| Abstract|| |
Background : Baksi sloppy hinge elbow is an all metal prosthesis having 7 0 - 10 0 varus - valgus inherent laxity at the hinge section with minimal motion bearing contact area. Due to the presence of laxity at it's hinge section, any strain on the prosthesis dissipates primarily to the surrounding soft tissues thus protecting the cement bone interfaces. However, from our long term clinical experiences on the use of our sloppy hinge design since 1984 and the knowledge of literature review of the results of using other semiconstrained (sloppy) or unconstrained designs, it was observed that radiolucency or loosening at the bone-cement interface occurred mainly around the humeral stem in the long run due to the continued effect of rotational torque of forearm and hand. Hence, an attempt in the improvement of the design concept is being made.
Methods : In this respect one flange each of one cm height and breadth and three mm thickness has been incorporated on either sides of the shank of humeral stem of the sloppy hinge at mediolateral (coronal) plane which will be seated in the corresponding longitudinal groove cut on either side of humeral shaft extending from its transverse cut end to become single assembly during the rotation of humerus.
Results : The preliminary results of clinical application of the modified sloppy hinge elbow in ten cases are found satisfactory.
Conclusion : The cyclical compression and distraction forces during flexion and extension of the elbow will be distributed over the larger bony area of lower end of humerus where flanges of the humeral shank being seated. The rotational torque effect of forearm and hand particularly with the arm in abduction will be minimised at the humeral bone cement interface as the humerus and the prosthetic stem act as a single assembly by the snugly fitting of the prosthetic flange in the humural shaft
Keywords: Mediolateral flange at humeral shank; Baksi sloppy hinge elbow.
|How to cite this article:|
Baksi D P. Modification of Baksi sloppy hinge elbow to minimize the stresses at the humeral bone cement interface- An early experience. Indian J Orthop 2005;39:240-3
|How to cite this URL:|
Baksi D P. Modification of Baksi sloppy hinge elbow to minimize the stresses at the humeral bone cement interface- An early experience. Indian J Orthop [serial online] 2005 [cited 2020 Mar 29];39:240-3. Available from: http://www.ijoonline.com/text.asp?2005/39/4/240/36598
| Introduction|| |
Total elbow arthroplasty is performed in different clinical conditions like traumatic arthritis or ankylosis, nonunion of intercondylar and supracondylar fractures, recent type IV intercondylar fractures, rheumatoid arthritis, post-burns ankylosis, osteoarthritis of elbow joint and unstable elbows. Total elbow prosthesis may be rigid hinge (linked but having no laxity), sloppy hinge (linked and having laxity) and unlinked joints  . A minority of implants incorporate a replacement for the head of radius.
Among the complications aseptic loosening is the common cause of failure. The development of 2 mm or more radiolucency at bone cement interface was considered loosening of prosthesis only when it was symptomatic or there was migration of prosthetic components. In the review of literature there was overall 9% prosthetic loosening with 14% radiolucency at bone cement interface  . The constrained prosthesis which do not allow dissipation of forces have shown unacceptably high rate of early loosening , . They showed 21% radiolucency and 11% loosening . The unlinked implants showed 10% loosening with 11% radiolucencies. They showed higher rates of loosening than that of sloppy hinge devices  . The semiconstrained or sloppy hinge showed 15% radiolucency and 5% component loosening ,,,,, However, the numbers of posttraumatic cases reported were small as their mean follow up was also small  .
Baksi's sloppy hinge prosthesis in 2.5-16.5 years (Av.11.5 Years ) follow up showed 12.5% radiolucency in post traumatic ankylosed and 18.75% among unstable elbows and loosening rate 3.4% in post traumatic ankylosed and 6.25% in unstable elbows, predominantly among younger individuals  . This loosening was seen commonly around the humeral and rarely around the ulnar stem , . Hence an attempt has been made to redesign the Baksi sloppy hinge prosthesis to reduce the stress around the bone cement interface and loosening around the humeral stem.
| Materials and Methods|| |
The original Baksi's rigid hinge prosthesis was designed in 1977 (Indian Patent No.146175, dated 11.8.1978) and has been used in clinical practice since 1977  . Its physical properties were extensively studied with the help of a newly designed Elbow Joint Simulator during the period from 1978 to 1983. This study concluded that if 7 0 -10 0 laxity is added in the hinge section, that will reduce the metal dust liberation and hinder the strain of loosening in the bone cement interface , . On the basis of these facts, the original rigid hinge prosthesis was redesigned into sloppy hinge elbow prosthesis in 1983 (Indian registered design no 161541). Baksi sloppy hinge [Figure - 1]a has got 7 0 -10 0 varus valgus laxity at its hinge section with the motion bearing component being shorter in length compared to its fixed part and having potential gap between motion bearing surfaces resulting in partial articular contact during elbow motions and minimal metal dust liberation. Because of the presence of laxity in the hinge section any strain occurring at the hinge will be dissipated primarily to the surrounding soft tissues protecting the bone cement prosthesis interface  .
The sloppy hinge design has been recently redesigned (Indian registered design no.200755) where two flanges each of one cm height and breath and three mm thickness has been incorporated on either side of the shank of humeral stem of the sloppy hinge at medio-lateral (coronal ) plane [Figure - 1]b which will be seated in the corresponding longitudinal groove cut on either side of the humural shaft extending from its transverse cut end [Figure - 1]c to become single assembly during the rotation of humerus .
Patients: The newly modified sloppy hinge design was used in ten patients of which four had post traumatic ankylosis, two recent intercondylar, two ununited intercondylar and one supracondylar fractures humerus and one case of giant cell tumour of lower end of humerus .
Methods: The operation technique followed was as reported  , with the addition of insertion technique of flange of humeral stem of the newly designed modified sloppy hinge into the shaft of humerus [Figure - 1]c. With patient in supine position and the arm under tourniquet control, a posteromedial incision was made over the elbow. Following the subfascial dissection, the ulnar nerve was isolated and flexor carpi ulnaris was mobilised. The medial epicondyle, coronoid and olecranon process and the lateral epicondyle were exposed detaching the soft tissues around them. The head of the radius was excised, lower end of humerus was cut transversely just proximal to olecranon fossa and subarticular L shaped resection of upper end of ulna was done preserving the insertions of triceps and brachialis. Then the isolated bony components of the elbow joint was excised. The medullary canals of ulna and humerus were prepared. A longitudinal groove of one cm height was cut on either side of the humeral shaft at mediolateral (coronal) plane extending from its transverse cut end [Figure - 1]c for sitting of the newly added flanges of humeral stem. The ulnar stem was fixed with the help of bone cement. Then the humural stem along with the flanges was snugly fitted within the medullar canal with the help of bone cement while the flanges sit in the corresponding longitudinally grooves cut over the side of humerus.
Then assembly of the prosthesis was done by the main linking screw and secured firmly with the locking screw .The wound was closed in layers along with the suction drain around the prosthesis. Routine postoperative regimen was followed as reported previously  .
| Results|| |
Early results were satisfactory [Figure - 2]a,b with recovery of painless elbow motions in all cases. Since the follow up period is less than a year, it is not judicious to draw any conclusion regarding its final outcome in respect of incidence of loosening in long term follow up.
| Discussion|| |
Aseptic loosening of bone cement interface is one of the common complications of total elbow arthroplasty. Three factors were considered for aseptic loosening as follows: the biomechanics of the joint, prosthetic design and surgical technique providing uniform cement mantle. The sloppy hinge design having varus-valgus laxity at its hinge section reduced the rate of loosening by dissipation of forces across the prosthesis to its surrounding soft tissue, so that less strain will be borne by the bone-cement interface , . Moreover movements in sagittal plane (flexion and extension) causes a cyclic compression and distraction load directed anteriorly, superiorly and posteriorly which are responsible for early loosening  . For absorbtion of such loads applied to the lower end of humerus Mayo- modified Coonrad elbow prosthesis has got special design concept by the incorporation of anterior flange extending upwards from the lower end of humeral stem, which significantly reduced the problems of the "wind shield wiper'' effect of stem loosening within the distal humerus  . A piece of bone graft is placed in between the flange and adjacent anterior surface of lower cut end of humerus for further extracortical bony anchorage of the stem to the shaft of humerus while the humeral stem is inside the medullary canal  . The bone graft may get anchored to the humeral cortex but unlikely to the metallic flange. Hence, in the later designs, titanium spray was used to the posterior surface of the flange for the bone in-growth and better anchorage of the bone graft. However, this bone grafting procedure usually constitutes a weak bond to counteract the rotational stress around the humeral stem and moreover there lies the possibility of impingement of the graft by the flange leading to its structural failure due to repetitive cyclical compressive load during each flexion of the elbow joint.
Our clinical experience with the sloppy hinge design showed that the aseptic loosening occurred commonly around the humeral and rarely around the ulnar stem ,, . The circumferential thinning out of a segment of humeral cortex was noted with thickening and sclerosis of the opposite cortex [Figure - 3] which indicated that the rotational stress predominantly concentrated around the humeral stem during movements of elbow with the abducted arm and shoulder. This rotational stress being concentrated around the shank of the prosthetic stem was distributed across the humeral bone-cement interface, resulting in occurrence of radiolucency at the initial period followed by clinical loosening of prosthetic stems. When the aseptic loosening sets in, the additional superior thrust during every flexion of elbow resulted in subsidence of humeral stem which was responsible for approximation of humeral and ulnar bone ends [Figure - 3], penetration of the humeral cortex by the tip of the stem or breakage of the humeral stem or fracture of shaft of humerus.
To achieve the solution of above problems in the newly modified design of Baksi sloppy hinge, the flange was incorporated, one on each side of the shank of humeral stem in mediolateral plane which are firmly seated in the corresponding groove made over the sides of humeral shaft [Figure - 1]c and fixed with the bone cement. It counteracts the rotational stress as well as superior thrust exerted during elbow motions. Furthermore because of addition of the flanges, the transverse diameter of the humeral prosthetic shank being wider, the cyclical compression and distraction forces during flexion and extension of the elbow will be distributed over the larger bony area of lower end of humerus. Therefore, force per unit area will be much less resulting in lesser concentration of compression, distraction as well as rotational stresses around the lower end of humerus. Thus the rotational torque effect of forearm and hand particularly with the arm in abduction will be minimized at the humeral bone-cement interface. Furthermore, the humerus and the prosthetic stem acts as a single assembly against the rotational torque by the snugly fitting of the prosthetic flange in the humeral shaft. The above factors protect the bone cement interface; thereby will reduce the rate of loosening of humeral stem.
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D P Baksi
DA-3, Sector 1, Salt Lake City, Kolkata -700064
Source of Support: None, Conflict of Interest: None
[Figure - 1], [Figure - 2], [Figure - 3]