The Biomechanics and Clinical Application of Novel Dual-headed Pedicle Screws in the Corrective Surgery for Spinal Deformity

Kyphosis or scoliosis is a deformity of the three-dimensional structure of the spine caused by lateral or posterior displacement of the spine. According to the population incidence statistics, the number of patients with spinal deformity in China is more than 3 million. The continuous progression of untreated or improper spinal deformity not only causes severe deformity, but also compresses the heart, lung, neurospinal cord and other important organs, leading to cardiopulmonary failure, paraplegia and even death, bringing huge economic and social burden to the country and society.

Conventional spinal orthosis uses common pedicle screws and rods to straighten the twisted spine by turning the rods and pushing and pulling screws (multiple screws one rod mode). Compared with developed countries, there are a large number of patients with moderate to severe spinal deformity in China, and there are many problems in conventional orthopaedic techniques :(1) severe spinal deformity stiffness, often requiring three-column osteotomy, spinal cord distortion and shearing during orthopaedic and closure, and the incidence of neurological complications significantly increased (8%-23%); (2) The stress between screws and rods is concentrated, and the risk of fracture after traditional internal fixation is higher (5%-22%); (3) Complex spinal malformations (such as lumbosacral or cervix and chest malformations), the installation of orthopedic rod is difficult, time-consuming and laborious operation. Therefore, it is necessary to invent new orthopaedic internal fixation devices and techniques for the safe and effective treatment of numerous patients with moderate and severe complex spinal deformity in China.

The development of three-dimensional orthopaedic theory of spinal deformity and the application of three-column spinal osteotomy have significantly improved the surgical efficacy of severe spinal deformity. In recent years, the most widely used three-column spinal osteotomy mainly includes pedicle subtraction osteotomy (PSO) and total vertebral column osteotomy (VCR), both of which can achieve good effect. The correction rate was about 50%-70% . However, with the wide development of three-column spinal osteotomy and orthopaedic surgery, the number of patients needing revision surgery due to the complications of internal fixation failure such as broken rod is increasing. Previous literature reported that the complication rate of broken rod after severe spinal deformity was about 3.7%-15%. Suk et al. reported that the incidence of rod breakage after posterior VCR for severe spinal deformity was 7.1% (5/70). Smith et al. reported that the incidence of broken rod after PSO in adult spinal deformity patients was 15.8%. Lv et al. discussed the risk factors and management strategies of rod breakage after posterior VCR surgery for severe spinal deformity, and found that the incidence of rod breakage after surgery was 3.7%. Wang Fei et al. retrospectively analyzed 7 patients with rod breakage after posterior VCR, and found that rod breakage occurred mostly within 2 years after surgery, and residual kyphosis was an important risk factor for rod breakage after surgery. Other risk factors included trauma, anterior column defect, swing gait, single rod fixation and titanium mesh displacement, etc. In patients with tri-column spinal osteotomy, the osteotomy area and adjacent upper and lower segments are mostly stress concentrated areas, and the stable osseous fusion has not been formed in the short term after surgery, and the internal fixation rod is prone to fatigue fracture. Therefore, reinforcement of the osteotomy area and adjacent segments is required to reduce the risk of rod fracture. In the past, in order to reduce the occurrence of postoperative complications of rod breakage, the commonly used improvement methods mainly include anterior support, increasing implant density and using thick rods. Anterior titanium mesh support has disadvantages such as difficulty in placement and postoperative subsidence, while increasing implant density and using thick rods will increase the cost of patients, and still cannot completely avoid the occurrence of postoperative rod breakage.

In the international scoliosis surgical orthosis mode, a long rod on the left and right sides of the spine is generally inserted into a single slot screw internal locking fixation. However, a major feature of severe scoliosis is the severe imbalance of the spine and the stiffness of the malformed segment, which makes the above traditional orthopaedic model difficult to achieve. Currently, scholars have widely recognized that the use of satellite rod technology in the periosteotomy area can reduce the risk of postoperative internal fixation failure in spinal deformity orthopaedic surgery. Scheer et al. showed in vitro biomechanical results that, compared with posterior internal fixation alone, the application of satellite rod technology in the peripherally fractured rod area could restore spinal strength during flexion, extension and left-right rotation, and the position of satellite rod was independent of spinal strength after revision. In 2012, the applicant team took the lead in applying the satellite rod technology in the three-column osteotomy for severe spinal deformity in China. The pedicle screws in the upper and lower segments of the osteotomy area were replaced by double-headed screws, making the screw placement simple. In addition, this technique does not increase the implant density of pedicle screws, and the satellite rods are mostly cut short rods, so there is no additional cost for patients. Relevant research results have been published in Spine and Chinese Journal of Orthopedics, etc.

However, in the traditional satellite rod technology, the fixation and connection of the main rod and the satellite rod rely on the traditional single slot screw and the traditional double slot connector, which are separated and not a whole in the mechanical structure, the stability of the rod is relatively poor, and the stability of the screw is insufficient. Applicants team early in order to solve the clinical problem, invented a kind of brand-new monomers double groove pedicle screws and internal fixation device, the new screw is always screw, on the basis of the traditional single slot screws and double groove connector combinations, constructed a new kind of monomers double groove pedicle screws (hereinafter referred to as new type double screw). The double-headed nail comprises a fixed end and a connecting end, and the connecting end is provided with two connecting slots: the main connecting slot and the auxiliary connecting slot. During the middle and posterior column osteotomy, double-headed nails were implanted at the head and tail of the osteotomy area, and satellite rods were placed across the osteotomy area and locked in the auxiliary connecting groove to keep the osteotomy area in a stable state during the osteotomy process. Then, the osteotomy point was closed by adjusting the satellite rods, and the main fixation rod was finally assembled.