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The Cementless Total Hip Replacement

Long- and mid-term results of the Link® cementless hip prosthesis system in the combination of the ribbed stem and the screw-in acetabular cup type „V"

S.Schill, H. Thabe Orthopädische und Rheumaorthopädische Abteilung Diakonie Krankenhaus, Ringstr. 58-60 55543 Bad Kreuznach Chefarzt Dr.med. H. Thabe

Introduction

We present a retrospective study of 437 cementless hip replacements in 385 patients performed from 1986 -1994. In all cases the cemtless anatomically shaped „Ribbed Stem" and the Screw-in Acetabular Cup Typ „V" were used. Our indication (11) for cementless total hip replacement depends on the biological age (< 60 y.), sufficient bone stock without large zystic destruction and aceptable femur anteversion. The cementless Link® hip prosthesis system in combinatoion of the „Ribbed Stem" and the Screw-in Acetabular Cup Typ „V" is in clinical application since 1984.

Surgical technique

We generally use the dorsal or so called southern approach for primary and revison hip surgery.

Srew-in cup Type `V`

The typ `V` srew in cup consits of a threated metall casing and different polyethylen inlays with variable shoulder hights. The deep self cutting threads of the metall casing combine sherical and cylindical threads. The sherical threads pull the implant in the required direction, while the end cylindrical threads, together with the depth of the treads, will garantee the high primary stability. Active acetabular bottom stabilisation is possible by a large central perforation in the metall ring. For cup implantation we prefer an anatomic position in straight alignment with the bony edge of the acetabulum. Experimental cadaver studies have proven the best primary stability when implanting the srew in cup in anatomical possition with maximum bone contact. In surgical procedure only the outer cortical layer of the lateral, tensile loaded acetabular bone is removed until slight bleeding cancellous bone is visible. The central pressure loaded pelvic floor is preserved. Acetabular reaming is finished two millimeters beyond the acetabular rim when good congruity with the osseus acetabular margin is achieved. We always try to use a srew in cup 4mm larger in diameter than the final reamer. The srew-in cup is inserted until its edge touches the bony rim of the acetabulum and cannellous bone chips from the reamers and the resected head are filled through the central perforation to ensure rapid bone graft remodelling. The entrance plane of the acetabulum is then corrected by the insertion of the polyethylen inlay. Optimum functional relationship to prosthesis stem is achieved by rotational position options in 30° steps and variable shoulder heights of 5 and 10mm. The final position of the polyethylen-inlay should cover the maximum extension-flexion plane.

The ribbed stem

Constructive features of the Link® ribbed stem are the anatomical s-shaped stem with deep longitudinal ribs, the use of a collar and the trochanteric tenson banding with a traction srew. The anatomical s-shaped stem provides an excellent adaptation to the natural shape of the medullary cavity. 19 different stem sizes for right and left side are available for an optimal prosthesis adaptation to the individual femur dimensions.
The surface profile with deep ribbs in the proximal third ensures interlocking and impaction of the cancelous bone between the ribs and provides in combination with the HX-coating a secure osteointegration. The prosthesis collar transfers physiological stress to the proximal femur in alignment with the trajectorial structures. Tension banding with a traction srew at the calcar, ensures additional anchoring and prestressing in the intertrochanteric region in order to minimize the zero passage and to achieve greatest possible primary stability. Static and dynamic labaratory testings after implantation have shown that longitudinal and transverse elasticity of the ribbed system closly corresponds to the original bone. This is additionally garanteed by the choise of titanium as implant material, the surface structure and the capacity for preloading by means of tension banding at the trochanter.
Femur resection is carried out as high as possible for reason of bone preservation and intertrochanteric tension banding. We use a curved sissel to obtain a groove for the lateral wing of the prosthesis. The groove is made in the maximum diameter of the proximal femur. The medullary canal is prepared with flexible reamers until slight cortical contact with gliding fit in the distal femur is achieved. Press-fit as well as undersizing in the distal femor shaft should be avoided. We choose the smallest size broach within the reamer diameter (size E) and drive it into the femur. The optimal broach size (size D,B) is achieved when cortical contact with the longitudinal ribbs is obtained in the proximal femor. A calcar reamer is used to prepare the resction plane and achieve optimal collar seating. We now have the oportunity for trial reposition to determine the correct head-neck length and to controll luxation tendency. Finally the original prosthesis is driven into the femor. Bone chips may be applied into the femur before impaction. After final seating of the prosthesis the collar is removable for additional bone grafting. Finally the traction screw is placed with a two millimeter drill hole in the lateral corticalis.
Summarizing the main principals for implantation technique of the Link® ribbed stem are excellent proximal upsite, distal gilding fit, cortical adaptation of the ribbs in the proximal femur and preservation of cancellous bone. The screw-in cup type `V`should be inserted in anatomic postion with maximum bone contact. The entrance plane is then corrected by the polyethylen inlay.

Material and methods

A detailed clinical and radiological examination was carried out for each patient preoperativly and in regular postoperative intervals. The clinical evaluation included objective and subjective parameters according to the rating system of Merlé d`Aubignè . Radiographic analysis used Gruen`s (2) zonal method for the stem and the zonal classification of De Lee and Charnley (1) for the cup implanats. We devided our results into two subgroupras with different stem and cup designs as well as stem implantation techniques. Group I was operated during 1986 and 1991. In this time peroid we used a toothed Ribbed Stem and a cobald-chrom screw in cup type „V". The UHMWPE-inlay was not metal-backed. Our implantation technique of the stem was characterized by distal undersizing. Operations in the second group were performed during 1992 and 1994. We used a HAtm-coated toothed Ribbed Stem and a Screw-in Acetabular Cup Type „V" made Form Tilastan® Titanium Alloy. The UHMWPE-inlay was now metall-backed. Distal stem undersizing was changed in favour of the „Gliding-fit" technique with slight cortical contact distally.
Between 1986 and 1991 272 THR were performed in 242 patients. 232 hips were available for clinical and radiological examination. 24 patients had died since surgery and 12 patients were lost. The responder rate was 85,3%. The average follow-up was 10,2 years.
The second group consits of 143 patients with 165 THR which were operated between 1992 and 1994. 135 patients were available for clinical and radiological follow-up. 6 patients had died since surgery and we were not able to contact 8 patients. . The responder rate was 95,1%. The mean follow-up was 6,2 years.
The dominat diagnosis was Osteoarthritis in 52%, respectivly 55% in both populations. The second frequent reason for THR was chronic inflammatory joint desease in 20%.

Results

The Merle dÀubigne score for group I (ribbed stem without HA) averaged 6,9 points preoperativly and 15,4 points postoperativly. The total scoring result was good and excellent in 82,3%. The Merle dÀubigne score for the HA-coated stems in group II increased from 7,2 points preoperativly to 16,9 points at follow-up. The absolute functional scoring was excellent and good in 97,4%.
The most benefit was achieved in pain relief and hip function for both populations. 62,5 % of the patients operated in group I were pain free or complained slight pain in 29,8% which did not interfere with daily activities. Load bearing pain was still evident in 9% of the operated patients. The HA-coated stem group II proved a distinct better pain relief in 95,7%, leaving 1% which still complained load bearing pain.
Range of motion increased in all directions for both populations. The gain in extension/flexion was +44° respectivly 40° in group II.
Sumarizing the HA-coated devices proved a significant better pain relief and functional score result. This difference in pain relief is strongly correlated to the higher incidence of stem migration in groupI.

Radiological analysis

The zonal radiographic analysis for the ribbed stem without HA-coating revield in 14,6 % varus stem migration with progedient radiolucent lines > 2mm in zone 5 und 6. Paralell to distal radiolucency a proximal subsidance with breakage or dislocation of the traction screw was noted in 12%.
Half of the patients with varus stem migration complained slight pain or constant loads bearing pain. For this reason we changed stem implantation technique from distal underszising to distal gliding fit. The HA-coated ribbed stems only showed small radiolucent lines less 1mm in 2-3% for zone 3, 5 and 6. Varus stem migration or progredient radioluucencies were not noted at all.
The radiographical cup analysis of the chrom-cobalt screw-in cup with non metal backed PE-Inlay revieled in 22% radiolucent lines less 2 mm for zone II. 10 out of 272 cup implants showed progredient radiolucent lines mainly in zone II and III, corresponding to cup migration and aseptic loosening.
In the metall-backed inlay group II radiolucent lines in zone II were only observed in 0,7%. Cup migration or aspsptic loosening was not observed.

Revisons

We had a total revision rate of 5,6% for aseptic loosening after 10 years in group I. Total hip exchange arthoplasty was performed in 8 patietns, a single stem revision in three cases and single cup revison twice. Additionally 2 total hip replacments undervent two-stage exchange arthroplasty for late periprosthetic infection.
In the second population we observed one periprosthetic infection. The estimated cummulative sucess rate for the Link® ribbed system without proximal HA-coating was 87,6% after 13 years and 95,1% for the HA-coated ribbed stem including septic revsion after 8 years.

Conclusion

In conclusion total hip arthroplasty using the Link® ribbed system and screw in cup is a reliable procedure in long- and midterm follow-up. 82% of the patients achieved good and excellent scoring results after an average follow-up of ten years. Varus stem migration in 20% caused critical evaluation of implantation technique with distal undersizing as a techniqual error. After changing our implantation to gliding fit technique of the distal stem and the introduction of HA-coated ribbed stem design stem subsidance was significantly reduced. The comparision between the two implantation periods using different stem design and implantation technique proved the superiority in subjective and objective parameters of the HA-coated group.

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