint capsule. The muscular tissue of each specimen was removed. Then, the upper part of the femur and the distal part of the tibial and fibula were dissected. The remaining femur and tibial and fibula were both approximately 25 cm in length. The specimens were maintained in standby packages to prevent dehydration and kept at −20°C for cryopreservation.

Establishment of Tibial Fracture Malunion Model

The cadaver specimens were thawed at room temperature for 12 hours before the experiment. The middle and upper tibial fracture models were created, and fixed using locking plates and screws at various residual valgus deformities (5°, 10°, or 15°), neutral position (0°, anatomically reduced), or varus deformities (5°, 10°, or 15°), respectively. A horizontal incision approximately 3–4 cm in length was made at the level of the joint space on each side. The subcutaneous fat was separated, the bursa was cut, and the joint space was exposed. The anterior and posterior cruciate ligaments and the medial and lateral menisci were preserved so as not to affect the normal distribution of the contact stress of the knee joint.

Specimen Assembled to Biomechanical Testing Machine

The specimen was first erected, and the femur end was fixed perpendicularly to the homemade clamp with the use of denture base resin and solution (type II self‐setting dental powder and tray water). After the dental powder and tray water solidified, the tibial end of the specimen was fixed in the same way. Then the two‐ends of the clamp were assembled to the biomechanical testing machine (Electroforce 3520‐AT, Bose company, USA), and the fixed position of the femoral and tibia stumps was adjusted so that the lower limb mechanical axis was close to the position when standing naturally (Fig. 2).

Figure 2.

The specimens were assembled to the BOSE Electroforce 3520‐AT biomechanical testing machine, and the position of the femoral and tibia stumps was adjusted so that the lower limb mechanical axis was close to the position when standing naturally.

Stress Measurement and Data Collection

An ultra‐low‐pressure sensitive film (0.5–2.5 MPa) was used to measure the contact pressure on the medial and lateral plateaus of the tibia. The pressure sensitive film was trimmed to a suitable shape and sealed in a polyethylene film bag. The total thickness of the pressure sensitive film and the polyethylene film bag was controlled to 250 μm. The pressure sensitive films were carefully placed under the knee meniscus15 separately through the medial and lateral incisions (Fig. 3). The incisions were then sutured tightly to close the joint capsule. After stabilization, the specimen was pressurized to 200 N at 10 N/s to eliminate creep. A vertical load was applied on the specimen at a rate of 10 N/s to 400 N for 2 min. Then, the pressure sensitive film was carefully removed from the knee joint.

Figure 3.

The ultra‐low‐pressure sensitive films were carefully placed under the knee meniscus separately through the medial and lateral incisions.

The stress values of the ultra‐low‐pressure sensitive films were read with the use of an FPD‐305E densitometer and an FPD‐306E pressure transducer (Fuji Company, Japan). The contact area of each pressure sensitive film was divided into four quadrants (front outer, front inner, rear inner, and rear outer). Five points were selected in each quadrant for stress value reading. A total of 20 values in each film were recorded and the average was calculated for final analysis.

Statistical Analysis

SPSS 21.0 software (SPSS, Chicago, IL, USA) was applied for statistical analysis. The variables in this study include stress values and angles of varus or valgus deformity. The normality of the stress data is verified using the Shapiro–Wilk test, and the variance consistency was verified using the Levene test. Normally distributed measurement data was recorded as mean ± standard deviation. The stress values on the medial and lateral plateau of the tibia at different positions were calculated and recorded, respectively. Analysis of variance (ANOVA) for random block groups was used to compare the stress values on the medial or lateral plateau at different angles of tibia varus or valgus deformity under vertical load. The Student–Newman–Keuls test was applied to make pairwise comparisons between the multiple sample measurements. Differences in the stress data between medial and lateral plateaus were tested using the paired samples t test. A P‐value less than 0.05 indicated a statistically significant difference.

Results

Stress Distribution on the Medial Plateau of the Tibia Measured at Different Varus and Valgus Deformities

The stress values on the medial plateau of the tibia measured at different varus and valgus deformities under 400 N vertical load are summarized in Table 1. ANOVA for random block groups revealed statistically significant differences in the stress values on the medial plateau among different varus and valgus deformities (including the neutral position) (F = 304.097, P < 0.001). The Student–Newman–Keuls test revealed statistically significant differences in the stress values on the medial plateau between pairwise comparisons among 15°, 10° and 5° varus deformities, the neutral position (0°), and 5°, 10° and 15° valgus deformities. The stress values on the medial plateau measured at 5°, 10°, and 15° varus deformities were significantly higher than that measured at neutral position (all P < 0.05). In contrast, the stress values on the medial plateau measured at 5°, 10°, and 15° valgus deformities were significantly lower than that measured at neutral position (all P < 0.05). From 15° of valgus deformity to 15° of varus deformity, the stress values on the medial plateau of the tibia increased proportionably (Fig. 4).

Table 1.

The stress values (Mpa) on the medial and lateral plateau of the tibia measured on the models of different varus and valgus deformities under 400 N vertical load

Deformity angle	Medial plateau (n = 14)	Lateral plateau (n = 14)	t‐value	P‐value
Valgus 15°	0.814 ± 0.120	1.436 ± 0.094	−15.25	<0.001*
Valgus 10°	0.925 ± 0.140	1.318 ± 0.089	−8.873	<0.001*
Valgus 5°	1.031 ± 0.152	1.190 ± 0.108	−3.186	0.004*
Neutral position (0°)	1.137 ± 0.139	1.041 ± 0.117	−1.973	0.045*
Varus 5°	1.255 ± 0.115	0.937 ± 0.103	7.705	<0.001*
Varus 10°	1.372 ± 0.110	0.829 ± 0.115	12.81	<0.001*
Varus 15°	1.489 ± 0.097	0.715 ± 0.115	19.25	<0.001*
F‐value	304.097	668.663	—	—
P‐value	<0.001†	<0.001‡	—	—

^* Comparison of differences in the stress data between medial and lateral plateau of the tibia at different deformed angles.

^† Comparison of differences in the stress data on the medial plateau of the tibia at different deformed angle.

^‡ Comparison of differences in the stress data on the lateral plateau of the tibia at different deformed angle.

Figure 4.

Color changes of the ultra‐low‐pressure sensitive films measured on the medial plateau at different angles of varus and valgus deformities of the tibia: (A) valgus 15°; (B) valgus 10°; (C) valgus 5°; (D) neutral position 0°; (E) varus 5°; (F) varus 10°; and (G) varus 15°.

Stress Distribution on the Lateral Plateau of the Tibia Measured at Different Varus and Valgus Deformities

The stress values on the lateral plateau of the tibia measured at different varus and valgus deformities under 400 N vertical load are recorded and summarized (Table 1). ANOVA for random block groups revealed statistically significant differences in the stress values on the lateral plateau among different varus and valgus deformities (including the neutral position) (F = 668.663, P < 0.001). There were statistically significant differences between pairwise comparisons among 15°, 10° and 5° varus deformities, neutral position, and 5°, 10° and 15° valgus deformities according to the Student–Newman–Keuls test. The stress values on the lateral plateau measured at 5°, 10°, and 15° valgus deformities were significantly higher than that measured at neutral position (all P < 0.05). Conversely, the stress values on the lateral plateau measured at 5°, 10°, and 15° varus deformities were significantly lower than that measured at neutral position (all P < 0.05). From 15° of valgus deformity to 15° of varus deformity, the stress values on the lateral plateau of the tibia decreased proportionably (Fig. 5).

Figure 5.

Color changes of the ultra‐low‐pressure sensitive films measured on the lateral plateau at different angles of varus and valgus deformities of the tibia: (A) valgus 15°; (B) valgus 10°; (C) valgus 5°; (D) neutral position 0°; (E) varus 5°; (F) varus 10°; and (G) varus 15°.

Comparison of the Stress Values Between Medial and Lateral Plateaus of the Tibia

Under 400 N vertical load, the stress on the medial plateau of the tibia was 1.137 ± 0.139 MPa at the neutral position, which was significantly higher than the value of 1.041 ± 0.117 MPa on the lateral plateau of the tibia (P < 0.05). Under 400 N vertical load, the stress values on the medial plateau of the tibia were significantly higher than the corresponding values on the lateral plateau at 5°, 10°, and 15° varus deformity positions in addition to neutral position, respectively (all P < 0.05), and significantly lower than the corresponding values on the lateral plateau at 5°, 10°, and 15° valgus deformity positions, respectively (all P < 0.05).

Discussion

Tibial fractures are common injuries in traumatic orthopaedics. Open or closed reduction and internal fixation are the standard treatment options for displaced tibial fractures. However, during closed reduction and intramedullary nailing, fractures cannot always be reduced anatomically, and various residual fracture deformities and complications may occur. In Milner (2002)17, 47 (29%) patients with tibial shaft fractures were found to have an angular deformity of more than 5° in the coronal plane of the tibia after the operation. The change in the mechanical axis of the lower limbs after fracture malunion can lead to changes in the tension of the ligaments and joint capsules, and patella malacia in the long term. If not corrected in time, this can affect the biomechanical characteristics of knee and ankle joints and, ultimately, result in TA, followed by impaired knee function. Van der Schoot et al.18 found that 20% (18/88) of patients with malunion of tibial fractures had symptoms of arthritis of the knee and ankle joints in the affected lower extremities. Studies have suggested that degenerative changes of the knee joint due to malalignment of the mechanical axis of lower extremities are directly related to the unreasonable distribution of stress in the joint19, 20. Therefore, malunion and malalignment of the lower limb is an important factor contributing to traumatic arthritis of the knee joint.

Alignment of the lower limb is a prerequisite for ensuring a reasonable distribution of joint stress. Under normal conditions, the lower limb mechanical axis is from the center of the hip joint to the center of the ankle joint, passing through the center of the knee joint, so that the load and stress on the knee joint can be reasonably dispersed21. When malunion and malalignment occurs after a tibial fracture, the contact characteristics of the knee joint will be altered. However, no quantitative studies were found in the literature that investigate the changes of the stress on the medial and lateral plateaus after tibial fracture malunion and explore the relationship between the stress distribution and the residual varus or valgus deformity. Therefore, we conducted this biomechanical study on the stress distribution of the knee joint. In the current study, the malunion of proximal tibial fractures was established to simulate the varus and valgus deformities. The stress on the medial and lateral plateau of the tibia was measured on both normal and deformed knee joints using the pressure sensitive film technique. It was found that the stress on the medial and lateral plateau of the tibia changed significantly when valgus and varus deformity occurred.

In patients with knee varus deformity, the mechanical axis is offset medially from the center of the knee joint. Our findings reveal that the stress on the medial plateau of the tibia is significantly higher at varus deformities when compared with those measured at the neutral position, and significantly higher than the corresponding stress values on the lateral plateau. When the knee is at valgus position, the mechanical axis is offset laterally and the lateral compartment load increases. We also find that the stress on the lateral plateau is significantly higher at valgus deformities than the stress on the lateral plateau measured at the neutral position, as well as the corresponding data measured on the medial plateau. High stress load on the joint is an important factor causing cartilage degeneration22. Stress concentration will accelerate the wear of articular cartilage, stress microfracture, ischemia, and necrosis. Sharma et al.23 found that the risk of knee osteoarthritis increased by 3.59 times in the medial compartment for patients with varus deformity, and the risk of knee osteoarthritis increased by 4.85 times in the lateral compartment for patients with valgus deformity. Therefore, to reduce the risk of long‐term arthritis in patients, orthopaedic surgeons should reduce the fractures anatomically to avoid malalignment and malunion of lower limbs fractures24.

In the current study, the specimen received a vertical load up to 400 N for 2 min. Ahmed et al.25 applied a pressure of 890 N when measuring specimen stress on the knee joint. Riegger‐Krugh et al.26 selected 1960 N vertical load in the stress measurement of the knee joint, which is equivalent to three times the average body weight. Different from Ahmed and Riegger‐Krugh's study, we chose 400 N vertical load applied on the single limb to simulate the static press on the knee joint of the normal body mass.

This study has some limitations. First, the sample size is small due to the limited source of specimens, which will reduce the credibility of the experimental data (type II error). Second, this study is based on cadaver specimens and cannot take the action of muscles into consideration. Third, dynamic load on the knee joint was not taken into consideration and the current model cannot simulate the stress distribution during knee motion. However, the current findings guarantee further biomechanical studies on the stress distribution of the knee joint under dynamic load, clinical studies on the quantitative relationship between residual malunion and traumatic arthritis, and the endeavor to reduce complex fractures anatomically in closed fashion or in a minimally invasive way.

In conclusion, tibial fracture malunion with residual varus or valgus deformity can lead to significant changes in the stress distribution and contact characteristics of the knee joint. Therefore, orthopaedic surgeons should reduce the fractures anatomically to avoid malunion of the fractures and malalignment of the affected lower extremities, to improve the functional recovery and reduce the risk of traumatic arthritis in the long term.