GB2030867A - Elbow joint prosthesis - Google Patents

Abstract

A humero-ulnar elbow joint prosthesis comprises a metallic ulnar component 14 in the form of a generally oval head having a curved stem corresponding to the curve of the upper medullary canal of the ulna 16, and a plastics humeral component 10 in the form of a cup having a trough at the inner surface for cooperation with the oval head of the ulnar component and a stud on the outer surface of the cup for location of the humeral component relative to the humerus 13, the arrangement being such that in use the oval head glides within the trough during elbow flexion permitting both a linear change in carrying angle and a limited degree of cyclic rotation. Preferably, the ulnar component 14 comprises a vitallium alloy and the humeral component 10 comprises high density polyethylene. <IMAGE>

Description

SPECIFICATION Elbow joint prosthesis This invention relates to an elbow joint prosthesis.

Review of literature shows that the linked hinge arthroplasty is not suitable as the incidence of loosening increases with time. The reason would appear to be the rigid hinge axial movement in one axis not allowing for the rotational and gliding movement normally occurring. Also the salvage procedure is difficult due to the wide excision of bone necessary to insert the prosthesis. Therefore, the rigid hinge prosthesis is not the answer for total joint replacement of the elbow.

The human elbow is not a simple hinge joint.

There is an element of rotation and also a linear change in carrying angle during elbowflexion. There are "three joints in one" when one considers the elbow joint design, namely humero-ulnar, humeroradial and upper radialo-ulnar. These three joints function in a very complex manner in conjunction with other joints of the upper limb to achieve the upper limb functions, some of which are: Balance, locomotion, feeding, manipulation, sensory activity, defence-aggression, communication, personal hygiene, etc.

These features make the design of an elbow joint prosthesis very difficult.

E. Y. Chao studied the three-dimensional static force analysis of the hand and elbow during isometric functions. According to Chao' the forearm exhibited a linear change in carrying angle during elbow flexion. Small cyclic rotation existed during flexion. There was no movement of the ulna during pronation and supination with the elbow extended.

The axis of flexion was confirmed to be approximately at the centre of the trochlea.

Analysis of the four major flexors showed that the brachioradialis was the most efficient muscle, with the biceps usually second. The triceps was more efficient than the anconeus in elbow extension. Joint constraint forces were higher in extended position than in flexed position. At maximum isometric lifting with elbow extended, joint contact force extended two time body weight. This force was slightly less at flexed position. When the el bow was loaded from extended to flexed position the joint force reversed its direction. Clinically this is significant as the oscillatory bending force might be the main factor causing stem fatique fracture in the prior linked hinge elbow prosthesis.

Also the sheering force may be responsible for loosening of the hinge prosthesis as the incidence of loosening increases with time in a linear fashion.

Nicol A. C. and Paul J. P. analysed the biomechanics of the elbow joint during eating, dressing, pulling a heavy object and assisted standing from the seated position. Their results indicate that during dressing and eating activities the muscle force did not exceed 135N. Both sidesofthetrochlearnotch were loaded throughout these activities where resultant compres sive force of 300N were encountered. Flexor muscles provided the motor power for the table pull exercise, where the tension in the biceps gradually decreased from 650N to 0.

Tension of 1 900N in the medial ligament was characteristic of this strong flexor activity.

During the assisted seat rise tension in the triceps muscle reached 340N. Maxima of 1700N and 1800N occurred at the medial and lateral sides of the joint respectively. Therefore, although the elbow joint is not a weight bearing joint the forces that act upon it for its function are far in excess of what one would conjecture.

Bearing the above factors in mind, an ideal elbow joint prosthesis should have the following characteristics: 1. The prosthesis should provideatotal replacement.

2. It should provide a nearly full range of movements to maintain the function of the artifical joint.

3. The carrying angle must be maintained as in the normal joint.

4. The joint must be stable during the ordinary course of movements and must allow for the rotational movement to annul strain which ordinarily leads to loosening of the rigid hinge elbow prosthesis.

5. When in situ, the prosthesis should not cause any abnormal pressure on the surrounding soft tissue throughout the range of movement.

6. The operative procedure to insertthe prosthesis should cause minimum trauma to local tissue and the instrumentation should be simple.

7. The fixation of the prosthesis must be secure.

8. The materials used must not induce any untoward reaction to the host tissues.

9. The prosthesis must be suitable for the poor quality of rheumatoid bone especially.

10. In cases of failure, a salvage procedure should be available as an alternative to give a useful arm.

It is, therefore, an object of the invention to provide an elbow joint prosthesis which fulfils or can be designed to fulfil these characteristics to a larger extent than heretobefore.

As stated previously, the flexion and extension movements occur at the humero-ulnar compartment, the lateral compartment plays the passive role; the axis of flexion lying approximately at the centre of trochlea.

The pronation and supination movements occur at proximal radio-ulnar joint. The annular ligaments is an important structure for stability of this articulation. In extended elbow position the radial head rotates, the ulna being stationary. Whilst during flexion there is some cyclic rotation allowing for the linear change in carrying angle, this occurring at both radio-ulnar and humero-ulnar compartments.

The medial side of the joint exhibited more tension than the lateral side during the strong flexor activity and also during assisted seat rise action (biceps muscle action chiefly) according to Nicol and Paul.

Therefore, the medial compartment of the joint plays a dominant role in flexion and extension of the joint, the lateral being passive. From the experience of excision arthroplasty of the humero radial com partment where the radial head is excised the annu lar ligament being left undisturbed, the patients can manage well. The lateral compartment of the joint transmits the axial load along the forearm. Taking into consideration the higher tension occurring at the medial compartment during flexion and extension, it is logical to conclude that during the active use of the elbow i.e. in mid pronation and semi-flexed position most of the loading would occur at the medial side rather than the lateral as one hardly gets off the chair whilst pushing down with elbows fully extended and fully supinated except perhaps during the athletic activity in a gymnasium.

Therefore, in common with previous designs, these factors are exploited and the present invention is concerned with a humero-ulnar joint.

According to the present invention there is provided a humero-ulnar elbow joint prosthesis comprising a metallic ulnar component in the form of a generally oval head having a curved stem corresponding to the curve of the upper medullary canal of the ulna, and a plastics humeral component in the form of a cup having a trough at the inner surface for cooperation with the oval head of the ulnar component and a stud on the outer surface of the cup for location of the humeral component relative to the humerus, the arrangement being such that in use the oval head glides within the trough during elbow flexion permitting both a linear change in carrying angle and a limited degree of cyclic rotation.

Preferably, the ulnar component comprises a vitallium alloy and the humeral component comprises high densisty polyethylene.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figs 1A and 1 B are side and front views respectively of the ulnar component, Figs 2A and 2B are front and side views respectively of the humeral component, Fig 3 shows the ulnar component fitted to the ulna, Fig 4 shows the humeral component fitted to the humerus, and Figs shows how the two components cooperate in use.

The elbow joint prosthesis to be described has been developed to relieve the disability of patients suffering from degenerative conditions affecting the elbow joint (distal ends of the humerus and the proximal end of the ulna). This prosthesis does not allow replacement of the radial head, but as a part of the procedure it can be excised if the head of the radius is hypertrophic or totally destroyed.

Referring now to the drawings, the humeral component 10 is made out of high density polythene plastics material generally in the shape of a cup having a trough 11 at its inner surface. There is a small stud 12 on the outer surface of the cup which helps to find the angle of insertion of the prosthesis relative to the humerus 13 (fig 4) and also acts as an additional fixation key. The humeral component 10 is fixed with acrylic bone cement to the humerus 13.

The ulnar component 14 is made out of metalvitallium alloy and has a stem 15 which takes the appropriate curve of the upper medullary canal of the ulna 16 (fig 3). At the proximal part of the stem 15 the end is shaped generally like an oval head 17 which engages in the trough 11 of the humeral component 10 when in situ (fig 5). The ulnar component 14 is fixed with acrylic bone cement. As a part of the procedure the head of the radius is excised but the annular ligament is left undisturbed.

The technique for inserting the prosthesis is as follows: The patient is anaesthetised with general anaesthetic and relaxed with scolin. The patient is positioned on the table in prone position with the elbow resting on the arm rest at 900 of flexion. The limb is draped to have free movements of the elbow during operation. The elbow is exposed through standard Campbells Post approach. The tongue of triceps through aponeurosis is reflected distally leaving its attachment to olecronon intact. The origin of triceps on distal humerus is reflected subperiosteally. The square block of bone from lower humerus is excised, extending proximally and including the olecronon fossa. The two condyles are not disturbed. The lower end of the humerus 13 is reamed for 1" to receive the stud 12 on the outer surface of the humeral component 10.

The ulna 16 is distracted from the humerus 13 and dislocated. The articular surface of the trochlear notch is curretted and the proximal ulna is reamed with 3/16" drill, care taken not to perforate the Cortex.

The radial head is excised and the annular ligament is left undisturbed. The ulnar component 14 is inserted first and fixed with bone cement. The humeral component 10 is cemented in situ with bone cement, the extra bone cement is trimmed, joint is reduced. The stability is tested, the triceps is resutured, a suction drain is inserted and the wound closed in layers. The elbow joint is immobilised in 900 of flexion and mid-prone-position with posterior plaster splint. The drain is removed after 24 hours.

The sutures are removed after ten days and the wound being healed, the elbow is mobilised in a sling which is discarded after two weeks.

The described prosthesis makes provision for the accessory movements which ordinarily occur in the human elbow joint. This artificial joint is not rigid hinge joint. In full extension, the prosthesis provides for the carrying angle, the stability maintained by tension of soft tissue in front and also the inferior surface of the humeral component gliding overthe post surface of the cup and snugly fitting.

Even when this elbow joint is in full extension some restricted adduction and abduction of the ulna is possible.

During flexion, the head 17 of the ulnar component 14 glides downwards and backwards until full flexion which is restricted by the two flexor surfaces coming in close contact. During this movement the carrying angle is masked as there is some rotation occurring at this joint. Thus this prosthesis has a certain features which allow the centre of the axis of flexion to be located at the original site of trochlea, there is an element of rotation and side to side movement as in the ordinary human elbow.

The materials used are the standard high density polyethylene and vitallium alloy which are proved not to invoke any untoward local or distant reaction to the host.

The fixative agent is the acrylic bone cement which is extensively used for the fixation of artificial joints.

Therefore even though the articular surfaces are reversed in configuration as compared to the ordinary human elbow i.e. instead of hollow of olecronon-notch of proximal ulna this joint has an ulnar head and a trochlear cup rather than solid mass of pully. The joint function and movements are physiologically the same as the ordinary human elbow joints.

Claims (3)

1. A humero-ulnar elbow joint prosthesis comprising a metallic ulnar component in the form of a generally oval head having a curved stem corresponding to the curve of the upper medullary canal of the ulna, and a plastics humeral component in the form of a cup having a trough at the inner surface for cooperation with the oval head of the ulnar component and a stud on the outer surface of the cup for location of the humeral component relative to the humerus, the arrangement being such that in use the oval head glides within the trough during elbow flexion permitting both a linear change in carrying angle and a limited degree of cyclic rotation.

2. A humero-ulnar elbow joint prosthesis as claimed in claim 1, wherein the ulnar component comprises a vitallium alloy and the humeral comprises high density polyethylene.

3. A humero-ulnar elbow joint prosthesis substantially as herein described with reference to or as illustrated in the accompanying drawings.