WO2020161374A1

WO2020161374A1 - Partial prosthesis for the scaphoid bone

Info

Publication number: WO2020161374A1
Application number: PCT/ES2020/070078
Authority: WO
Inventors: Patricia GÓMEZ BARBERO; Daniel MONTANER ALONSO; José Luis RODRIGO PÉREZ
Original assignee: Universitat De València; Fundacion Para El Fomento De La Investigación Sanitaria Y Biomédica De La Comunitat Valenciana
Priority date: 2019-02-08
Filing date: 2020-02-03
Publication date: 2020-08-13
Also published as: ES2778573A1; ES2778573B2

Abstract

The invention relates to a partial prosthesis for the scaphoid bone, which is adapted to replace a proximal portion of the scaphoid bone following its removal, thereby protecting at least the distal pole of the entire scaphoid bone and maintaining the integrity of the scaphotrapeziotrapezoid joint. It has the advantage of not having radial anchoring that passes through the radiocarpal joint. Instead, the prosthesis is kept in position by means of: (a) distal anchoring to the remaining scaphoid bone using a blind hole, for example, by means of a threaded Morse taper; and (b) proximal stabilisation by means of proximal channels that enable a tendon graft, formed by the palmaris longus flexor muscle or the plantaris muscle, to pass to the semilunar bone.

Description

PARTIAL PROSTHESIS FOR THE SCAFOID BONE

The present invention relates to a partial scaphoid bone prosthesis, adapted to replace, after exeresis of a proximal portion of the actual scaphoid, said removed proximal portion, thereby keeping at least the distal pole of the entire scaphoid bone undamaged and furthermore with integrity of the scapho-trapezius-trapezoid joint.

Background of the invention

The wrist, and especially the carpal bones, form a joint joint subjected to significant forces of shear, torsion and compression. The scaphoid bone is the first external bone of the first row (proximal row) of the carpus. It has a cuboid shape, with six faces, of which four are articular, and presenting a proximal pole and a distal pole. It articulates with the radius, lunate bone, large bone, trapezoid bone, and trapezius bone. Within the carpus, the scaphoid plays a fundamental role as it acts as a connecting link between the two rows. Any alteration in the same entails a modification of the biomechanics of the wrist, causing an abnormal transmission of loads between the radius and the scaphoid that leads to pain symptoms with loss of mobility and in the future to joint instability that will lead to a carpal osteoarthritis.

On the other hand, after an impact on the hand or wrist, scaphoid bone fractures are the most common, representing 80% of carpal fractures. The importance of its correct diagnosis and treatment in the acute phase is fundamental for the prognosis and evolution, since the lesions of this bone not treated or incorrectly managed will have long-term complications such as the absence of consolidation or pseudoarthrosis, avascular necrosis up to 50 % of cases or chronic instability that will progressively lead to the development of carpal osteoarthritis.

Chronic “non-union” of the scaphoid and post-traumatic or idiopathic avascular bone necrosis (NOA) (Preiser's disease) are closely related to its anatomy, since the scaphoid bone is covered in more than 80% of articular cartilage where the supply main blood comes from the dorsal radial artery (70-80%), through the ligamentous attachments, although the blood supply at the proximal pole level depends only on intraosseous flow, which explains the delayed consolidation rates, recalcitrant pseudoarthrosis or NOA. Faced with any of these injuries, it will be indicated to carry out a surgical gesture in order to start the osteogenesis process again, correct bone deformities and avoid carpal collapse secondary to non-union.

The primary surgical options include the use of vascularized and non-vascularized bone grafts, with or without supplemental fixation with needles and screws. Unfortunately, these techniques are not always successful, and in some cases we find episodes of pseudoarthrosis or advanced NOA where, secondarily, an instability of the midcarpal joint has been generated that leads to the development of osteoarthritis and carpal collapse, following the patterns called SNAC wrist ( “Scaphoid nonunion advange collapse ').

The progression of the SNAC wrists is slow, presenting arthritic changes from 5-10 years after the initial injury in the scaphoid. In the SNAC wrist with the presence of a proximal scaphoid fragment, the joint between it and the radius remains preserved for many years, acting as a second lunate, which on many occasions does not prevent a progressive degenerative process of its cartilage, fragmentation and trabecular collapse. The problem of advanced carpal collapse is that it poses additional treatment difficulties, in many cases not well resolved, since in most cases salvage surgical techniques are chosen (proximal carpectomy, scaphoid excision or midcarpal arthrodesis such as the four corners or of the luno-large joint or total arthrodesis), where pain is controlled but sacrificing a great joint range with the consequent functional limitation in their activities of daily living. In addition, all these increasingly aggressive surgeries carry a high risk of complications such as infection, pseudoarthrosis, failure of the support system, prominence of the system with irritation of the soft tissues or rigidity associated with a long time of immobilization. In fact, in four-corner arthrodesis or carpectomy, the midcarpal joint is removed, and revision of these interventions requires a total wrist arthrodesis or arthroplasty.

The mission of an orthopedic surgeon should be to try to relieve pain while maintaining the greatest range of motion and strength possible.

In most joints of the human body, the use of arthroplasty is established in cases of osteoarthritis or NOA in order to recover the previous joint situation, controlling pain and trying to preserve the greater joint range. However, at the level of the joint radio-scaphoid, reconstructive surgery gives way to salvage surgery, where the patient's pain is effectively controlled, but at a very high price. In these cases, an alternative should be available where more mobile wrists without pain could be offered to the patient, with prosthetic replacement being a feasible alternative.

Scaphoid excision represents an alternative in the treatment of advanced carpal osteoarthritis. Scaphoidectomy is a relatively simple technique for those patients with symptoms secondary to chronic nonunion of the scaphoid or bone necrosis of the proximal fragment, a fragment with a more precarious vascularization where this pathology generally takes root.

Scaphoid excision can be partial at the distal pole, at the proximal pole, or complete. Isolated complete scaphoidectomy is a technique currently ruled out as it presents problems associated with progressive carpal collapse with pain, significant loss of strength and mobility. Therefore, if it is carried out, it requires complementary techniques that stabilize the rest of the bones so that they transmit the load through the lunate bone.

The idea of stabilizing the carpus after excision of the scaphoid with arthroplasty has been tried repeatedly. Its defenders argue that it is a simple and less aggressive technique than salvage techniques where the anatomy and biomechanics of the carpus are recovered without sacrificing other joints. Furthermore, in case of failure there is always the possibility of resorting to a four-corner arthrodesis or proximal carpectomy.

Currently, some models of prostheses designed to replace the entire scaphoid bone are known. For example, European patent No. EP3170477 and WO201886765 describe respective scaphoid prostheses manufactured in a compatible biomaterial, where this material can comprise at least one element from the group consisting of titanium or be a polymer, such as a polyetherketone or a ceramic material. , eg, a ceramic material containing zirconium. However, for the stabilization of these scaphoid prostheses they have a central channel along their axis. In this, the tendon plasty is passed, which is fixed by drilling a channel (110) through the lunate where the tendon is passed again according to the Henry / Corella technique and the dorsal and palmar ligaments of the scapho-lunar are biomechanically reconstructed to ensure the physiological movement of the prosthesis. They are also known from patent document No. US4198712 and US4936860 respective complete scaphoid prostheses. In the first case (US4198712) a complete anatomical replica prosthesis of the scaphoid is described, with a stem (78) that is inserted in the trapezius (that is, in the bone of the distal row of the carpus) and is fixed to the lunate with a suture Transosseous with 2-0 Dexon to provide stability to the implant, or in cases where the anatomy of the patient does not allow the use of stems, it can be shortened or even eliminated and fixed by means of a transosseous suture to the trapezius. It is made of an elastic material. In the second case (US4936860) a complete anatomical replica prosthesis of the scaphoid is also described, but in this case metallic, and it presents a shaft (104) that extends inside the implant body. It is positioned on the wrist with the shaft that is inserted into the trapezius to provide stability to the implant with a procedure similar to that described in patent No. US4198712. The first published results of the first prosthesis (US4198712) were very encouraging, however, in the long term it was observed that the carpal collapse progressed, possibly associated with the development in most patients of a synovitis due to silicone particles (88% of the patients). 8 years) 12, 13. This fact led to implant removal in some series in 90% of patients due to pain requiring partial arthrodesis interventions in approximately 25-30% of cases.

Failures in attempts to replace the scaphoid have been attributed to the material, synovitis secondary to silastic, and implant instability. Implant instability has been observed in many of the current models, regardless of the material used, with pain, implant flexion and carpal collapse with deformity in DISI.

The pyrolytic carbon material has been used in arthroplasties of the trapeziometacarpal, interphalangeal or lunate joint.

A prosthesis, in this case partial, in the shape of an ellipsoid for the proximal pole of the scaphoid, made of pyrolytic carbon, is known from patent document No. US6090145. This prosthesis is known as APSI “Adaptive Proximal Scaphoid Implant” (Bioprofile®, Grenoble, France), and is used for cases of avascular necrosis or proximal nonunion without arthritic signs in the proximal radioscaphoid joint. This prosthesis is inserted between the lunate bone and the distal scaphoid remnant after an exeresis of the proximal pole, its stability being based on the three-dimensional adaptation of the implant to the movements of the wrist. Therefore, it does not provide any type of fixing element to the rest of the bones or ligaments of the carpus and only acts as a mere spacer, that is, it is in mobile contact with at least one bone surface and the implant remains free. It has been found that, in the short term, this prosthesis allows a recovery of grip strength similar to that of the contralateral wrist, a symmetrical mobility to the contralateral side. However, it is difficult to understand that an implant that dispenses with the support provided by the intrinsic carpal ligaments can be kept in place simply with the help of the joint capsule or its three-dimensional morphology, so with this implant the problem of Instability. There are currently long-term studies with a mean follow-up of 8.7 years that corroborate this, presenting 21% of implant dislocations or deformity in DISI. Therefore, the problem of stabilizing a partial scaphoid prosthesis persists in an attempt to keep the implant in position to minimize the risk of implant dislocation and for it to function according to carpal kinematics.

In view of the above, there is an evident need for a partial prosthesis of a proximal part of the scaphoid bone, which allows stabilizing the prosthesis to keep it without moving and that it works according to carpal kinematics, thus reducing and even completely eliminating the risk of a dislocation of the prosthesis.

Description of the invention

In the description of this invention, the term implant or prosthesis is used interchangeably, with the same meaning.

In the description of this invention the term "proximal" is used as the part of the scaphoid closest to the radius, and the term "distal" as the part of the scaphoid furthest from the radius.

The object of the present invention is to provide a partial prosthesis for a proximal part of the scaphoid bone of a patient, which solves the aforementioned drawbacks and has the advantages that are described below.

According to a first aspect, the present invention provides a partial prosthesis of the scaphoid bone, adapted to replace, after an exeresis of a proximal part of the real scaphoid of a patient, said removed proximal part, while the remaining part of the real scaphoid corresponding to the side distal remains in place without retreating as bone remnant, characterized in that it comprises a body: with a shape and external dimensions that correspond substantially to the actual morphology of the part removed from the scaphoid corresponding to a portion of the proximal side of the scaphoid, which, on the distal side, comprises a blind hole that extends along the prosthesis, where this blind hole comprises at least one threaded portion so that a fixation element can pass through the scaphoid bone remnant in a retrograde direction to stabilize the prosthesis distally, and that, on the proximal side , comprises through channels for the passage of a proximal tendinous plasty with the Flexor Palmar Largo muscle or with the Plantar Delgado muscle up to the lunate bone, so that the prosthesis is anatomically stabilized at the proximal level with the lunate bone.

Furthermore, the prosthesis is made of a material that comprises a biocompatible material.

The prosthesis of the invention is based on maintaining the intrinsic capacity of the wrist at the distal pole, which is not affected in most cases, and making an anatomical partial prosthesis that only requires stabilization at the proximal level. In other words, instead of completely replacing the scaphoid, the distal third of the scaphoid is maintained together with the trapezius, so the prosthesis of the invention respects the intrinsic ligaments of the wrist, which play an important role in carpal kinematics during the different movements, especially in the “throwing darts” movement. Furthermore, this distal scaphoid remnant will be used for the distal anchoring of the prosthesis, so that in future wrist movement they act as a single block, imitating the previous movement of the scaphoid.

In other words, the prosthesis of the invention advantageously does not act as a simple spacer (as is the case of patent No. US6090145) that, due to its curved surfaces and the low coefficient of friction that pyrolytic carbon presents, allows the prosthesis to slide , which is undesirable since it leads to a risk of implant dislocation. This type of prosthesis type "spacers" were placed where previously the damaged bone was and filled its space to avoid collapse. The problem is that these systems were not fixed to other carpal bones trying to reproduce the normal anatomy of the wrist where there are multiple intrinsic and extrinsic ligaments that allow it to act in a coordinated and synchronous manner.

Furthermore, another advantage is that this invention does not have any anchor at the level of the radius that passes through the radiocarpal joint, which is also undesirable. On the contrary, the implant of the present invention is optimally held in position in the patient, thanks to:

- (a) its anchorage distally to the scaphoid bone remnant thanks to the provision of a single blind hole at the distal level, for example, by means of a threaded morse cone system, and

- (b) to its stabilization at the proximal level in a totally anatomical way with the lunate bone thanks to the provision of proximal channels for the passage of a tendinous plasty formed by the Flexor Palmar Largo muscle or with the Plantar Delgado to the lunate bone.

Another advantage is that, for prosthetic stabilization, this invention does not use the flexor carpi radialis, a tendon involved in carpal kinetics that allows flexion of the hand on the forearm and also acts in part of the pronation and abduction movements. Failing that, the Long Palmar Flexor (Palmaris Longus) is used, an inconstant, narrow and elongated muscle that does not contribute to the kinetics or kinematics of the carpus, so that it may even be absent in 20% of patients, thus specifying the use of another donor tendon that does not alter the normal biology or mechanics of the organism, such as the Plantar Delgado.

Preferably, the knockout hole is arranged in a substantially central position within the body, and in the desired orientation.

Preferably, the threaded portion of the blind hole is disposed at the proximal end of said blind hole. The aforementioned threaded portion of the blind hole comprises, according to a preferred embodiment, a thread in the part of the proximal end where it will engage the coil of the compression screw, which has a different thread pitch and a different diameter between the proximal and distal portion to allow compression. at the time of positioning the scaphoid. In the real scaphoid bone remnant, self-drilling is performed by passing the self-tapping and self-drilling screw mentioned above. Optionally, the surface of the prosthesis that contacts the scaphoid bone remnant comprises a rough material, i.e. not polished. This, advantageously, achieves a correct osseointegration with the bone remnant and avoids micromotion.

According to a second aspect, the present invention provides the above-described partial prosthesis that further incorporates the fixation element, to anchor the prosthesis to the distal scaphoid bone remnant. Preferably, the fixation element is a cannulated screw with a length between 18 and 26 mm. This cannulated screw is inserted through the bone remnant in a retrograde direction (ie distal to proximal), stabilizing the prosthesis. During the intervention, it is preferably guided by a Kirschner wire that allows the remnant to be connected to the prosthesis. The effects of this connection to the bone remnant consist of a decrease in the “microgap” between both components (prosthesis and scaphoid bone remnant). This fact is verified, from a mechanical point of view, in a decrease in the implant-bone remnant micro-movements, supported by an increase in their contact surface, with the consequent improvement in stability.

As initially discussed, the prosthesis comprises proximal channels that allow a proximal (biological) tendon plasty to pass with the long palmar flexor or, failing that, with the plantar thin one up to the lunate bone. In the process of designing the proximal plasty, it is used, as in the 3LT technique by Marc García Elias, the reconstruction of the dorsal lunate ligament and in the same way the ulnar traction of the lunate and pyramidal is decreased by tightening the joint to the radiopyramidal ligament.

This proximal tendon plasty that passes through channels from the proximal pole of the scaphoid prosthesis to the lunate bone simulates the scapholunate ligament. With this, it is intended that the prosthesis simulates the scaphoid in carpal kinematics, avoiding its flexion and subluxation when applying loads.

According to a preferred embodiment of the invention, these proximal through channels of the prosthesis body are two channels that intersect each other, forming a "T", so that there are three openings in the body. These channels are preferably straight. These channels are of such a diameter to allow a tendon plasty to pass. The operation to anatomically stabilize the prosthesis with the lunate bone at the proximal level is detailed below:

1. Extraction of the proximal part of the scaphoid bone, keeping a distal part not extracted.

2. Drilling a blind hole in the distal side of the scaphoid bone remnant for the passage of the fixation element.

3. To be able to anatomically stabilize the prosthesis with the lunate bone at the proximal level, the lunate bone must have previously been drilled through two openings (opening from dorsal to palmar and opening from the lunate side to converge with the other perforation), so the doctor opens two channels in the lunate bone connected to each other.

4. The partial denture is placed.

5. The partial denture is anchored distally with a retrograde fixation screw.

6. It begins to stabilize it proximally by passing the plasty through the most dorsal opening of the scaphoid and the plasty is removed from the side of the lunate. Then the doctor introduces the plasty to the lunate through its palmar opening, leaving through the dorsal.

7. Then again the doctor introduces the plasty through the same dorsal opening of the scaphoid and this time takes it out through the most distal opening to finally suture the tendon on itself without losing traction.

Preferably, the shape and exterior dimensions of the prosthesis body correspond substantially to the actual morphology of a part of the distal side greater than the proximal pole itself. According to a preferred embodiment of the invention, the shape and exterior dimensions of the body of the prosthesis correspond substantially to the actual morphology of the 2/3 parts of the proximal side of the actual scaphoid removed from a patient, while the remaining distal 1/3 of the scaphoid remains in place without retreating. However, a partial prosthesis of a proximal part can be made with a portion other than 2/3 parts as long as it is sufficient to allow proximal stabilization of the lunate with a plasty through the transosseous channels mentioned above and allows the step of the retrograde screw exerting distal stabilization.

The body of the prosthesis presents a narrowing at the waist level of the resulting scaphoid, simulating the part of the waist of the real scaphoid removed. It also has a base on the distal side, substantially flat, for attachment to the distal bone remnant. The shape and external dimensions of the body of the prosthesis will depend on the anatomical shapes and sizes of the scaphoid of each patient. The prosthesis is manufactured, preferably, in three different sizes, and a prosthesis for the left side and others for the right side. However, this prosthesis can be made to measure, according to the specific dimensions of each patient. It can be manufactured according to known manufacturing techniques, for example by 3D printing.

After numerous measurements carried out by the holders, it has been seen that the optimal exterior dimensions of the prosthesis can vary according to the following parameters: length of the prosthesis body from the proximal end to the distal end added to the length of the bone remnant between 25 and 31 mm. thickness at the level of the scaphoid girdle of the prosthesis is between 8 and 1 1, 9mm.

Due to this variability in length and thickness, it has been decided to make the prosthesis of the invention in three different sizes, in order to mold the prosthesis to the best anatomy of the patient.

Preferably, internally, the blind hole of the prosthesis extends along the prosthesis with a total length between 10 and 20mm. and it has a diameter between 3 to 4mm., and more preferably 3.5mm.

Also preferably, the proximal channels of the prosthesis have a diameter of between 2 and 4mm, and more preferably between 2.5 and 3.5mm.

Optionally, the exit of the central hole and the proximal channels can be rounded or blunt, so as not to damage the plasty and prevent it from breaking or cutting.

Preferably, the body of the prosthesis is made of an outer portion of biocompatible ceramic. For example, it can be formed of a metallic alloy with at least one outer portion of biocompatible ceramic. You can choose pyrolytic carbon as ceramic, although the use of other ceramics is also possible, for example alumina. ceramic that has been used successfully in hand implants. Pyrolytic carbon is a totally biocompatible material, highly resistant to deterioration by friction, practically indestructible and chemically inert, without causing bone wear. Its coefficient of friction is very low, which allows the implant to slide without causing bone wear in order to achieve the least resistance. Furthermore, its Young's modulus, identical to that of bone, allows it to be perfectly tolerated.

The prosthesis of the present invention is used, preferably, in cases of recalcitrant pseudoarthrosis of the scaphoid, avascular necrosis thereof, Preiser's disease, acute fractures with great comminution or displacement where it is impossible to perform an anatomical reduction and primary osteosynthesis, or in cases of Incipient osteoarthritis of the wrist secondary to advanced collapse due to scaphoid nonunion, provided there is no compromise of SNAC III-IV type intercarpal degeneration.

The most significant thing that differentiates the prosthesis of the invention from other implants is that it remains stabilized to the rest of the carpal bones, imitating the native scaphoid and, therefore, fully integrated with the carpal kinematics. In this way, there is an implant that replaces the non-viable part of the scaphoid while maintaining the distal third of it, which allows prosthetic anchorage and behaves as a unit. In addition, the prosthesis is also stabilized at the proximal level to recreate 100% carpal anatomy and kinematics.

Thanks to the advantageous configuration of the prosthesis, they make it unique and different from what currently exists on the market.

Brief description of the figures

For a better understanding of what has been explained, some drawings are attached in which, schematically and only as a non-limiting example, practical cases of different embodiments are represented.

Figures 1 to 5 represent different views of a possible embodiment of the prosthesis of the invention, where the shape and external dimensions can be seen, as well as the three openings of the two proximal channels for the passage of the plasty and the opening on the distal side. the central knockout hole. Figure 6 schematically represents the carpal bones, where the scaphoid bone has not been removed.

Figure 7 schematically represents the carpal bones in a position other than that of Figure 6, where the most proximal part of the scaphoid bone has been removed and has been replaced by the partial prosthesis of the invention and with the plasty inserted.

Figures 8 and 9 schematically represent respective detailed views of the prosthesis of the invention once positioned in a patient and with the plasty inserted.

Figure 10 schematically represents a possible embodiment of the cannulated fixation screw.

Description of some examples of realization

Hereinafter, some embodiments of the present invention are described with reference to Figures 1 to 10.

The kinematic behavior of the prosthesis (10) of the invention has been empirically tested by the owner on cadavers, with satisfactory results. The tests carried out were:

First, the cadaver forearm piece is prepared (with 4 to 6 specimen pieces). Almost all soft tissues are removed both proximally and distally, leaving only four axes of traction that correspond to the flexor carpi radialis, flexor carpi lunaris, extensor carpís lunaris and extensor carpís radialis longus and brevis that work together. The forearm is attached to a support by means of screws anchored in the ulna and radius and connected to a photogrammetry system with several dynamometers in the 4 main tendons that allows to apply flexion-extension forces and radial and ulnar deviation and analyze them.

- Next, the 2/3 of the carpal scaphoid is replaced by the previously designed prosthesis that is stabilized and the same kinematic analysis is repeated. During the movement everything is captured with cameras and load cells to be able to extract the frames and calculate the relative position of the markers and the tensile forces, the Euler angles and the instantaneous centers of rotation.

In addition, the study is completed with radiographic measurements of flexion-extension, prono-supination, and lateralization movements.

As shown in Figure 6, where the situation of the carpus is represented in which the prosthesis (10) has not yet been placed, the complete real scaphoid bone (1) is located in the carpus being in contact with the trapezoid (6), the trapezoid (5), the large one (4), the lunate (3), and the radius (7). The scaphoid bone (1) has a length "A" and a thickness at the level of the waist "B", the parameters of which will depend on each patient.

On the other hand, Figures 7 to 9 represent the situation of the carpus in which the prosthesis (10) of the invention has already been placed in place, keeping the bone remnant on the distal side of the scaphoid bone (2). In said Figures it can be seen how the prosthesis (10) is firmly attached to the bone remnant (2) by means of a screw (in Figures 7 to 9 only the hole (12) is shown while in Figure 10 the screw (18 ), which is arranged within the central blind hole (12). Proximally, the prosthesis is anatomically stabilized with the lunate bone (3) by means of a tendon plasty (23) with the Flexor Palmar Largo muscle or with the Plantar Delgado This tendon plasty (23) is arranged by forming two loops that pass through the channels (16 and 17) following the operations described in the part of the Description, and being fixed by means of a suture point (22), see Figures 8 and 9. Preferably, the final end and the proximal end of the plasty (23), once the two loops have been passed, are sewn on the same plasty (23) already passed.

As can be seen in the attached Figures 1 to 5, this particular embodiment of the prosthesis (10) of the invention presents a body with a shape and external dimensions that substantially correspond to the real morphology of 2/3 parts of the proximal side that have previously been removed from the scaphoid of a patient. It has a surface (1 1) on the distal side that is substantially flat, where an opening (12a) is provided, which extends to form the central blind hole (12). The body of the prosthesis is solid and at the proximal level it has three openings (13, 14, and 15) that form two channels (16, 17) that intercept each other for the passage of the plasty (23). The first channel (16) extends from the first opening (13) to the second channel (17), and this second channel (17) extends from the second opening (14) until reaching the third opening (15). It can be seen in Figure 5 how these two channels intersect forming a "T" shape.

Returning again to Figures 7 to 9, in them a possible configuration of the blind central hole (12) of the prosthesis can be seen, which has a thread (13a) at the proximal end of the central hole (13), and in the continuation of the central blind hole (12). In the bone remnant (2), a second thread will be created that will self-tapping when the retrograde screw is inserted, which is self-drilling and self-tapping (13b).

Figure 10 shows a possible configuration of the screw (18) that is screwed inside the central blind hole (12) of the prosthesis and the continuation of the central blind hole (12) of the bone remnant (2). Said screw (18) has a slightly conical body, and is characterized in that its proximal and distal threads have different size and different thread pitch. Thus, it has a lower portion (screw head area) (21) equipped with a larger and larger thread, an intermediate portion (20) without thread, and an upper portion (screw tip) (19) provided with a smaller pitch and smaller thread, wherein the upper portion (called the first thread pitch) (19) is adapted to be threaded into the internal thread (13a) of the hole (12) of the prosthesis (10).

Although reference has been made to a specific embodiment of the invention, it is clear to a person skilled in the art that the partial scaphoid prosthesis described is susceptible to numerous variations and modifications, and that all the mentioned details can be substituted by others. technically equivalent, without departing from the scope of protection defined by the appended claims.

Claims

1. Partial prosthesis (10) of the scaphoid bone, adapted to replace, after an exeresis of a proximal part of the real scaphoid of a patient, said proximal part removed, while the remaining part of the real scaphoid corresponding to the distal side remains in place without being removed as a bone remnant (2), characterized in that it comprises a body: with a shape and external dimensions that correspond substantially to the actual morphology of the removed part of the scaphoid corresponding to a portion of the proximal side of the scaphoid, which, on the distal side , it comprises a blind hole (12) that extends along the prosthesis, where this blind hole (12) comprises at least one threaded portion (13a) so that an element can pass through the scaphoid bone remnant in a retrograde direction fixation system to distally stabilize the prosthesis, which, on the proximal side, comprises through channels (16, 17) for the passage of a proximal tendon plasty al (23) with a band of the Flexor Palmar Largo muscle or with the Plantar Slim muscle up to the lunate bone (3), so that the prosthesis (10) is anatomically stabilized proximally with the lunate bone (3), and made of a material comprising a biocompatible material.

Prosthesis (10) according to claim 1, in which the shape and external dimensions of the body correspond substantially to the actual morphology of a part substantially greater than the proximal pole of the scaphoid, while the remaining distal part of the scaphoid remains in your site.

Prosthesis (10) according to claim 2, in which the shape and external dimensions of the body correspond substantially to the actual morphology of the 2/3 parts of the proximal side of the actual scaphoid removed from a patient, while the 1 Remaining distal / 3 of the scaphoid remains in place.

Prosthesis (10) according to claim 1, in which the through channels (16, 17) are two that converge together forming a "T", so that there are three openings (13, 14 and 15) in the body.

5. Prosthesis (10) according to claim 1, in which it also comprises a cannulated screw (18) as a fixation element, which has a length between 18 and 26 mm.

Prosthesis according to claim 1, in which the threaded portion (13a) of the blind hole (12) is arranged at the proximal end of the central hole (12).

Prosthesis (10) according to the preceding claim, in which the threaded portion (13a) of the blind hole (12) comprises a thread at the proximal end with a smaller thread pitch, and in the bone remnant of the scaphoid ( 2) Seats a second thread with a greater thread pitch, to allow for compression when positioning the scaphoid.

Prosthesis (10) according to claim 1, in which the body is made of a metallic alloy with at least one outer portion of ceramic.

9. Prosthesis (10) according to the preceding claim, in which pyrolytic carbon is chosen as the ceramic.

10. Prosthesis (10) according to claim 1, in which the part (1 1) of the prosthesis that contacts the bone remnant of the scaphoid (2) comprises a rough material, that is to say not polished, to achieve the osseointegration with the bone remnant and avoid micromotion.

1 1. Prosthesis (10) according to claim 1, in which the length of the prosthesis body from the proximal end to the distal end added to that of the bone remnant (2) is between 25 and 31 mm.

12. Prosthesis (10) according to claim 1, in which the blind hole (12) extends along the prosthesis (10) with a total length between 10 and 20 mm.

13. Prosthesis (10) according to claim 1, in which the body has a narrowing at the waist of the resulting scaphoid, where the resulting scaphoid is the set of the proximal prosthesis plus the distal part remaining in the patient.

Prosthesis (10) according to the preceding claim, in which the thickness of the body of the prosthesis (10) at the level of the scaphoid girdle is between 8 and 11.9mm.

15. Use of the prosthesis according to any one of claims 1 to 14, in cases of recalcitrant pseudoarthrosis of the scaphoid, avascular necrosis thereof, Preiser's disease, acute fractures with great comminution or displacement where it is impossible to perform an anatomical reduction and primary osteosynthesis, or in cases of incipient osteoarthritis of the wrist secondary to advanced collapse due to scaphoid pseudoarthrosis, provided there is no compromise of SNAC III-IV type intercarpal degeneration.