The most important finding of this study was that patellar dislocation could cause changes in the electrophysiology and proprioceptors of VM and VL. Patellar reduction was beneficial to the rehabilitation of VM and VL. Moreover, NGF injection had a significant effect on the electrophysiology and pathology of muscle. Most of the studies on the restoration of knee proprioception focus on the basis of physical rehabilitation exercise, but the effect is not satisfactory. Although the biomechanical stability of the patellofemoral joint is irreplaceable, more attention is paid to proprioception and maintaining the dynamic stability of the normal patellar trajectory. It has been reported that proprioception is involved in the regulation of motion by the human nervous system [17], which is correlated with natural sensation of the knee joint [18].
Proprioception of the knee joint plays an important clinical role in controlling joint stability, correcting body posture, and maintaining body balance; proprioception of the knee joint mainly comes from the muscles around the joint [19]. Proprioceptive stress stimulation can make the body respond to joint stress faster and reduce the rate of joint wear and dislocation [20]. This study showed that after patellar dislocation, the mechanical proprioceptors of VM and VL were atrophied, deformed, and decreased (Group 3 data in Table 3). Proprioceptors were increased after patellar reduction (Group 1 and Group 2 data in Table 3). Mechanical stress is one of the most important factors affecting growth [21]. It is considered to be related to the loss of normal stress stimulation and VM and VL activity after patellar dislocation in growing rabbits. Specific immunohistochemical staining and microscopic observation of VM and VL in rabbits were conducted in this study. The distribution, types, quantity, and morphology of proprioceptors were then analyzed. The results suggested that patellar dislocation should be corrected as early as possible to minimize the occurrence of secondary changes.
The purpose of patellar reduction surgery is to stabilize the patella in the trochlear groove and strengthen the tension of the VM. Smillie [22] referred to the VM as the “key to the knee” to illustrate its importance in patellar stabilization. SEPs and EMG have been widely accepted as methods to detect the sensory state of knee joints in animals [14]. Statistically significant differences in electrophysiology and pathology in the reduction group and patellar dislocation group were noted. The reason may be that the flexion and extension of the quadriceps muscle stimulate nerve proprioceptors. In contrast, the indices of Group 3 were worse at the age of 6 months than at 4 months, highlighting the importance of maintaining the normal patellar trajectory. Any changes in muscle balance between the VM and VL can affect the patellofemoral joint, whether congenital or posttraumatic, depending on the severity of the disorder.
Proprioception is defined as the ability to perceive sensory stimuli such as touch, pain, pressure, and movement. Joint position sense and joint motion sense are common proprioception senses that enable individuals to accurately perceive joint position and motion. Proprioception is reported to play an important role in the regulation of the nervous system and motor performance [23]. The effects of joint proprioceptive information on motion control can be divided into two categories [24]: first, the appropriate response of the body to external environmental changes. Through the central adjustment in joint motions to adapt to the change in the external environment, for example, walking should be performed according to the road surface's situation to change the pace and direction. Although most of the sensory information descendants are produced by the visual system and central nervous system, body sensory information is accurate and fastest and plays an irreplaceable role. Second, they participate in the integration of higher orders in the sensorimotor center. In the process of movement, the motor center regulates skeletal and muscle activities, and proprioceptive information contains the necessary joint position and strength information [25]. Proprioceptors, the vestibule, and vision work together to control muscles. The control process of motion requires continuous sensory integration and information input by the proprioceptors of muscles. The proprioceptor information procedure may be interrupted when injury occurs; as a result, the sensorimotor center cannot quickly and accurately perceive the position, movement, muscle force, and other information of the joint.
Surgical treatment of patellar dislocation has achieved good clinical results. Patients may benefit from prompt surgical intervention to repair soft tissue injuries and restore patellar tracking, preventing further redislocations [26]. In patients with patellar dislocation, the mechanical stability of the knee joint must be ensured, but attention should also be paid to joint proprioception. Drug intervention is a hot research topic in proprioception rehabilitation. NGF plays an important role in proprioceptive repair [27]. Among the members of the neurotrophic factor family, NGF was the first to be discovered and is collectively known as a neurotrophic factor [28]. As early as the 1950s, R. Levi-Montalcini carried out pioneering studies on laboratory animals and isolated cells, focusing on the biological role of NGF [29]. These studies suggest that NGF protects the survival of degraded or damaged peripheral nerve cells and has a protective effect on the regulation of neurotransmitter and neuropeptide synthesis [30]. After muscle injury or degeneration, NGF can reduce the expression of injury-induced transcription Factor 3 and promote the reneutralization of target organs [31], thereby protecting proprioceptive neurons. Morphological and quantitative changes in proprioceptors in the VM and VL were observed. The study found that after surgical reduction, in the NGF-injected group, the proprioceptors were different from those in the group without NGF injection with small size and partial structural loss, and the indices of SEPs and EMG were statistically significant. Treatment with NGF was beneficial to the rehabilitation of knee proprioception.
The dynamic position of the patella is dependent on the vectors of VM and VL [6]. The proprioceptors of VM and VL can collect information on joint movement and position, which can be transmitted to the spinal cord and brain through afferent nerves. The information is analyzed and integrated through the efferent nerve, and the corresponding response is made to adjust the motion and stability of the knee joint. Patellofemoral joint instability is mainly manifested by poor balance and coordination ability, significantly decreased spatial position judgment ability, poor nerve reactivity, prolonged nerve response to sudden torsional stress and electrical stimulation, and weakened muscle strength [32]. After a first episode of patellar dislocation, most patients presented with soft tissue lesions [33]. Electrophysiological examination is an important means to monitor muscle balance. Electrophysiological tests were performed on VM and VL to demonstrate muscle imbalance in patellar dislocation and the effectiveness of patellar reduction. VM can prevent lateral dislocation of the patella by resisting the pressure generated by knee valgus angulations and VL, which is central to the stabilization of the patella [22]. It is suggested that special attention should be given to functional exercise of the muscle after patellar dislocation, and at the same time, drugs promoting proprioception rehabilitation can be used to facilitate its treatment.
This study demonstrated that functional instability and mechanical instability were not distinct but related to each other, and their joint persistent action ultimately led to chronic instability of the joint. In this study, the proprioception of the patellar dislocation model in growing rabbits was significantly improved after surgical reduction. The results indicated that patellar reduction could improve functional stability as well as mechanical stability. Early reduction is conducive to the rehabilitation of VM and VL muscle coordination and balance and is thus conducive to the stability of the patella. Clinically, functional instability should be considered as well as mechanical stability in the treatment and rehabilitation of patients with patellar dislocation, which causes not only changes in skeletal tissue but also soft tissue imbalance. An integrated treatment should be further investigated, and attention should be given to electrophysiological changes in muscles around the knee joint.
There were several limitations to this study. First, rabbit models have been widely used to study patellar dislocation, but rabbit knees are bent and cannot be equated to upright walking humans. Second, patellar dislocation surgery was performed in rabbits at 2 months of age, which mimics early patellar dislocation in humans but does not completely coincide with the time point of patellar dislocation in humans. In addition, the number of rabbits in the experiment was relatively small, and although it was enough to be statistically significant, large numbers of animals would be better. Finally, the integrated treatment of patellar dislocation needs to be further studied.