US20200289165A1

US20200289165A1 - A screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation

Info

Publication number: US20200289165A1
Application number: US16/630,479
Authority: US
Inventors: Xiangyang Ma
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-12-23
Filing date: 2016-12-19
Publication date: 2020-09-17
Also published as: US12150678B2; WO2017107883A1; US20210290272A1

Abstract

The invention discloses a screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation, comprising two pulling-screws (2), two supporting-screws (1), two variable cross section fixing rods (3), two lock nuts (4) and a bracing beam (5). Each of the variable cross section fixing rods (3) is configured to connect a supporting-screw (1) and a pulling-screw (2), the bracing beam (5) is configured to connect both of the variable cross section fixing rods (3); wherein each of the supporting-screws (1) comprises a first tail (11) and a first head (12), and each of the pulling-screws (2) comprises a second tail (21) and a second head (22); both the first tail (11) and the second tail (21) are provided with nail grooves, with U-shaped grooves on the sides; wherein each of the nail grooves is provided with internal thread inside, and the lock nut (4) is connected with the nail groove; each of the variable cross section fixing rods (3) passes through the U-shaped grooves and is fastened by the lock nut (4); the bottoms of the nail grooves of the supporting-screws (1) are 2-6 mm higher than the bottoms of the nail grooves of the pulling-screws (2). The aforementioned screw-rod instrument not only increases the safety and convenience of a surgery, but also improves the function of spondylolisthesis reduction of the screw-rod system through raising the bottoms of the nail grooves 2-6 mm higher.

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to a technical field of medical apparatus and instruments, and more specifically to a screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation.

DESCRIPTION OF THE RELATED ART

The posterior atlantoaxial transarticular screw (Magerl screw) fixation technique has been successfully developed since 1979, it was always considered as the golden standard of a surgery of atlantoaxial fixation. However, with the advent of the posterior atlantal lateral mass screw technique (Goel-Harms technique), the posterior short-segment screws and rods fixation technique composed of atlas screw combined with axis screw gradually replaces Magerl screw, and becomes a preferred method for the posterior atlantoaxial fixation, regarded as the new golden standard of the surgery. The posterior short-segment screws and rods fixation technique separately arranges screws in the atlas and the axis, it not only has advantages including small entry angle of screw, wide range of application and high fixation strength, but also can play a role in spondylolisthesis reduction during operation, so as to dramatically improve the effectiveness, safety, applicability and convenience of the treatment of atlantoaxial dislocation.
However, fixators with screws and rods currently used for atlantoaxial dislocation are not customized designed for the atlantoaxial dislocation, they are just general fixators for posterior cervical vertebra, and for both imported and domestic medical apparatus and instruments, a variety of defects of them have been observed in an operation process of the clinic treatment of the atlantoaxial dislocation. Therefore, it is necessary to further develop and improve related technologies, such as the improvement of the existing fixators with screws and rods.

SUMMARY OF THE INVENTION

For overcoming the defects of the existing medical apparatus and instruments in clinic application, the present invention provides a screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation. The screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation increases the range of application and implements adjustments according to atlantoaxial structure, so as to adapt to the operation of reduction for the dislocation of the atlas and the axis, improving the function of reduction, and simplifying surgical operation.
A screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation comprises two pulling-screws, two supporting-screws, two variable cross section fixing rods, two lock nuts and a bracing beam; wherein each of the variable cross section fixing rods is configured to connect a supporting-screw and a pulling-screw, the bracing beam is configured to connect both of the variable cross section fixing rods; wherein each of the supporting-screws comprises a first tail and a first head, and each of the pulling-screws comprises a second tail and a second head; both the first tail and the second tail are provided with nail grooves, with U-shaped grooves on the sides; wherein each of the nail grooves is provided with internal thread inside, and the lock nut is connected with the nail groove by the internal thread; each of the variable cross section fixing rods passes through the U-shaped grooves and is fastened by the lock nut; the bottoms of the nail grooves of the supporting-screws are 2-6 mm higher than the bottoms of the nail grooves of the pulling-screws. In traditional structure, the height of the bottom of the nail groove of a supporting-screw is the same as that of the bottom of the nail groove of a pulling-screw; the design according to the present application result in a higher bottom of the nail groove of the supporting-screw, 2-6 mm higher than traditional height. When a patient is in serious dislocation, the height of the bottom of the traditional nail groove would lead to a short distance for spondylolisthesis reduction, if we need to pull a greater distance, much degree of bend of the fixing rod will be required to improve the effect of spondylolisthesis reduction. However, the operation is difficult, and it is not easy to fix, and the misoperation is likely to cause damage to the patient. In this invention, the thickness of the bottoms of the screw slots is increased, so the lifting distance can be increased accordingly, for improving the function of spondylolisthesis reduction, and lowering the risk of operation, without adjusting other structures.
Furthermore, each of the variable cross section fixing rods is provided with an anti-rotation joint portion matched with the U-shaped grooves; the anti-rotation joint portion has a plane and/or a curved surface fitting the U-shaped grooves closely. As a whole, the traditional shape of a fixing rod is cylinder, and the fixing rod is in contact with the U-shaped grooves by spot contact, thus it is easy to cause rotation. Especially under the circumstance of bending the fixing rod, rotation is easy to happen, resulting in high difficulty to fix and complication to operate. In the present invention, the anti-rotation joint portion is provided, and not all of the fixing rod is cylinder-shaped, and the anti-rotation is achieved at the anti-rotation joint portion through the variable cross section of the fixing rod, thereby solving the abovementioned problem. The anti-rotation joint portion has a plane and/or a curved surface fitting the U-shaped grooves closely, for instance, when the cross section of the anti-rotation joint portion is rectangular structure (i.e. a plane that fits closely), the anti-rotation joint portion cannot rotate in the U-shaped grooves, so as to fix and lock; due to the curved surface of the bottoms of the U-shaped grooves, a fitted surface is configured on the contact segment between the anti-rotation joint portion and the bottoms of the U-shaped grooves, and a deadlocked fixation can also be formed. No matter a plane or a curved surface that fits well, they all can change the spot contact of the traditional structure into the surface contact, in order to increase contact area dramatically and prevent rotation.
Furthermore, the second tail is a long arm nail groove, with an annular recess configured on the middle part of the outer wall. The traditional structure is a short arm nail groove. In clinical application, if the dislocation of the atlas and the axis occurs, the atlas will slide forward, so that the screw slots of the atlas screw and the axis screw will not be in the same coronal plane, thus a doctor has to utilize the screw-rod lifting to implement the reduction. However, the existing screws for cervical posterior instrument are polyaxial screws with short arms, thus configuration and operation of rods are very difficult, especially for the clinical cases in serious dislocation of the atlas and the axis, a doctor always needs to carry out auxiliary operation by using a device for lifting the screw and pressing the rod in a narrow space. For overcoming the defect, we have designed a polyaxial lifting screw with long arm for atlas, a doctor can gradually tighten a nut to complete the spondylolisthesis reduction of the atlas and the lock of the fixing rod, thus simplifying the arrangement of the rod and the operation of reduction. In addition, in the present invention, an annular recess is configured on the middle part of the outer wall of the second tail, thus the second tail is separated into an upper part and a lower part by the annular recess. When a higher tail is required, it can be used normally; when using height is less than or equal to the height of the outer wall, the upper portion could be broken off at the annular recess by a user; accordingly, this kind of long arm structure expands the scope of application, and improves practicality and convenience.
As a preferred embodiment of the present invention, the pulling-screw is a polyaxial screw, with the second head connected to the second tail which has a tapered thread tip. As we know, the preparation of the screw path of atlantoaxial pedicle and the screwing of screws are so difficult, especially, if the height of the posterior arch of atlas at the screw entering point is too small, even though a doctor succeeds in the preparation of the screw path, the tapping process of a tapered thread tip or the screwing process of a screw can easily lead to bone fracture at the inferior wall of the posterior arch of atlas, resulting in the slide-out of a screw tap or a screw, difficult to access to the previous correct screw path. In this case, repeated adjustments are inevitable during a surgery, for the purpose of accurately screwing the screws, but repeated adjustments are not only time-consuming and arduous, but also take a risk of venous plexus and spinal cord injury. Therefore, we have designed the thread of the screw head as a tapered thread, wherein the diameter of the front end of the screw becomes smaller, so as to automatically enter the correct screw path, thus increasing the convenience and safety of a surgery.
As a preferred embodiment of the present invention, the first head of the supporting-screw is a single-axial screw which is fastened to the first tail, having a tapered thread tip; or the first head of the supporting-screw is a polyaxial screw which is fastened to the first tail, having a tapered thread tip. As a result, a single-axial screw or a polyaxial screw could be selected according to different usage requirements.
As a further preferred embodiment of the present invention, the bracing beam comprises lock-rod nuts, lateral combination nuts and a lateral connection plate; wherein both sides of the lateral connection plate are provided with oval openings; wherein one end of the lock-rod nut is connected to the first tail or the second tail through thread, the other end of the lock-rod nut is fastened by one of the lateral combination nuts after the other end passes through one of the oval openings. This kind of design makes the bracing beam to be directly fixed to the supporting-screw and/or the pulling-screw, instead of a traditional fixed structure on fixing rods, improving the effect of fixation, so it is more convenient to operate and implement.
Furthermore, the lateral connection plate is a straight plate or a curved plate, depending on various situations.
In conclusion, screws and rods for the posterior atlantoaxial vertebrae fixation according to the present invention not only have the biomechanical stability comparable to that of the Magerl screw fixation, but also overcome the aforementioned technical deficiencies of the Magerl screw, specifically, the beneficial effects of the screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation according to the present invention include:
(1) setting screws in the atlas and the axis is separately, anatomical reduction of the atlas and the axis is not required prior to surgery, and the screw-rod lifting can be implemented during surgery, so as to facilitate the reduction of the atlas and the axis;
(2) the entry angle of a screw is small, setting screws would be successful for a patient with cervical kyphosis or short neck or obesity;
(3) the operation can be implemented under direct vision, the repeated X-ray perspective operation is not required;
(4) in accordance with the individual anatomy of the patient, flexible and diverse screw fixing techniques for the atlas and the axis are selected respectively. If the height of anatomy for the posterior arch of atlas is not enough, and it is not appropriate for the screw fixing of the atlantoaxial pedicle, the fixation of the transpedicular screw of the atlas and the screw of lateral mass of atlas can be selected. If the axis transverse foramen is too large to fix the screw of the pedicle of the axis, the axial lamina screw, the screw of lateral mass of axis, C_2/3transarticular screw and so on can be selected for fixation. Different screw fixing techniques can be selected for left and right sides of the atlas and the axis, thus screw-rod fixation of the atlas and the axis can be implemented by flexible combination of different screw fixing techniques. The aforementioned advantages lead to significant improvement of the effectiveness of the reduction for the dislocation of the atlas and the axis and the fixation, increasing the range of application for people. The screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation according to the present invention thoroughly overcomes the deficiencies including that: traditional fixing rods tend to rotate, traditional screws tend to deviate from the correct screw path, short arm screws lead to difficulty of configuration of fixing rods. Consequently, the screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation not only increases the safety of a surgery and convenience of the operation, but also dramatically improves the function of spondylolisthesis reduction through the design of the supporting-screw for axis with higher nail grooves.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to illustrate the embodiments of the invention or technical solutions of the prior art more clearly, the embodiments or drawings that need to be used in the prior art will be further described in detail below. Obviously, the drawings described below are just some examples of the present invention, those skilled in the art could also obtain other drawings based on the drawings of the present invention, without creative efforts.

FIG. 1 is an assembly diagram of the whole screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation according to the present invention;

FIG. 2 is a schematic diagram of the structure of a supporting-screw according to the present invention;

FIG. 3 is a schematic diagram of the structure of a pulling-screw according to the present invention;

FIG. 4 is a schematic diagram of the structure of a variable cross section fixing rod according to the present invention;

FIG. 5 is a cross-section view of an anti-rotation joint portion according to the present invention, wherein FIG. 5 shows two different cross-section shapes;

FIG. 6 is a schematic diagram that shows the spondylolisthesis reduction with the screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation according to the present invention.

DETAILED DESCRIPTION

The solutions of the embodiments of the invention will be described explicitly and completely, in conjunction with the drawings. It is clear that the described embodiments are only a part of embodiments of the present invention, not all embodiments of the present invention. Based on the embodiments of the present invention, all the other embodiments obtained by those of ordinary skill without any creative works are within the protection scope of the present invention.
As shown in FIGS. 1-6, a screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation comprises two pulling-screws 2, two supporting-screws 1, two variable cross section fixing rods 3, two lock nuts 4 and a bracing beam 5. Each of the variable cross section fixing rods 3 connects with a supporting-screw 2 and a pulling-screw 1, the bracing beam 5 connects with both of the variable cross section fixing rods 4. Each of the supporting-screws 1 comprises a first tail 11 and a first head 12, and each of the pulling-screws 2 comprises a second tail 21 and a second head 22; both the first tail 11 and the second tail 21 are provided with nail grooves, with U-shaped grooves on the sides. Each of the nail grooves is provided with internal thread inside, and the lock nut 4 is connected with the nail groove by the internal thread. Each of the variable cross section fixing rods 3 passes through the U-shaped grooves and is fastened by the lock nut 4. The bottoms of the nail grooves of the supporting-screws 1 are 2-6 mm higher than the bottoms of the nail grooves of the pulling-screws 2, see H as shown in FIG. 2. The specific lifted height can be selected based on the actual situation, a specification can be set every 2 mm for preparation of products. Each of the variable cross section fixing rods 3 is provided with an anti-rotation joint portion 31 matched with the U-shaped grooves; the anti-rotation joint portion 31 has a plane (shown as a side in FIG. 4) and/or a curved surface (shown as an upper or lower portion in FIG. 4) fitting the U-shaped grooves closely. Shown as B in FIG. 5, all sides could be designed to be flat according to actual requirements; or shown as A in FIG. 5, three sides are flat, and only the bottom is a curved surface. An annular recess 23 is configured on the middle part of the outer wall of the second tail 21. The pulling-screw is a polyaxial screw, with the second head 22 connected to the second tail 21 which has a tapered thread tip. The first head 12 of the supporting-screw 1 is a single-axial screw which is fastened to the first tail 11, having a tapered thread tip; or the first head 12 of the supporting-screw 1 is a polyaxial screw which is fastened to the first tail 11, having a tapered thread tip. The connection type of a polyaxial screw includes: the polyaxial screw can pass through openings of the bottom of the nail groove of the first tail 11 or the second tail 21. The bracing beam comprises lock-rod nuts 51, lateral combination nuts 52 and a lateral connection plate 53; wherein both sides of the lateral connection plate 53 are provided with oval openings; wherein one end of the lock-rod nut 51 is connected to the first tail 11 or the second tail 21 through thread, the other end of the lock-rod nut 51 is fastened by one of the lateral combination nuts 52 after the other end passes through one of the oval openings. The lateral connection plate 53 is a straight plate or a curved plate, for different situations. It should be noted that, as the bracing beam 5 comprises lock-rod nuts 51, the first tail 11 or the second tail 21 is connected to the lateral connection plate 53, one end of the lock-rod nut 51 screws into nail grooves of the first tail 11 or the second tail 21, and the variable cross section fixing rod 3 is fastened, now the lock nut 4 has been replaced, and the function of the lock nut 4 has been achieved.
Experiments showing effect of using this product are described as follows:
General Information:
This group includes 6 patients (4 men, 2 women), who are 5-36 years, with an average age of 17. There exist one case of old odontoid fracture malunion atlantoaxial dislocation, two cases of fresh odontoid fracture and atlantoaxial dislocation, and three cases of os odontoideum and atlantoaxial dislocation. 6 patients have cervical region discomfort, wherein 2 patients suffer muscle force of arms and legs decrease, and the remaining 4 patients have normal kinaesthesis. American Spinal Injury Association (ASIA) classifies 6 patients into Class D (2 cases) and Class E (4 cases), as shown in Table 1. After admission to hospital, all the patients will have some x-ray plates from mouth opening normotopia position, cervical vertebra positive side position and over flexion-extension side position, atlantoaxial CT scan, cervical vertebra MRI scan and other examinations, in order to the situation of evaluate displacement fracture, atlantoaxial dislocation and spinal compression, and have a predetermined plan of screw fixing type.
Surgical Steps:
A patient undergoes general anesthesia with endotracheal intubation, and is in prone position, cervical vertebra flexes forward moderately, skull traction is maintained during surgery. A longitudinal incision which is approximately 6-8 cm long is made at the middle of the back neck, decollement is conducted from the external occipital protuberance to the inferior margin of C₂spinous process bone membrane, and the occipital bone, Q posterior arch and C2 lateral mass become visible, the attachment of semispinalis cervicis on C₂spinous process is maintained. The atlas and the axis are fastened by vertebral pedicle screws, after the screw entering point is determined, the opening is drilled, a hand drill is used to prepare the screw path by free hand, a probe is used to touch the integrity of the all walls, then one tapered tap with diameter of 3.0 mm and another tapered tap with diameter of 3.5 mm are used to tap successively, the pulling-screw is placed into the atlas, and the uniaxial supporting-screw is placed into the axis. Afterwards, the supporting-screw with pad height of 2 mm or 4 mm is selected according to the degree of dislocation, the variable cross section fixing rod with automatic anti-rotation function is trimmed and bent, wherein the elliptical portion is placed into the nail groove of the supporting-screw for axis, the cylindrical portion is placed into the nail groove of the pulling-screw for atlas, the nut of the supporting-screw for axis is tightened, and the orientation of the variable cross section fixing rod fixes automatically, then the nut of the pulling-screw for atlas is tightened, finally, the spondylolisthesis reduction is completed. After the reduction under fluoroscopy is satisfied, the long arm nail groove of the pulling-screw for atlas will be broken off, then the tail of screw for atlas is placed into the bracing beam. Cortex of bone of C₁posterior arch and C₂vertebral plate is removed by abrasive drilling, and the bone graft bed is prepared, cancellous particles of autogenous iliac bone are taken for bone grafting (since it is non-fusion fixation, preparation of the bone graft bed and Ilium grafting are unnecessary, the internal fixation apparatus can be removed in secondary operation after odontoid fracture healing, for recovering the atlantoaxial rotatory function). Negative pressure drainage is implemented, layered suture is carried out, and the surgery is completed.
The drainage tube will be removed 48 h after surgery, and the x-ray, CT and MRI reexamination will be conducted one week after surgery, in order to know situations of fracture reduction, reduction for the dislocation of the atlas and the axis and cord compression relieving.
Result:
The surgery of screws and rods for the posterior atlantoaxial vertebrae fixation and reduction has been performed successfully on 6 patients in this group, wherein there are 3 cases of non-fusion fixation and 3 cases of fusion fixation (see Table 1). The surgery time is 60-150 min (average 95 min), bleeding volume during surgery is 30-80 ml (average 50 ml), without spinal and vertebral artery injuries. Screws are placed into the screw path successfully during surgery, without slip-out and offset. The fixing rod can be inserted once for all, and it can prevent rotation automatically after the screw nut is tightened, it is not necessary to utilize a rod clamp and other anti-rotation tools to maintain direction. The fixing rod is placed into the nail groove of the screw for atlas, without auxiliary devices including lifting screws and pressing rods, the atlas and the axis can complete reduction automatically after the nut of the pulling-screw for atlas is tightened, fluoroscopy during operation shows that the reduction is satisfactory. The bracing beam can be implanted successfully, bending and repeated adjustment are unnecessary, the bone graft space between the atlas and the axis cannot be influenced. Postoperative x-ray plates and CT reexamination demonstrate that the implant position is good, and the reduction for the dislocation of the atlas and the axis is satisfactory; 2 patients in Class D (ASIA classification) all recovered to Class E.

TABLE 1

Patient		Age	Preoperative	Neurological	ASIA	Surgical
Number	Gender	(Years)	Diagnosis	Symptoms	Classification	Method

1	male	19	fresh odontoid	cervical	E	surgery of screws and
			fracture and	pain		rods for the posterior
			atlantoaxial			atlantoaxial vertebrae
			dislocation			fixation and reduction
						(non-fusion)
2	female	36	fresh odontoid	cervical	E	surgery of screws and
			fracture and	pain		rods for the posterior
			atlantoaxial			atlantoaxial vertebrae
			dislocation			fixation and reduction
						(non-fusion)
3	male	5	old odontoid	neck	E	grinding and breaking
			fracture malunion	discomfort		off odontoid malunion
			atlantoaxial			position through
			dislocation			transoral approach +
						surgery of screws and
						rods for the posterior
						atlantoaxial vertebrae
						fixation and reduction
						(non-fusion)
4	female	10	os odontoideum	neck discomfort	D	bone graft fusion of
			and atlantoaxial	and muscle		screws and rods for
			dislocation	strength of		the posterior
				lower		atlantoaxial vertebrae
				extremities is		fixation
				level 5−
5	male	10	os odontoideum	neck discomfort	D	bone graft fusion of
			and atlantoaxial	and muscle		screws and rods for
			dislocation	strength of		the posterior
				limbs is		atlantoaxial vertebrae
				level
4+		fixation
6	male	21	os odontoideum	cervical	E	bone graft fusion of
			and atlantoaxial	pain		screws and rods for
			dislocation			the posterior
						atlantoaxial vertebrae
						fixation

The above descriptions are only the preferred embodiments of the invention, not thus limiting the embodiments and scope of the invention. Those skilled in the art should be able to realize that the schemes obtained from the contents of the specification and drawings of the invention are within the scope of the invention.

Claims

1. A screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation, wherein the screw-rod instrument comprises two pulling-screws, two supporting-screws, two variable cross section fixing rods, two lock nuts and a bracing beam;

wherein each of the variable cross section fixing rods is configured to connect a supporting-screw and a pulling-screw, the bracing beam is configured to connect both of the variable cross section fixing rods;

wherein each of the supporting-screws comprises a first tail and a first head, and each of the pulling-screws comprises a second tail and a second head; both the first tail and the second tail are provided with nail grooves, with U-shaped grooves on the sides;

wherein each of the nail grooves is provided with internal thread inside, and the lock nut is connected with the nail groove by the internal thread; each of the variable cross section fixing rods passes through the U-shaped grooves and is fastened by the lock nut;

the bottoms of the nail grooves of the supporting-screws are 2-6 mm higher than the bottoms of the nail grooves of the pulling-screws.

2. The screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation according to claim 1, wherein each of the variable cross section fixing rods is provided with an anti-rotation joint portion matched with the U-shaped grooves; the anti-rotation joint portion has a plane and/or a curved surface fitting the U-shaped grooves closely.

3. The screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation according to claim 1, wherein the second tail is a long arm nail groove, with an annular recess configured on the middle part of the outer wall.

4. The screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation according to claim 1, wherein the pulling-screw is a polyaxial screw, with the second head connected to the second tail which has a tapered thread tip.

5. The screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation according to claim 1, wherein the first head of the supporting-screw is a single-axial screw which is fastened to the first tail, having a tapered thread tip; or the first head of the supporting-screw is a polyaxial screw which is fastened to the first tail, having a tapered thread tip.

6. The screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation according to claim 1, wherein the bracing beam comprises lock-rod nuts, lateral combination nuts and a lateral connection plate; wherein both sides of the lateral connection plate are provided with oval openings; wherein one end of the lock-rod nut is connected to the first tail or the second tail through thread, the other end of the lock-rod nut is fastened by one of the lateral combination nuts after the other end passes through one of the oval openings.

7. The screw-rod instrument specially used for posterior atlantoaxial vertebrae fixation according to claim 6, wherein the lateral connection plate is a straight plate or a curved plate.