IE45680B1 - Prosthetic elbow joint - Google Patents

Description

This invention relates to a prosthetic elbow joint.

The early attempts at total replacement of the elbow made use of a fully constrained hinge prosthesis cemented into the medullary cavities of the humerus and ulna. The immediate results were dramatically successful but a longer follow-up has shown that this type of joint can give rise to major problems. Chief among these is the high incidence of loosening of the cement plug in the humerus which can result in such a degree of instability as to render the arm largely useless. it is thought that the reason for this loosening is the repetitive transmission of twisting loads to the cement bone interface in the humerus. The type of activity likely to generate such a force can be exemplified in its most extreme form when a patient attempts to hold a fairly heavy object between the palms of his hands with the elbows flexed to 90°.

The loosening of the humeral component is often associated with much thinning of the surrounding cortex of the bone so that in the event of a fall the patient is very liable to,sustain a severe comminuted fracture.

Most of the early hinge implants involved the removal of the epicondyles of the humerus, thus incurring a serious risk of gross instability should the prosthesis ever have to . be removed. An object of this invention is to obviate or mitigate the aforesaid disadvantages. - 2 45680 According to this invention there is provided an elbow joint prosthesis consisting of co-operable humeral and ulnar components having complementary bearing surfaces, in which the humeral component includes a shaped body having a surface portion shaped to mimic the natural bearing surface of the humerus and pair of general laterally extending fixation protrusions projecting from opposed sides of the body and directed so as to be capable of engaging, in use, in excised recesses in the medial epicondyle and capitulum of the humerus.

Preferably the humeral component is generally stirrupshaped with the shaped body forming the base of the stirrup and the sides of the stirrup being members conforming generally in shape to the medial and lateral supracondylar ridges of the humerus.

The ulnar component preferably includes a shaped body having a bearing surface portion complementary to and engageable with the bearing surface of the humeral component, and preferably, a seating surface portion presenting a pair of adjacent substantially perpendicular surfaces for seating on a complementary prosthesis 20 receiving seat excised in the ulna, and a stem portion locatable within the medullary canal of the ulna.

The humeral component may be fabricated of a plastics material and the ulnar component of metal or vice versa or other bicompatible materials may be used, for example ceramic materials. 45S80 The prosthesis is normally cemented into the ends of the humerus and ulna after surgically forming suitable cavities in the bone end. Surgical cement is used for this purpose. However, as an alternative, the prosthesis may be fixed without cement by providing on bone-engaging surfaces of the prosthesis a porous surface into which the bone tissue may grow, this tissue invasion being sufficient to lock the prosthesis in position. To provide the porous surfaces sintered metal may be used in the fabrication of the prosthesis or a coating of porous material may be applied thereto. - 4 45630 The prevision cf laterally extending fixation protrusions on the humeral component obviates the need for long intramedullary fixation stems common in prior devices of this type. The insertion of these long stems into the medullary cavity of the bone is one source of post-operative infection and the shallower cavity required by the device of this invention is clearly an advantage in this regard. The capitulum engaging fixation flange is preferably of cylindrical shape and may have a roughened or grooved surface to key with the cement used in fixing the device in the excised recess.

The medial epicondyle-engaging fixation protrusion is preferably generally elliptical in cross-section and may also be grooved to key with the cement.

Thus the present invention proposes the use of a two15 component, preferably metal on plastics, partially constrained joint, fixed locally within the expanded extremities of the humerus and ulna so that the amount of bone resection is minimal and ligamentous stability hopefully preserved.

From a preliminary morphological study of the elbow, it 2o is apparent that the alignment and contour of the humeral/ulnar articulation are far from simple and the criteria observed in the design of the implant of this invention have been not only to fashion the alignment and contours of the articular surfaces as closely as possible to the pattern of the normal trochlea . but also to determine the siting.and dimensions of the fixation flanges by reference to the exact position of the surrounding bony buttresses, namely the medial epicondyle and the capitulum. -545680 The invention will now be described, by way of example, with reference to the accompanying drawings.

Figs. 1 to 5 illustrate the humeral component of the elbow joint prosthesis and Figs. 6 to 12 show a complementary ulnar component.

Fig. 1 shows the humeral component with an outline of the humerus bone shown in broken line to illustrate the location of the prosthesis in the bone, Fig. 2 is a lateral view of the humeral component, construction of the Fig. 3 is a drawing detailitgthe/bearing surface of the component, Fig. 4 is a plan view defining dimensions and Fig. 5 is a front perspective view defining dimensions.

Fig. 6 is an elevation of the radial component showing the ulna bone 'shown in outline, Fig. 7 is a plan view of Fig. 6, Fig. 8 is a superior view, Fig, 9 is an inferior view, construction of the Fig. 10 is a drawing detailing the/bearing surface of the ulnar component, Fig. 11 is a plan view defining dimensions, and Fig. 12 is an elevation defining dimensions.

The Humeral Component (Figs. 1 to 5) Referring to Figs. 1 and 2, the humeral component 1 of an elbow joint prosthesis is generally stirrup shaped, the base 2 -645680 of the stirrup being prosthesis for the trochlea. The trochlea portion has a bearing surface 3 which mimics that of a natural trochlea. The sides of the stirrup 4, 5 can form in general directional shape to the outline of the medial 8 and lateral 9 supracondylar ridges respectively of the humerus. First 6 and second 7 laterally extending protrusions are provided on the prosthesis. When the prosthesis is implanted undercuts are made surgically in the capitulum to receive the protrusion 6 and in the medial epicondyle 11 to receive the protrusion 7.

The top 12 of the stirrup-shaped prosthesis is located in the intramedullary canal of the humerus. The whole prosthesis is cemented into place but the protrusions bearing on the actual bone in the capitulum and medial epicondyle undercuts which bear the load.

Figs. 3, 4 and 5 are given to define the shape and size of the prosthesis. In these Figs, the various symbols are defined by way of example, as follows. 1 Radii R κ 23.5 mm Angles a = 7.5° 2 R ° 6.0 mm β = 10° 3 R “ 23.5 mm γ - 8° $ = 22.5 £ = 37.5 Dimensions a “ 36-45 mm f = 22.5-24.5 mm b = 35-44 mm g - 36-45mm c = 11-14 mm h = 14 mm d » 9-11 mm e 22-24 mm k = 46.56 mm -7“ The Ulnar Component (Figs. 6 to 12) Referring to Figs. 6 to 9, the ulnar component of an elbow joint prosthesis has a stem portion 13 and a head portion 14 mounted on one side of the stem. The head portion 14 has a bearing surface 15 designed to complement and co-act with the bearing surface 3 of the humeral component. The head 14 in side elevation has two sides 16,17 at right angles which abut portions of the ulna when implanted. A curved tail portion 20 of the head extends along and is attached to the stem 13.

When implanting the prosthesis the surgeon excises a portion of the head of the ulna by cutting through the coronoid process in 18 a direction generally tranverse to the longitudinal axis of the bone and then cuts at right angles through the olecranon 19 to provide a right-angled seat for the sides 16,17. The stem 13 is inserted into the intramedullary canal. The prosthesis is cemented into place.

Figs. 10, 11 and 12 are given to define the shape and size of the prosthesis. The various symbols are defined, by way of example, as follows. ,4 Radii 23.5 mm R = 10.5 mm _6 R = 23.5 mm .8 Angles =10.0° R = 7.75 mm 6.0 mm (humeral construction') η ° 30.0° Dimensions m 24-29 mm n ° 8.7-10.7 mm s 0 = 11-15 mm p = 16-21 mm q ° 16-20 nun = 42.5-53mm t - 11-14 mm -845680 Articular Surfaces 1. The prosthesis is a two component metal on plastic assembly replacing only the trochlear articulation.

The head of the radius will in most cases be excised. 2. The contour and alignment of the bearing surface of the humeral component mimic very closely those of the normal trochlea, and like the latter are designed in such a way that on fixation of the prosthesis the shoulders of the trochlea will be set at an oblique inclination to the sagittal plane. 3. The articular surfaces of fche humeral canponent of tine prosthesis will Ine convex in the sagi ttal plant but in the coronal pl.me will exhibit a surface whose three-dimensional curvatures are specified to reproduce the corresponding contour of the normal trochlea, i.e. its medial surface is convex, while its lateral surface is concave. The ulnar component will be generally complementary to this and again will mimic the appearance and contours of the trochlear notch of the normal ulna.

The closely matched saddle resulting from the above design should confer valuable inherent stability on the prosthesis and yet allow a certain amount of release glide” in both compartments when the ulna is subjected to any axial twisting action. Any excess displacement, however, would be checked as in a normal -9'45680 elbow by the opposing medial or lateral ligament. Thesaddle contour is modelled on the saddle of the normal elbow. 4. By means of design criteria 2 and 3 the normal role of the ligaments in preserving stability during functions of the elbow is preserved. The surfaces are specified to give relative movements compatible with the ligamentous restraints.

Fixation of Implant 1. Twisting forces transmitted from the flexed forearm lever”, are the main causes of loosening in the • humerus. The retention of the medial and lateral ligaments in the proposed arthroplasty should help to reduce the stressing of the prosthesis and inter15 face by these loads but, in addition, the fixation protrusions have been designed in such a way that the humeral component is secured over as wide an area of the intercondylar bone as possible.

The normal trochlea is heavily buttressed on its lateral side by the large bulk of the capitulum and excavation of the latter provides an ideal cavity in which a fairly long and substantial fixation protrusion can be cemented. At the moment this is envisaged as being circular in outline but might well incorporate conventional grooves to improve the hold of the cement. -1045680 The medial side of the normal trochlea is less well buttressed, as the medial epicondyle comes off well posteriorly and fairly high up on the medial trochlear lip. Nevertheless, by siting a slim elliptical fixation protrusion at an appropriate level on the prosthesis, good fixation can also be achieved medially. It is to be noted that the normal anatomy precludes the medial and lateral flanges being in the same coronal plane.

The lateral can indeed be essentially coronal in align10 ment but the medial flange must be set back from this plane. 2. . To impart resistance to rotational forces about the coronal axis a stirrup-shape has been incorporated to secure the humeral component to the supracondylar ridges.

Again the normal anatomy of the latter has determined the design of this shape. The supracondylar arch does not itself lie in the coronal plane but is positioned posteriorly relative to the coronal axis of the trochlea on its medial side so as to become confluent with the medial epicondyle. Moreover, its lateral limb ascends almost vertically from the capitulum while its medial limb lies much more obliquely as it comes round the margin of the olecranon fossa. Thus, a symmetrically fashioned Y-fixation device would not measure up to the stringent anatomical criteria which have been imposed -114δ6θθ by the inventors in designing the prosthesis. It is envisaged that three sizes may be necessary so as to ensure that the dimension of the stirrup arch matches any given supracondylar ridge sufficiently accurately to allow the humeral component to be counter-sunk in the bone and thus avoid the excessive bone cement loading which would result if it were too superficial.

These design criteria coupled to the saddle” contour of the articular surfaces will also mean that right and left sided conformations will be necessary. 3. Experience with rheumatoid elbows has shown that the olecranon is frequently so eroded as to consist only oi the posterior cortex of the original bone. Thus it is preferable to provide a short ulnar stem inclined to the radial side of the longitudinal axis of the main articular surface to match the lie of the medullary cavity of the proximal ulna. The length of this stem can be assessed by mechanical testing.

The prosthesis of this invention has been designed to mimic the normal anatomy of the articular surface of the human elbow and in so doing to preserve as far as possible normal ligamentous action. The proposed method of fixation is part and parcel of a uniform anatomical concept in that the siting and dimensions of these fixation flanges are accurately matched to the various bony buttresses which surround the trochlear articulation.

Claims (5)

1. CLAIMS:1. An elbow joint prosthesis consisting of co-operable humeral and ulnar components having complementary bearing surfaces, in which the humeral component includes a shaped 5 body having a surface portion shaped to mimic the natural bearing surface of the humerus and pair of general laterally extending fixation protrusions projecting from opposed sides of the body and directed so as to.be capable of engaging in use, in excised recesses in the medial epicondyle and capitulum of the humerus. 10

2. A prosthesis according to Claim 1, in which the humeral component is generally stirrup-shaped with the shaped body forming the base of the stirrup and the sides of the stirrup being members conforming generally in shape to the medial and lateral supracondylar ridges of the humerus. 15

3. A prosthesis according to Claim 1 or Claim 2, in which the ulnar component includes a Seating surface portion presenting a pair of adjacent substantially perpendicular surfaces for seating on a complementary prosthesis- receiving seat excised in the ulna, and a stem portion locatable within the medullary canal of the ulna.

4. An elbow joint prosthesis having a humeral component substantially as hereinbefore described with reference to and 25 as shown in Figs. 1 to 5 of the accompanying drawings. -1345680

5. An elbow joint prosthesis substantially as hereinbefore described with reference to and as shown in Figs. 1 to 12 of the accompanying drawings.