Effect of posterior cruciate ligament rupture on biomechanical features of the medial femoral condyle

Abstract

Objective:  To investigate the biomechanical impact of rupture of the posterior cruciate ligament (PCL) and its various bundles on the medial femoral condyle.

Methods:  Twelve fresh human cadaveric knee specimens were divided into four groups: PCL intact, anterolateral band (ALB) rupture, posteromedial band (PMB) rupture and PCL complete rupture groups according to the purpose and order of testing. Strain in the middle of the medial femoral condyle was measured under different loads (200–800 N) at 0°, 30°, 60°, and 90° of knee flexion.

Results:  At 0° of knee flexion, compared with the PCL intact and ALB rupture groups, strain on the medial femoral condyle increased in the PMB rupture and PCL complete rupture groups under all loading conditions. There was no statistical difference between the PMB rupture and PCL complete rupture groups. At 30°, 60° and 90° of knee flexion, compared with the PCL intact group, increase in strain on the medial femoral condyle was noted in the ALB rupture group under higher loading conditions (600 N and 800 N) and PCL complete rupture group under all loading conditions. The PCL complete rupture group had higher strain on the medial femoral condyle than did the ALB rupture group under most loading conditions.

Conclusion:  At 0° of knee flexion, PMB rupture or PCL complete rupture can cause increase in strain on the medial femoral condyle. However, at 30°, 60° and 90° of knee flexion, ALB rupture or PCL complete rupture can cause increase in strain on the medial femoral condyle.

Introduction

The posterior cruciate ligament (PCL) of the human knee is widely recognized to be the primary restraint against posterior tibial displacement. It has been estimated that 520% of all knee ligament injuries and as many as 60% of ligament injuries treated in emergency rooms involve the PCL ¹ , ² , ³ . After PCL injury, cartilage degeneration is predominantly found in the medial compartment of the tibiofemoral joint and in the patellofemoral joint ³ , ⁴ , ⁵ , ⁶ , ⁷ . However, the mechanism of cartilage degeneration of knee following by PCL injury is not fully understood.

Numerous studies investigating the effects of PCL injury on the biomechanics of the knee joint have been published. Some studies have primarily assessed the altered kinematics of the knee joint after PCL injury during flexion with weight bearing. Other research has evaluated the effects of PCL injury and different PCL reconstruction techniques on the biomechanics of the knee. In addition, Hoher et al. measured the force in the PCL in response to simulated muscle loading conditions and reported that PCL forces were maximal near 90° of flexion of the knee ⁸ . Skyhar et al. applied quadriceps loading to cadaveric knees and measured contact pressure in the patellofemoral joint before and after resection of the PCL ⁹ . An increase in contact pressures was observed after PCL resection.

However, reports on the biomechanical function of the PCL and its various bundles and the biomechanical impact of PCL rupture on the other parts of knee joint are scarce, especially in regard to the effects of partial or complete rupture of the PCL on the medial femoral condyle under different physiological loading conditions. In this study, strain on the middle part of the medial femoral condyle under different conditions, including anterolateral bundle (ALB) rupture, posteromedial bundle (PMB) rupture and PCL rupture, were calculated. Our aim was to investigate the impact of rupture of PCL and its various bundles on the biomechanical features of the medial femoral condyle using biomechanical testing under diverse function load and flexion angles. We also aimed, if possible, to further explore the influence of the mechanism of PCL rupture on the medial femoral condyle cartilage and to provide effective therapy.

Materials and methods

Specimen preparation

Twelve fresh frozen cadaveric knees (six left and six right, asymmetry), all from adult males, were used in this study. All the cadaveric knees underwent macroscopic inspection and the posterior drawer test to rule out gross anomalies, degeneration, fracture, tumor and PCL damage. The average age of the cadavers was 30.6 years, with a range from 25–48 years. The femur and tibia were each cut at approximately 30 cm from the joint line, keeping the skin and soft tissue intact. All specimens were covered with pledget soaked in 0.9% saline and sealed with double plastic bags, then preserved at −70°C. Approval for the study was obtained from the local Hospital Ethics Committee.

Group of experiments and test procedures

The specimens were divided into four groups according to the test order: PCL intact (control), ALB rupture, PMB rupture and complete PCL rupture groups.

Before the experiments, the specimens were kept at 4°C for 24 h and then at room temperature for another 24 h, after which they were used for biomechanics testing. The soft tissues of the proximal end of the femur were removed whereas the remaining soft tissues surrounding the knee joint were left intact. The ends of the femur and tibia were then fixed in cylinders to enable rigid fixation during testing.

The twelve specimens were each fixed in clamps with the femur inferiorly and tibia superiorly. The quadriceps and proximal thigh muscles were fixed with wire to the femur holding clamp at a tension of 400 N and 200 N. A 5 mm vertical incision was made on the inner side of the patella and a 5 mm × 4 mm area in the middle of the medial condyle of femur was exposed. The surface layer of cartilage was removed from the point to be tested and the medial condyle burnished using abrasive paper followed by degreasing. Once dry, a strain foil was fixed vertically with α‐cyanoacrylate in a direction consistence with the load. The joint capsule was then closed with the strain foil inside connected to a static strain indicator (DH‐3818, Dong Hua Test, Jiangshu, China). An extra strain foil was also connected as a temperature compensation chip. A load of 250 N was repeated 20 times at a speed of 0.5 mm/s to eliminate the influence of the innate viscosity of the specimens.

The specimens were then positioned at 0°, 30°, 60° and 90° of flexion (Fig. 1) and the static strain measuring device was calibrated in balance. A continuous axial load (200–800 N) was loaded at a speed of 0.5 mm/s, with an interval of 10 minutes to allow restoration of elasticity. Data at 200 N, 400 N, 600 N and 800 N from every channel of the static strain measuring device were documented. After finishing the tests above, the specimens were separated randomly into an ALB rupture group of six in which the ALB was severed, and a PMB rupture group of six, in which the PMB was severed. The procedure described above was then followed in both of these groups. Next the PCLs of all 12 specimens were severed completely, modeling complete rupture of the PCL, and the test procedure described above repeated.

Figure 1.

The knee specimens were fixed and used for biomechanical testing at (a) 0°, (b) 30°, (c) 60° and (d) 90°.

During the whole test, gauze soaked in 0.9% saline was used to cover the surface of the knee joint, ensuring that the specimens were kept at a humidity of at 60–80%. The whole testing procedure described above was conducted at a temperature of 25°C.

Statistical analysis

SPSS13.0 for Windows was used for data management and statistical analysis. One‐way analysis of variance was used to compare the means of different groups. Data were expressed as mean ± SD. A P‐value less than 0.05 was taken as statistically significant.

Results

Strain analysis at 0° of flexion

At 0°, the strain value in the middle of the medial condyle of femur was negative, indicating that the strain in that position is a compression strain (Table 1). Under different loads, the strain values in the ALB rupture group were not statistically different from those in the PCL intact group (P > 0.05). However, the strain values in the PMB rupture and PCL complete rupture groups were markedly greater than those in the PCL intact group (P < 0.05). The PMB rupture and PCL complete rupture groups also had significantly increased strain values (P < 0.05) in comparison to the ALB rupture group. However, when compared to the PMB rupture group, the strain values in the PCL complete rupture group were not significantly greater (P > 0.05).

Table 1.

Strain in the middle of the medial femoral condyle in all groups under different loading conditions at 0° of flexion (, µε)

Group	Strain under different loads (µε)
	200 N	400 N	600 N	800 N
PCL intact group	−39.58 ± 11.49	−55.08 ± 14.10	−91.92 ± 19.49	−110.00 ± 11.65
ALB rupture group	−45.83 ± 15.22	−56.83 ± 7.76	−99.50 ± 10.99	−109.67 ± 7.20
PMB rupture group	−63.33 ± 14.84 [Link] , [Link]	−109.50 ± 13.68 [Link] , [Link]	−147.67 ± 10.27 [Link] , [Link]	−169.83 ± 17.30 [Link] , [Link]
PCL complete rupture group	−67.16 ± 15.81 [Link] , [Link]	−123.33 ± 16.44 [Link] , [Link]	−150.58 ± 12.23 [Link] , [Link]	−167.00 ± 14.50 [Link] , [Link]
	F = 9.07, P= 0.000	F = 60.96, P= 0.000	F = 42.43, P= 0.000	F = 58.25, P= 0.000

µε, micro‐strain; ^a, P < 0.05 compared with PCL intact; ^b, P < 0.05 compared with ALB rupture.

Strain analysis at 30° of flexion

When flexed to 30°, the strain values in the middle of the medial condyle of the femur were also negative, indicating that the strain in that position is a compression strain (Table 2). Compared with the PCL intact group, the ALB rupture group did not have significantly increased strain values under loads of 200 N and 400 N (P > 0.05), whereas under loads of 600 N and 800 N the strain values were markedly increased (P < 0.05). Under different loads, compared with the PCL intact group, the PMB rupture group did not have significantly increased strain values (P > 0.05), whereas in the PCL complete rupture group the strain values were markedly increased (P < 0.05). In comparison with the ALB rupture group, under a load of 600 N, the strain values of the PCL complete rupture group were not significantly increased (P > 0.05), while the opposite results were obtained under loads of 200 N, 400 N and 800 N (P < 0.05). However, compared with the PMB rupture group, the strain values in the ALB rupture group showed a non‐significant increase under loads of 200 N and 400 N (P > 0.05), whereas significant increases were found under loads of 600 N and 800 N (P < 0.05). Also, significant increases was found in the PCL complete rupture group compared with the PMB rupture group under different loads.

Table 2.

Strain in the middle of the medial femoral condyle in all groups under different loading conditions at 30° of flexion (, µε)

Group	Strain under different loads (µε)
	200 N	400 N	600 N	800 N
PCL intact group	−34.33 ± 10.04	−51.25 ± 10.19	−83.00 ± 13.89	−95.00 ± 12.63
ALB rupture group	−35.66 ± 4.63	−56.50 ± 10.52	−130.50 ± 13.66 [Link] , [Link]	−139.83 ± 17.42 [Link] , [Link]
PMB rupture group	−41.17 ± 9.75	−63.00 ± 17.65	−93.67 ± 17.59	−107.33 ± 4.94
PCL complete rupture group	−67.58 ± 13.32 [Link] , [Link] , [Link]	−105.92 ± 16.26 [Link] , [Link] , [Link]	−138.00 ± 12.46 [Link] , [Link]	−159.83 ± 11.04 [Link] , [Link] , [Link]
	F = 23.36, P= 0.000	F = 36.18, P= 0.000	F = 45.97, P= 0.000	F = 49.36, P= 0.000

µε, micro‐strain; ^a, P < 0.05 compared with PCL intact; ^b, P < 0.05 compared with ALB rupture; ^c, P < 0.05 compared with PMB rupture.

Strain analysis at 60° of flexion

When the knees were flexed to 60°, the strain values in the middle of the medial condyle of the femur became positive, demonstrating that the strain in that position is tension strain (Table 3). When compared with the PCL intact group, under loads of 200 N and 400 N, strain values in the ALB rupture group showed a non‐significant increase (P > 0.05), whereas significant increases were observed under loads of 600 N and 800 N (P < 0.05). Non significant increases were noted in the PMB rupture group under different loads, whereas in the PCL complete rupture group the increase in strain values was significant (P < 0.05). What's more, in comparison with the ALB rupture group, except under loads of 600 N, the strain values (200 N, 400 N and 800 N) in the PCL complete rupture group were all significantly increased. Also, when compared with the PMB rupture group, strain values in the ALB rupture group under loads of 200 N and 400 N showed non‐significant increases (P > 0.05), whereas under loads of 600 N and 800 N, the increases were significant (P < 0.05). Still, significant increases were found in the PCL complete rupture group compared with the PMB rupture group under different loads (P < 0.05).

Table 3.

Strain in the middle of the medial femoral condyle in all groups under different loading conditions at 60° of flexion (, µε)

Group	Strain under different loads (µε)
	200 N	400 N	600 N	800 N
PCL intact group	16.25 ± 4.11	35.92 ± 13.34	71.92 ± 13.46	91.67 ± 12.72
ALB rupture group	28.33 ± 10.11	41.17 ± 9.68	127.67 ± 10.27 [Link] , [Link]	133.00 ± 12.08 [Link] , [Link]
PMB rupture group	20.83 ± 11.48	36.83 ± 7.76	79.50 ± 10.99	89.67 ± 7.20
PCL complete rupture group	44.66 ± 14.72 [Link] , [Link] , [Link]	103.33 ± 16.44 [Link] , [Link] , [Link]	133.92 ± 8.76 [Link] , [Link]	150.17 ± 12.85 [Link] , [Link] , [Link]
	F = 15.08, P= 0.000	F = 65.16, P= 0.000	F = 52.06, P= 0.000	F = 62.60, P= 0.000

µε, micro‐strain; ^a, P < 0.05 compared with PCL intact; ^b, P < 0.05 compared with ALB rupture; ^c, P < 0.05 compared with PMB rupture.

Strain analysis at 90° of flexion

When the knees were flexed to 90°, strain values in the middle of the medial condyle of the femur were positive, demonstrating that the strain in that position is a tension strain (Table 4). When compared with the PCL intact group, under loads of 200 N and 400 N, the strain values of the ALB rupture group showed non‐significant increases (P > 0.05), whereas significant increases were observed under loads of 600 N and 800 N (P < 0.05). Non significant increases were noted in the PMB rupture group under different loads, whereas in the PCL complete rupture group the increase in strain values was significant (P < 0.05). What's more, in comparison with the ALB rupture group, except under a load of 800 N, strain values (200 N, 400 N, 600 N) of the PCL complete rupture group were all significantly increased. Also, when compared with the PMB rupture group, the strain values of the ALB rupture group under load of 200 N and 400 N showed non‐significant increases (P > 0.05), whereas under loads of 600 N and 800 N, the increase became significant (P < 0.05). Still, significant increases were found in the PCL complete rupture group compared with the PMB rupture group under different loads (P < 0.05).

Table 4.

Strain in the middle of the medial femoral condyle in all groups under different loading conditions at 90° of flexion (, µε)

Group	Strain under different loads (µε)
	200 N	400 N	600 N	800 N
PCL intact group	9.75 ± 3.55	17.92 ± 5.18	41.00 ± 8.90	76.75 ± 11.89
ALB rupture group	11.00 ± 4.19	20.00 ± 7.51	70.66 ± 17.91 [Link] , [Link]	112.50 ± 16.03 [Link] , [Link]
PMB rupture group	15.50 ± 6.12	25.67 ± 6.71	49.83 ± 10.91	72.67 ± 33.53
PCL complete rupture group	39.08 ± 9.35 [Link] , [Link] , [Link]	52.00 ± 8.60 [Link] , [Link] , [Link]	92.92 ± 10.97 [Link] , [Link] , [Link]	123.50 ± 13.62 [Link] , [Link]
	F = 48.27, P= 0.000	F = 54.39, P= 0.000	F = 43.30, P= 0.000	F = 18.61, P= 0.000

µε, micro‐strain; ^a, P < 0.05 compared with PCL intact; ^b, P < 0.05 compared with ALB rupture; ^c, P < 0.05 compared with PMB rupture.

Discussion

In this study, strain values in the middle of the medial condyle of the femur increased with increasing load and were all negative at 0° and 30° and positive at 60° and 90°, regardless of diverse loads. Nevertheless, both compression and tension strain altered with position and force. In the process of knee joint flexion, the tibiofemoral joint is involved in both roll and glide movements. Flexion of the knee joint initially results in glide of the femoral condyle, this being followed by increasing roll movements that peak at 60°. Relative roll movements of the femoral condyle on the tibial plateau shift the point of contact between them, avoiding impact and enhancing flexion extent. Dennis and Challaghan have pointed out that the femoral condyle is located 6.5 mm ahead of the midpoint of the tibial plateau at 0° and 7.8 mm after the midpoint when flexed to 90°, with an average shift of 14 mm ¹⁰ , ¹¹ . In our study, as the degree of flexion increased, the contact point moved backwards and the strain on the femoral condyle gradually changed. Consequently, when the knees were flexed to 60°, the change in curvature radius and shift of contact point together with tension on the joint capsule and transverse ligament of the knee converted compression strain into tensile strain.

The biomechanical impacts of partial and complete rupture of the PCL on the femoral condyle were investigated in this study. First, our biomechanical results showed that complete rupture of the PCL increased medial femoral condyle strain under different loads at 0°, 30°, 60° and 90° of flexion. It has been reported that the PCL not only constrains posterior tibial translocation, but also limits external tibial rotation ¹ , ¹² , ¹³ , ¹⁴ . Both abnormal posterior translocation and internal/external tibial rotation may be major factors causing increase in contact pressure. Secondly, our results showed that PMB rupture increases medial femoral condyle strain under different loads at 0°, whereas ALB rupture has no influence on it. This may because the PMB is tight and the ALB slack when the knee is at 0° of flexion ¹⁵ . Because PMB rupture causes posterior tibial translocation in extension, after resection of the PMB medial femoral condyle strain increases dramatically at 0° of flexion under different loads. Thirdly, at 30°, 60° and 90° of flexion, the ALB rupture group showed obvious increase in the strain on the medial femoral condyle with high loads but not with low loads, whereas the PMB rupture group demonstrated no obvious biomechanical impact on the medial femoral condyle. In flexion the ALB is tight, whereas the PMB is slack ¹⁵ . Therefore, ALB rupture can increase posterior tibial translocation in flexion, which results in increase in strain on the medial femoral condyle after resection of the ALB at 30°, 60° and 90° of flexion under high loads.

In addition, at 30° and 60° of flexion, strain values in the PCL complete rupture group did not significantly increase under a load of 600 N, whereas opposite results were noted under loads of 200 N, 400 N and 800 N compared with those in the ALB rupture group. At 90° of flexion, strain values of the PCL complete rupture group did not significantly increase under a load of 800 N, whereas opposite results were noted under loads of 200 N, 400 N and 600 N compared with those in the ALB rupture group. With increase in knee flexion, the ALB becomes more vertically oriented, decreasing its ability to resist posterior tibial translation ¹⁶ . In contrast, the orientation of the PMB becomes more horizontal, increasing its ability to resist posterior tibial translation ¹⁶ . These factors may help explain why complete rupture of the PCL has a stronger influence than does ALB rupture under most loads in flexion. We propose two possible explanations for why complete rupture of the PCL does not have stronger influence than does ALB rupture under a load of 600 N at 30° and 60° of flexion or under a load of 800 N at 90° of flexion. The first reason is posterior tibial translocation and external tibial rotation. After resection of the ALB, with increasing load the ability of other parts of the knee joint to resist posterior tibial translation and external tibial rotation gradually decreases. The second reason is the location of the contact point in the process of knee joint flexion. With increasing knee flexion, the contact point moves backwards and strain on the medial femoral condyle gradually changes.

In conclusion, the current data suggest that partial and complete rupture of the PCL can cause increase in strain on the medial femoral condyle, which may explain the degenerative changes that occur in the cartilage of the knee after PCL injury.

Disclosure

The authors declare that they have no competing interests.