CN222398597U - Centrally-mounted positioner - Google Patents

Abstract

The utility model belongs to the technical field of unicompartmental knee replacement instruments, and specifically relates to a central positioner, comprising: an anchor plate, the anchor plate is used to be inserted into the medial tibiofemoral joint gap of the knee joint; a movable plate, the movable plate is attached to the surface of the anchor plate and slides axially until it is attached to the medial femoral condyle of the knee joint, a plurality of positioning holes are equidistantly distributed axially on the upper surface of the movable plate, a T-shaped slot is axially opened in the middle of the upper surface of the movable plate, the two sides of the T-shaped slot extend toward the two sides of the anchor plate respectively, a guide rod fixedly connected to the movable plate is inserted at the opening of the T-shaped slot, and sliders are integrally installed on both sides of the guide rod located at one end of the inner side of the T-shaped slot, and the two sliders are respectively adapted to the parts of the T-shaped slot extending toward both sides. The utility model utilizes a movable central positioner to play a positioning and measuring role in the central positioning process of the medial femoral condyle, so that the femoral prosthesis can be located in the center of the medial femoral condyle, and the femoral prosthesis is perpendicular to the tibial plateau prosthesis.

Description

Centrally-mounted positioner

Technical Field

The utility model belongs to the technical field of unicondylar knee joint replacement instruments, and particularly relates to a centrally-mounted positioner.

Background

Since 1982, the Oxford unicondylar knee system has been successfully used to treat medial osteoarthritis of the knee. If surgical treatment is performed early in the disease, surgery may slow the rate of arthritis progression in other compartments of the knee joint and may provide long-term relief of symptoms. The product is improved repeatedly, the 10-year survival rate is up to 90%, and the average annual wear is 0.03mm.

The traditional unicondylar knee system mainly includes a tibial component and a meniscal pad as follows:

The tibia component is also made of cast cobalt-chromium-molybdenum alloy, after a patient limb is bent by about 90 degrees, a skin incision on the inner side of the patella is made from the inner side edge of the patella to a position 3cm below a joint line, then a joint capsule incision of about 2com is made along the skin incision in an adaptation way, the lower half part of the whole incision exposes a tibia nodule to the front inner side edge of a tibia platform, after osteophytes are removed, a femur clearance measurer, a tibia saw guide and a G-shaped clamp are used, an osteotomy plane can be accurately positioned, a first cutter osteotomy is carried out on the tibia platform on the knee joint, and a tibia template is fixed by a screw at the position after the treatment is finished;

the femur component is made of cast cobalt-chromium-molybdenum alloy, and the unique spherical design can increase the strength, the wear resistance and the biocompatibility of the prosthesis and mainly comprises a head nail and a headless nail;

The meniscus liner is made of ultra-high molecular weight polyethylene which is directly molded, has the characteristic of high wear resistance and can slide on a tibia template.

The unicondylar knee joint replacement operation adopts a metal plate with radian to penetrate into the gap between the tibia and the femur, and a height-adjustable measuring block is arranged on the head of the metal plate to measure the gap in real time, and after the gap is calibrated, the punching position of the unicondylar of the femur cannot be given and guided, so that medical staff often judge through experience, and errors are easy to occur. Therefore, we propose a medial tibial femoral joint gap of the knee joint with a centrally positioned locator defined for the medial femoral condyle punch location.

Disclosure of utility model

The utility model aims to provide a centrally-mounted positioner, which is used for playing a positioning and measuring role in the medial femoral condyle centrally-mounted positioning process, so that a femoral side prosthesis can be positioned in the center of the medial femoral condyle, and the femoral side prosthesis is perpendicular to a tibial plateau prosthesis.

The technical scheme adopted by the utility model is as follows:

a centrally located positioner, comprising:

an anchor plate for insertion into the medial tibiofemoral joint space of the knee;

The movable plate is attached to the surface of the anchoring plate and slides along the axial direction until being attached to the medial femoral condyle of the knee joint, a plurality of positioning holes are distributed on the movable plate at equal intervals along the axial direction, when the femur is drilled, the movable plate slides along the middle of the upper surface of the anchoring plate and gradually approaches the medial femoral condyle, after the movable plate is aligned with the midline of a femoral osteotomy zone, a designated positioning hole is utilized to pass through a micro-drill, a guide hole is drilled at the midline position of the femoral osteotomy zone, two mounting holes are symmetrically drilled at the upper and lower positions of the guide hole respectively for the double columns of the femoral condyle prosthesis to be respectively embedded into the medial femoral condyle, thus, a movable central positioner is formed by the anchoring plate and the movable plate, the positioning and measuring functions are exerted in the processes of the tibia section, the medial femoral condyle drilling guide way device, the punching position of the medial femoral condyle is limited, the femoral side prosthesis can be positioned at the center of the medial femoral condyle, and the femoral side prosthesis is perpendicular to the tibial plateau prosthesis.

T-shaped grooves are formed in the middle of the upper surface of the movable plate along the axial direction, two sides of each T-shaped groove extend towards two sides of the anchor plate respectively, guide rods fixedly connected with the movable plate are inserted into the openings of the T-shaped grooves, and when the holes are drilled, the movable plate drives the guide rods to move and slide along the openings of the T-shaped grooves, so that the movable plate is limited and moves along the central line of the anchor plate.

The guide rod is located the inside both sides of one end of T shape groove and all integrally installs the slider, two the slider respectively with the T shape groove towards the partial adaptation of both sides extension.

Four ribs of the anchor plate are uniformly provided with hammer heads, and the four hammer heads respectively extend towards the middle of one side of the anchor plate where the hammer heads are positioned.

And slopes are integrally arranged on one sides of the four hammer head parts, which face the middle of the anchor plate.

And the sides of the four hammer head parts, which are far away from the middle of the anchor plate, are provided with round corners.

The upper surface of the anchoring plate is also provided with a step groove, and the step groove is from one end of the anchoring plate to the middle of the anchoring plate.

Four the upper edge that the anchor board was kept away from to tup portion is all integrally installed the bank protection, the bank protection all has the radian towards one side of anchor board.

The utility model has the technical effects that:

A middle-set positioner for femur is prepared as bending knee joint by about 45 deg., drilling bone marrow by electric drill, inserting intramedullary rod until intramedullary rod is propped against internal surface of femur, bending knee joint to 90 deg., inserting anchoring plate 1, sliding movable plate 2 along middle of upper surface of anchoring plate, gradually closing to internal condyle of femur, aligning central line of femur osteotomy region, punching several guide holes by means of designated locating hole 3, punching two installing holes for inserting two columns of femoral condyle prosthesis into internal condyle of femur, forming movable middle-set positioner by anchoring plate and movable plate, positioning and measuring internal condyle of femur, defining central punching position of internal condyle of femur, and making internal condyle prosthesis be perpendicular to platform prosthesis.

According to the centrally-mounted positioner, when the hole is punched, the movable plate drives the guide rod to move, and the guide rod slides along the opening of the T-shaped groove, so that the movable plate is limited, and the movable plate moves along the central line of the anchoring plate.

According to the middle-set positioner, the step groove forms a height difference at the two hammerheads, so that the middle-set positioner can be inserted into the knee joint inner side tibiofemoral joint gaps with different opening degrees, can be used in two sizes, and has more various functions.

According to the centrally-mounted positioner, two adjacent revetments can be in an arc shape at the hammerhead, so that the two sides of the medial condyle of the femur can be dragged, the centering of the T-shaped groove is facilitated, manual alignment is not needed, and the position of the guide hole can be conveniently and rapidly calibrated.

Drawings

FIG. 1 is a front view of a centrally located positioner of the present utility model;

FIG. 2 is a side view of a centrally located positioner of the present utility model;

FIG. 3 is a side view of the fly leaf of the present utility model;

FIG. 4 is a front view of an anchor sheet of the present utility model;

FIG. 5 is a side view of an anchor sheet of the present utility model;

Fig. 6 is a top view of an anchor sheet of the present utility model.

In the drawings, the list of components represented by the various numbers is as follows:

1. The device comprises an anchor plate, a movable plate, a positioning hole, a T-shaped groove, a guide rod, a sliding block, a hammer head, a slope, a step groove, a slope protection and a slope protection.

Detailed Description

The present utility model will be specifically described with reference to examples below in order to make the objects and advantages of the present utility model more apparent. It should be understood that the following text is intended to describe only one or more specific embodiments of the utility model and does not limit the scope of the utility model strictly as claimed.

As shown in fig. 1-6, a centrally located locator, primarily for use in unicondylar knee replacement surgery, comprises:

the anchoring plate 1 is used for being inserted into a tibial and femoral unicondylar gap in the knee joint;

The movable plate 2 is stuck to the surface of the anchor plate 1 and axially slides until the movable plate 2 is stuck to the medial condyle of the knee joint femur, and a plurality of positioning holes 3 are axially and equidistantly distributed on the movable plate 2;

The unicondylar knee joint replacement is suitable for treating knee joint medial osteoarthritis, the patient has to have full-layer cartilage loss at two sides of knee joint medial compartment, and bone-to-bone contact is confirmed by imaging examination or arthroscopy, and the operation is specifically developed, and comprises the following steps:

Step one, preoperative X-ray template measurement, namely drawing the contour of the medial condyle of the femur on an X-ray sheet by using a template, wherein the extension line of a middle post of the prosthesis is bent by 10 degrees compared with the long axis of the femur shaft, the outer surface of a schematic diagram of the prosthesis is positioned at the outer side of the X-ray image by about 2mm, which is the thickness of articular cartilage, and for the prosthesis with accurate model, the proximal end part of the prosthesis is positioned at the outer side of the proximal bone surface of the femoral condyle by about 2mm, so that the surface of the prosthesis is flush with the reserved proximal cartilage;

Step two, limb treatment of a patient, namely placing a patient limb after the towel spreading on a thigh support after the thigh tourniquet is inflated, and bending hip joints by about 30 degrees to enable lower limbs to be in a sagging position;

Bending the knee joint to 90 degrees, and making a skin incision on the inner side of the patella from the inner side edge of the patella to the position 3cm below the joint line, and incising a joint capsule through the skin incision, wherein the upper end of the joint capsule incision is required to extend proximally for 2cm to the inside of the thigh muscle;

then the joint capsule incision extends downwards to the inner side of the patella ligament along the inner side edge of the patella, the lower half part of the incision exposes the tibial tuberosity to the front inner side edge of the tibial plateau, and as much of the inner meniscus is resected as possible without loosening the inner collateral ligament;

While the surgeon in the learning process should enlarge the surgical incision for better exposure;

Step three, the osteophyte is resected, namely all the osteophytes on the inner side edge of the inner condyle of the femur and the inner side edge of the intercondylar fossa must be resected, the operation assistant stretches the knee joint to enable the incision to move up and down so as to be convenient for exposing all the osteophytes in the visual field, and the osteophytes in front of the ACL dead center of the tibial plateau and the osteophytes on the top of the intercondylar fossa must be resected to correct and fix buckling deformity;

A 6mm narrow osteotome is used for cutting off the osteophyte from the lower part of the medial collateral ligament to the rear lateral edge of the medial condyle of the femur, and enough space can be provided for inserting a saw blade into the intercondylar fossa in the next step after the osteophyte is cut off;

step four, the tibia platform cuts bones, namely placing the knee joint in a buckling position, and inserting one end of a femoral condyle measurer according to the pre-operation estimated model, wherein the initial thickness is 1mm;

Taking out all the drag hooks, evaluating ligament tension, and adaptively selecting a femoral condyle measurer with ultra-small size, medium size or large size to obtain proper ligament tension; if proper ligament tension is not obtained, replacing the femoral condyle measurer with a different model until proper ligament tension is obtained, checking the relation between the front of the femoral condyle measurer and the approximate cartilage surface position of the patient before arthritis is caused to determine the optimal model of the femoral component, inserting the correct femoral condyle measurer into the center of a compartment on the inner side of the knee joint, inserting a tibial saw guide, and enabling the long axis of the guide to be parallel to the long axis of the tibia;

The femur condyle measurer, the tibia saw guide and the G-shaped clamp are used simultaneously to accurately position the osteotomy plane;

then, a swing saw and a hard narrow saw blade are used for performing sagittal plane osteotomy of the tibia;

Step five, femur drilling and alignment, namely bending the knee joint by about 45 degrees, drilling the bone marrow of the femur by using a 4mm electric drill, wherein the operation needs to use a 5mm opening cone, the femur drilling position is required to be 1cm in front of the front edge of the femur and is clung to the inner side of the inner wall of the femoral malleolus fossa, and the drilling direction is towards the anterior superior iliac spine;

Then inserting the intramedullary rod until the intramedullary rod abuts the inner surface of the femur;

The knee joint is bent to 90 degrees, an anchor plate 1 is inserted, a movable plate 2 slides along the middle of the upper surface of the anchor plate and gradually approaches to the medial femoral condyle, after the center line of a femoral resection area is aligned, a designated positioning hole 3 is utilized to pass through a micro-drill needle, a plurality of guide holes are drilled at the center line position of the femoral resection area, two mounting holes are respectively drilled at the upper and lower positions of the guide holes and are used for respectively embedding the double columns of the femoral condyle prosthesis into the medial femoral condyle;

Simultaneously inserting the femoral drill guide, evaluating joint gap thickness, inserting the intramedullary rod connector onto the intramedullary rod and inserting the other end thereof into the left/outer hole of the femoral drill guide, so as to ensure proper alignment of the guide;

Step six, femur osteotomy, namely inserting a femur rear osteotomy guide block into the drilled hole, and striking and fixing;

Inserting a drag hook to protect MCL, cutting off the posterior articular surface of femoral condyle by using a sagittal saw with the width of 12mm, pressing down the saw blade to slightly bend the saw blade so as to ensure that the saw blade is tightly abutted against the lower surface of a rear osteotomy guide block to perform osteotomy, carefully operating to avoid damaging medial collateral ligaments and anterior cruciate ligaments, and taking down the guide block by using a striking hammer to ensure that the guide block is taken down parallel to a femoral drilling guide hole to avoid damaging the drilling holes;

good exposure has now been achieved behind the knee, resecting the residual medial meniscus;

in the MCL area, a small meniscus should be left to avoid damage to the MCL by the tibial component, and the posterior meniscus corner must be completely resected;

Wherein, the femur osteotomy area can be ground for several times and thoroughly clean bone fragments;

Step seven, testing the model, namely bending the knee joint for 100 degrees at the moment, inserting a tibia template and testing the model by using a single-column femur component so as to confirm the balance of bending and stretching gaps;

Maintaining the meniscus pad trial in place, moving the knee joint in a full range of joint motion, confirming knee joint stability, pad fixation and no impact;

Step eight, finally preparing, namely inserting a tibia template with a proper model, in order to ensure that the proper model is selected, placing the rear edge of the template and the rear bone Pi Zhidui of the tibia when placing the tibia template, enabling a universal osteotomy hook to pass through the rear cortex of the tibia to help to complete the operation, enabling the tibia template to be aligned with the medial cortex of the tibia or slightly overhang, and using a small-size tibia assembly if the overhang exceeds 2 mm;

step nine, fixing the prosthesis, namely firstly placing a small amount of bone cement on the tibia osteotomy face and trowelling to manufacture a bone cement thin layer covering the whole tibia osteotomy face, inserting a tibia template and compacting downwards, compacting the rear part, compacting the front part, and extruding redundant bone cement in front of the tibia;

Extruding a second portion of bone cement into the femoral drill hole, coating the bone cement on the concave surface of the meniscus gasket, placing the meniscus gasket coated with the bone cement on the medial condyle of the femur at 45 degrees with the long axis of the femur, compacting the meniscus gasket by using a beater, scraping off superfluous bone cement at the edge by using a Woodson bone cement spatula, bending the knee joint at 45 degrees and keeping the posture, inserting a proper depth finder to pressurize the bone cement, and not completely straightening or bending the knee joint, or possibly shaking the prosthesis and loosening the prosthesis;

Step ten, ending, closing the incision conventionally.

Meanwhile, a double-column femoral prosthesis is used in the femur section, the inner cambered surface of the double-column femoral prosthesis is filled with porous titanium plasma spraying and hydroxyapatite double-coating, and the tibia template is provided with PPS and HA double-coating on all bone/prosthesis contact surfaces.

Among other things, the patient has the following notes:

1) The anterior and posterior cruciate ligaments must be functional, and anterior cruciate ligament defects are a contraindication of unicondylar knee replacement;

2) If the knee joint side position sheet shows bone loss after knee joint or uncorrectable side subluxation occurs on the everting stress position sheet, prompting the damage of Anterior Cruciate Ligament (ACL), and if the integrity of the ACL is doubtful, evaluating by using a crochet needle in the operation process;

3) The compartment outside the knee joint should be well functioning, with complete meniscus and full-thickness articular cartilage;

4) The varus deformity in the joint must be passively corrected to a pre-disease state, but cannot exceed a normal range, and the valgus stress position slice of the knee joint can be shot for confirmation;

5) The passive correction of joint deformity after the application of valgus stress is more important than the degree of intra-articular deformity, and the valgus deformity exceeding 15 degrees can be rarely corrected to a neutral position, so that soft tissue loosening can not be performed absolutely;

6) Knee joint flexion deformity should be less than 15, if the flexion deformity is greater than 15, the anterior cruciate ligaments will typically have been broken;

7) Under the anesthesia state, the knee joint of the patient must be capable of buckling to more than 110 degrees, so that the femoral condyle can be treated in the operation;

8) Age, weight or activity level of the patient is not a surgical contraindication, while cartilage calcification is also not a contraindication for unicondylar knee replacement;

9) Unicondylar knee replacement is strictly prohibited in all types of inflammatory arthritic patients;

10 After cutting the knee joint and performing a direct examination, a final decision is made as to whether a unicondylar knee arthroplasty can be performed.

As shown in fig. 2 and 3, a T-shaped groove 4 is axially provided in the middle of the upper surface of the movable plate 2, two sides of the T-shaped groove 4 extend toward two sides of the anchor plate 1 respectively, a guide rod 5 fixedly connected with the movable plate 2 is inserted into the opening of the T-shaped groove 4, and when the hole is punched, the movable plate 2 drives the guide rod 5 to move, slides along the opening of the T-shaped groove 4, and plays a limiting role on the movable plate 2, so that the movable plate 2 moves along the central line of the anchor plate 1.

As shown in fig. 2 and 3, the two sides of the guide rod 5, which are located at one end inside the T-shaped groove 4, are integrally provided with the sliding blocks 6, and the two sliding blocks 6 are respectively matched with the parts, extending towards the two sides, of the T-shaped groove 4, so that the guide rod 5 is attached to the T-shaped groove 4, the guide rod 5 can be horizontally righted, and the skew degree is reduced.

As shown in fig. 4, 5 and 6, four ribs of the anchor plate 1 are integrally provided with hammer heads 7, and the four hammer heads 7 extend towards the middle of one side of the anchor plate 1 where the hammer heads are located respectively, so that two hammer heads are formed at two ends of the anchor plate 1 respectively, and the hammer heads are convenient for medical staff to hold.

As shown in fig. 4, 5 and 6, the four hammer heads 7 are provided with slopes 8 towards one side of the middle of the anchor plate 1 in an integrated manner, so that the two hammerheads can better fit with the radian of the hand, the holding of medical staff is facilitated, the edge angle of the middle-arranged positioner is reduced, and the hand is prevented from being cut.

As shown in fig. 4, 5 and 6, the sides of the four hammer heads 7 far away from the middle of the anchor plate 1 are provided with round corners, so that the edges and corners of the middle positioning device are further reduced, the hands of medical staff can be prevented from being cut, and tissues at the opening can be prevented from being cut.

As shown in fig. 4, 5 and 6, the upper surface of the anchor plate 1 is further provided with a step groove 9, the step groove 9 is formed from one end of the anchor plate 1 to the middle of the anchor plate 1, the step groove 9 forms a height difference at two hammerheads, can be inserted into the gap between the tibia and the thigh at the inner side of the knee joint with different opening degrees, can be used for playing the purposes of two sizes by a middle-arranged positioner, and has more various functions.

As shown in fig. 4, 5 and 6, the slope protection 10 is all integrally installed at the upper edge that four hammer heads 7 kept away from anchor plate 1, and slope protection 10 all has the radian towards one side of anchor plate 1, and two adjacent slope protection 10 can be the arc in the tup department that belongs to and drag the both sides of the medial condyle of femur, make things convenient for T-shaped groove 4 to center, need not the manual work to find, be convenient for the position of quick calibration guide hole.

As shown in fig. 1 and 2, the surface polishing treatment is performed on the anchor plate 1, the movable plate 2 and the positioning hole 3, so that the surface roughness is small, the femur and tibia are not worn, and the surfaces of the T-shaped groove 4, the guide rod 5, the sliding block 6, the hammer head 7, the slope 8, the step groove 9 and the slope protection 10 are free of defects such as sharp edges, burrs and attachments, so that the whole middle-set positioner is ensured not to cause unnecessary damage to surrounding tissues in the use process. In addition, the roughness of these surfaces should be controlled within a certain range to further reduce irritation and friction.

As shown in fig. 2 and 3, the spacing between the positioning holes 3 is 3mm, leaving enough space for more densely selectable positioning of the positioning holes 3, so that both mounting holes have enough space for drilling in the medial femoral condyle.

The utility model has the working principle that when the femur is drilled and aligned, the knee joint is bent for about 45 degrees, a 4mm electric drill is used for drilling the bone marrow of the femur, the operation needs to use a 5mm opening cone, the femur drilling position is required to be 1cm in front of the front edge of the femur and is clung to the inner side of the inner wall of the malleolus fossa of the femur, and the drilling direction is towards the anterior superior iliac spine;

Then inserting the intramedullary rod until the intramedullary rod abuts the inner surface of the femur;

The knee joint is bent to 90 degrees, the anchor plate 1 is inserted, the movable plate 2 slides along the middle of the upper surface of the anchor plate and gradually approaches to the medial condyle of the femur, after aligning to the midline of the femoral resection area, a plurality of guide holes are drilled at the midline of the femoral resection area by utilizing the designated positioning holes 3 through the micro-drilling needle, two mounting holes are drilled at the upper and lower positions of the guide holes respectively for the double columns of the femoral condyle prosthesis to be respectively embedded into the medial condyle of the femur, and thus, the movable middle-setting positioner is formed by the anchor plate and the movable plate, the positioning and measuring functions are exerted in the middle-setting positioning process of the medial condyle of the femur, the femoral drilling guide is guided to drill the medial condyle of the femur, and the punching position of the medial condyle of the femur is limited, so that the mounting precision of the femoral prosthesis is improved.

The foregoing is merely a preferred embodiment of the present utility model and it should be noted that modifications and adaptations to those skilled in the art may be made without departing from the principles of the present utility model, which are intended to be comprehended within the scope of the present utility model. Structures, devices and methods of operation not specifically described and illustrated herein, unless otherwise indicated and limited, are implemented according to conventional means in the art.

Claims (9)

1. A centrally located positioner, comprising:

An anchor plate (1), the anchor plate (1) being for insertion into a medial tibiofemoral joint space of a knee;

The movable plate (2) is stuck to the surface of the anchor plate (1) and slides along the axial direction until the movable plate (2) is stuck to the inner side condyle of the knee joint femur, and a plurality of positioning holes (3) are distributed on the movable plate (2) along the axial direction at equal intervals.

2. The centrally-mounted positioner according to claim 1, wherein a T-shaped groove (4) is formed in the middle of the upper surface of the movable plate (2) along the axial direction, two sides of the T-shaped groove (4) extend towards two sides of the anchor plate (1) respectively, and a guide rod (5) fixedly connected with the movable plate (2) is inserted into an opening of the T-shaped groove (4).

3. The centrally-mounted positioner according to claim 2, wherein the guide rods (5) are integrally provided with sliding blocks (6) on both sides of one end of the inside of the T-shaped groove (4), and the two sliding blocks (6) are respectively matched with the extending parts of the T-shaped groove (4) towards both sides.

4. A centrally located anchor assembly according to claim 1, wherein four ribs of the anchor sheet (1) are integrally provided with hammer heads (7), the four hammer heads (7) extending toward the middle of the side of the anchor sheet (1) where they are located.

5. A centrally located positioner according to claim 4, wherein the four hammerhead portions (7) are each integrally provided with a ramp (8) on a side facing the middle of the anchor plate (1).

6. A centrally located anchor assembly as claimed in claim 4, wherein the four hammerhead portions (7) are rounded at a side thereof remote from the middle of the anchor plate (1).

7. The centrally-mounted positioner according to claim 1, wherein the upper surface of the anchor plate (1) is further provided with a step groove (9), and the step groove (9) extends from one end of the anchor plate (1) to the middle of the anchor plate (1).

8. The centrally-mounted positioner according to claim 4, wherein the four hammer head parts (7) are integrally provided with slope protection (10) at the upper edges far away from the anchor plate (1), and the slope protection (10) has radians at the sides facing the anchor plate (1).

9. A centrally located positioner according to claim 1, wherein the spacing of the positioning holes (3) is 3mm.