A FORTY FIVE YEAR EXPERIENCE WITH THA M. Kerboull

1. A FORTY FIVE YEAR EXPERIENCE WITH THAM. Kerboull

2. Mc KEE –M.A Mc KEE – WATSON FARRAR My experience with THR began in 1965 with a metal on metal prosthesis in cobalt chromium alloy with a 41mm femoral head called Mc Kee –Merle d’Aubigné. It differed from the original Mc Kee in that the acetabular sprockets were shorter and the femoral component had a Moore stem whereas the original Mc Kee had a Thompson stem.

3. After 2 year experience with this cemented prosthesis, I noted there was in 10 % of the cases a complete cement-bone radiolucent line, meaning an actual or potential loosening of the acetabular component.

4. On the other hand, the femoral component fixation appeared to be strong and did not show any worrisome picture.

5. At that time, these early loosenings were ascribed to a high level of stresses on the acetabular component due to addition of a high friction coefficient and a large femoral head. The study of the retrieved components, moreover, showed that they had a high frictional moment, far higher than that of the low-friction Charnley prosthesis.

6. I gave up the Mc Kee-MA prosthesis in 1969 and adopted the Charnley prosthesis. Its small head and stainless steel polyethylene low friction torque seemed strong arguments to protect the acetabular component from harmful stresses.

7. 2 years later, the result was reversed but not better. If there was no radiological concern about acetabular fixation, I noted a relatively high rate (24 %) of early debonding of the stem sometimes associated with a transverse cement fracture at the stem tip level. Even though this debonding was asymptomatic or almost asymptomatic and did not prevent a satisfactory function, I thought it had to be regarded as a loosening of the stem and a failure of the cemented fixation. On a consecutive series of 220 THR (1970-1971) using the original Charnley flat back stem, followed up 20 to 25 years, despite a rate of early stem debonding of 24 %, only 3 % needed a revision of the stem.

8. The first event was a longitudinal crack in the supero medial part of the mantle due to high pressure transmitted by a stem thin and therefore elastic, strongly curved with a long offset and a stem neck angle relatively closed (125°).

9. The break of cement in this location widened the proximal part of the cement sheath, decreased the shear stresses along the stem and increased the vertical force on the distal cement which broke under tensile stress and allowed the stem to subside.

10. SURPRISINGLY,The cement fractures were especially seen in wide medullary canals through thick cement. Rarely observed when the stem filled a relatively narrow canal despite a thin cement layer. 36 % 6 %

11. And never when a narrow straight stem had been implanted in a dysplastic femur after reaming of the canal, despite a very thin cement layer and neither was seen any radiolucency at the cement bone interface although all the cancellous bone had been removed by the reaming. 0 %

12. These cement craks clearly showed that PMMA was a brittle material characterized by Relatively high compressive strength 93 Mpa Medium bending strength 64 Mpa and weakness in shear 42 Mpa and tension 35 Mpa

13. What Solution ? - In 1971, nobody knew how to improve the mechanical properties of PMMA. - A false solution : To stengthen the cement stem bond by giving the stem a rough surface, but such a surface finish, because of the strong adhesion of the cement to stem, would increase shear stresses at the cement bone interface and under these conditions of overload there would be a considerable potential for the cement bone interface to fail.I therefore rapidly gave up this idea.

14. A better solutionto avoid cement fractures and stem debonding was to subject the cement only to stresses it can resist (compressive stresses) and protect it against harmful stresses (bending, shear and tension).It seemed theoretically possible to obtain this condition by giving the stem such a shape that the stresses within the cement would be decreased to a level consistent with its poor physical properties. That was the reason why in 1972 I modified the Charnley stem.

15. I retained: - The polished surface with a Ra of about 0.04 µm (1.6 µi) to allow the stem to have micromotion inside the cement mantle without any risk of deteriorating the cement. - The collar to limit the micro subsidence of the stem under load - And a rectangular cross section which offers a better resistance the torsional moment about the stem. MK I 1972

16. I opened the stem neck angle to 130° to give the stem a straighter shape and decrease the pressure in the supero medial part of the cement mantle and thus prevent longitudinal crack in this location.

17. I widened and thickened the proximal part of the stem to give it a double tapered shape with a regular taper angle of 5°. Ch. Stem K. Stem

18. Which such a stem, I calculated that the shear stresses along it would be transformed, for the most part, into their pressure components and the vertical distal force reduced to a level lower than the cement tensile strength preventing thus the cement with push out fracture at this level.

19. Under these conditions: The cement mantle, as well as the cement bone interface, would no longer be subjected to shear stresses, but only to compressive stresses.Therefore, interdigitation between cement and cancellous bone was no longer necessary and that seemed particularly interesting in revision surgery.

20. Removal of the cancellous bone from the medullary canal appeared to have an additional advantage, that of giving the cement mantle subjected to compressive stresses (torsional moment about the stem) and tensile hoop stresses (double tapered stem) a firm, even and rigid constraint by the cortical bone of the femoral shaft and thus avoiding longitudinal cement cracks.

21. An other important requirement:To reconstruct a normal architecture of the artificial hip in every case, taking into account a broad variety of morphological aspects, a wide range of sizes was needed: 7 neck lengths 7 offsets 7 stem sizes 5 neck lengths 5 offsets 5 stem sizes standard series 18 dysplastic series 5

22. With this type of prosthesis, in every case, a well selected femoral component fits (but not actually fills) the medullary canal. The stem is automatically centered, so there is no need for a stem centralizer.

23. I believed and I still believe that these anatomical conditions were mechanically sounder than a thick cement mantle which requires a relatively thin stem with a rounded cross section, because a thin stem overloads the cement and a rounded section does not resist a high torsional moment.

24. With the Kerboull stemthe cement mantle was rather uneven, relatively thick anteriorly and posteriorly (> 2 mm), thin and sometimes very thin medially and laterally (< 1 mm) but I have never seen it incomplete or broken when revising a worn or loose socket with a well fixed femoral component.

25. It doesn’t matter whether the cement is thin or elsewere thick since it is only subjected to radial compressive and hoop tensile stresses which it can resist very well since the cement mantle is firmly constrained into a rigid cortical tube.

26. Clinical experience with this prosthesis (Howmedica MK I 1972-1988) confirms the soundness of my theoretical thinkings since- The early debonding has almost completely disappeared (1 % and always < 2 mm).- At 20 Y. follow up the rate of radiological loosening of the femoral component was always low : 1.5 % in patients over 60 years 1 % in patients under 40 years 1 % in patients with dysplastic femur- Never was seen femoral osteolysis except in calcar zone where it was related to polyethylene debris.

27. A transient experience with other stems different in surface finish (matte) and cross section (rounded) CMK II Sanortho (1984-96), CMK III Vector Orthopédique (1988-96) under influence of fashion and engineers was led concurrently with the polished types MK I (1972-88), MK III (1988 to now) and has strengthen our basic opinion.

28. These matte prostheses were not so successful. Dependent on the roughness of the surface, the rate of radiologic femoral loosening was at 15 Y. follow up. 0.6 % for the polished stem (Ra 0.04 µm) 3.3 % satin stem (Ra 0.9 µm) 10.3 % matte stem (Ra 1.7 µm)

29. The femoral loosening in these matte and satin stems occurred at the cement bone interface and was associated with cortical osteolysis. Cement cracks were seen with the satin finish in the proximal part of the cement mantle indicating an overload of the torsional moment at this level.

30. Cement cracks were exceptional with the rougher surface. In this latter case, the cement mantle was usually intact adherent to the stem indicating a fatigue fracture of the cement bone interface due to shear overstress in this location.

31. Therefore,the behaviour of the cemented stem could be completely different, dependent on the geometry and surface finish of the stem since in these series the cement mantle was quite similar and rather thicker and more even in CMK II and CMK III which were less successful than MK I and MK III.

32. In conclusionTo consider bone cement individually is a skewed approach of the problem. Bone cement is only a part of a composite structure in which the mechanical behaviour of the stem and not the cement layer plays the decisive role. It is indeed quite logical to try to improve the mechanical properties of bone cement but it will always remain a brittle material.

33. It seems more effective to protect it against high tensile and bending stresses and protect bone cement interface against shear stresses. That is what a highly polished, double tapered stem with a regular taper angle of 5° and a rectangular cross sectional area can do.

34. On the acetabular side, the problem is completely different. Because the socket is strongly fixed to cement, there is no micromotion in this interface, so all the stresses are concentrated in the cement bone interface.

35. These stresses are of pressure in the bearing zone and shear all around the socket. In order that bone fixation lasts, acetabular stresses must remain lower than the bone cement resistance.

36. Pressure forces located in the bearing zone essentially depend on the patient weight and the geometry of the femoral component. To reduce these forces it is therefore necessary to have a femoral component with a stem neck angle of 130°, an adequate offset, and implant the acetabular component in the right anatomical place.

37. Whatever the original aspect of the hip Normal

38. Subluxated Completely dislocated.

39. Shear stresses around the socket depend on:friction coefficientand friction moment which directly depends on the dimension of the femoral head. Therefore a small head is preferable.

40. But to avoid instability, the hemispheric socket has to be deepened by a 2 mm cylindrical segment and the neck diameter just below the head does not exceed 10 mm to avoid impingement between neck and socket. It is always necessary to retain the integrity of the peri articular muscles and sometimes to restore their balance.

41. How to resist shear stresses? To resist shear stresses 2 holes in pubic and ischium bone are enough and more efficient to resist valgus forces than a unique or multiple holes in the roof which are too close to the centre of displacement, and which furthermore have the disadvantage of impairing the rigidity of the roof.

42. To resist pressure forces we must retain the sclerotic bone of the roof. That means that when we transform an arthrotic ovalized cavity in a spheric cavity, the reference is not the long diameter (jagged line) of the oval which would lead to the destruction of the sclerotic roof and thinning of the anterior and posterior walls and therefore would decrease the resistance of the roof and increase the elasticity of the cavity.

43. The right reference is the small diameter of the oval or transverse diameter of the acetabulum (dotted line). Deepening the cavity in this location until the inner floor, save the bone of the anterior and posterior wall as well as the sclerotic bone of the roof.

44. The gap between bone and socket superiorly and anteriorly located can be filled with a structural bone graft in some destroyed or dysplastic acetabulum.

45. But if the gap is just 1 cm thick it can be filled with cement strongly impacted. A thick cement layer in bearing zone has the advantage of rigidifying the socket in an area subjected to high pressure forces.

46. Anywhere else the cement layer must be thin not to rigidify the polyethylene socket in an area where the acetabular cavity is elastic.

47. This concern for having an acetabular component in mechanical harmony with the bone cavity reproves, in my opinion, the high socket placement as well as the metal back cemented socket and a thick layer of cement.

48. With this type of acetabular cemented component implantation.There never has been any mechanical loosening of the socket. Unfortunately osteolysis due to polyethylene particulate debris has led at 20 y. to socket loosening with a rate of 5 to 12 % dependent on the age of the patient and etiology of the hip disease.

49. ConclusionA well cemented prosthesis is a safe means to give the patient a satisfactory function for a long time if this prosthesis is well adapted to cemented fixation and the implantation technique perfectly done.The only problem is the polyethylene wear and its detrimental consequences. However, we can hope that with the highly cross linked polyethylene we will see a considerable improvement of the long term results in the future.

50. These basic notions both mechanical and technical have guided our technique of THA since 1972. I am going to tell you about long term results of THR in 3 categories of patients: - THR in Primary hip degenerative disease - THR on young patients less than 40 y - THR on Crowe IV hip dysplasia The operative technique, the prosthesis used and result assessment method were the same for the 3 series.