Acetabular bone response to porous tantalum


G.A MACHERAS [1], D. MPALTAS [1], A. KOSTAKOS [1], K. TSIAMTSOURIS [1],
S. KOUTSOSTATHIS [1], K. KATEROS [2]
[1] Second Orthopaedic Syrhery Clinic of 1st IKA Hospital
[2] General Peripheral Hospital of Leivadia

Mailing address:
G.A. Macheras
18 Papadiamantopoulou St.
11528 Athens

ABSTRACT
We studied the acetabular bone behavior and its biological response to acetabular implants made of porous tantalum, as well as bone deficits filling capacity after the implantation of these acetabular protheses. Furthermore, we assessed the biological fixation of these protheses.
Between January 1998 and December 2001, 180 porous tantalum acetabular protheses were implanted in 172 patients. During the postoperative radiology control acetabular bed deficits were observed 1-4mm between protheses and acetabular wall in 25 cases which consisted our study material. Acetabular division in five zones was performed and we noted down which of the above zones presented a bone deficit. Radiology control was repeated six (6), twelve (12) and twenty-four (24) weeks later. All bone deficits were completely recovered twenty-four weeks (6 months) after the implantation. The study method as well as the time of bed deficits filling are described in detail and the degree of protheses biological fixation is assessed, as well as the porous tantalium osteoconducting behavior.
Key words: Porous tantalium, Hedrocel acetabular protheses, Osteoconductivity.

INTRODUCTION
Long term survival of cementless acetabular hip protheses is dependent of several factors, mainly including their initial fixation, metallic shell osteoconducting properties and the bone response, in order to fulfill any empty spaces remaining after the implantation with simultaneous bone penetration in porous protheses surface (protheses bone penetration mechanism) or with metallic shell bone overlapping (protheses bone overlapping mechanism) with a view to accomplishing the so called protheses biological fixation. It is well known that every implanted arthroplasty forms corrosion products that are likely to cause periprothetic osteolysis[4,11,22]. This osteolytic procedure can be decreased by preventing corrosion productsΥ penetration in the surface between bone and protheses. In cementless arthroplasties effective joint space decrease can be performed through improving bone penetration in porous protheses surface[1,8,10,12].
During the last 20 years several materials have been tested in order to produce a metallic shell of cementless arthroplasties, each having disadvantages and weak points. The commonest ones are several titanium alloys and cobalt-chromium alloys. External surface of metallic shell is processed via different techniques, like overlapped nodules, plasma sprays, microporous surface, metallic fibers, e.t.c., aiming at forming a porous surface in which bone will develop and penetrate around or inside it, in order to achieve the implant biological fixation[1,8,10,13,14].
Total porous surface of these materials consists 30% to 50% of their volume, thus resulting in total bone penetration decrease and consequently prothesesΥ total biological fixation decrease as well as decreasing the developing holding power between the two surfaces (bone and prostheses)[11,12]. Porous tantalium, a new biomaterial which has got to use in prosthetic Orthopaedic Surgery since 1997, has an unusually large surface which is consisted of several parts in combination and it comes up to 75-80% of total material volume. Its total geometry, shape and size is similar to that of spongy bone, that is to say, it is 2 or 3 times larger than any other material1,6,7. Porous tantalium elasticity is 3Gpa, that is, it stands between spongy (0.1Gpa) hypochondrial (2Gpa) and cortical bone (15Gpa) elasticity. On the contrary titanium (110Gpa) and cobalt-chromium alloys are much more inelastic materials.
The combination of tantalium large porous surface and its elasticity allows a larger bone penetration volume and thus resulting in a faster and more powerful implantsΥ biological fixation[2,3,6-8,25]. The porous surface and bone contact during the implantation, as well as the excellent transoperative stability are of vital importance because, in that way, bone penetration is maximized[5,8,24].
Direct full bone and protheses contact are comprising the desired target, however, it has been proven that bone penetration occurs even when there are deficits up to 2mm[6,7,10,14] in currently known cementless protheses. Experimental trials in dogs and mice have shown that in porous tantalium implants, up to 3mm deficit filling is taking place[2,6,7]. In current study, we examined the up to 4mm bone deficitsΥ filling potential, after the implantation of porous tantalium acetabular protheses.

Picture 1. Hedrocel acetabular implant in cross-section, showing the bonding between
the polyethylene and the tantalum shell.

2 3
Picture 2. Hedrocel acetabular implant in situ with a small gap between the
prosthesis and the bone in zone C.
Picture 3. Hedrocel acetabular implant in situ with a large gap between the
prosthesis and the bone.

4
Picture 4. Immediately post op picture, in which the measurement of the various
acetabular parameters and the gap.

5
Picture 5. The same patient as in picture 4 the 2nd month post op.

6
Picture 6. The same patient as in picture 4 the 6th month post op.

7
Picture 7.The same patient as in picture 3, in which the complete filling of the
| gap is clearly seen after 12 weeks.

1
Figure 1. The five zones in which the acetabulum was divided.

MATERIAL
From January of 1998 till December of 2001 we implanted 180 porous tantalum acetabulars without cement in 172 patients. One hundred and nine were women age 32-79 (average age 63yrs) and 63 men age 47 to 81yrs (average age 71yrs).
The diagnosis was
- Idiopathic O.A (Osteoarthritis) 154
- Aseptic necrosis 9
- Lower congenital dislocations 6
- Post traumatic arthritis
In all the patients we used Hedrocell cup (implex corp, ALLENDALE, N.J. 07401, USA) the shape of which is widened hemisphere with an increase of the diameter by 2mm in the circumference for application with press-fit technique. The diameter of the tantalum shell pores is 550μm, whereas the elasticity is 3Gpa. The polyethylene is embodied with the porous shell of the tantalum. The integration of the polyethylene in the porous tantalum is made with the process of direct infusion and compression. Due to this direct infusion and compression polyethylene penetrates within the shell by 2mm (figure 1).
This process offers the possibility of producing acetabulars with a minimum total thickness of polyethylene of 8.5mm in external diameter 48mm for the head 28mm (6.5mm without the shell and 2mm within the metal shell), whereas in the acetabulars with an external diameter 40mm the minimum total thickness of the polyethylene is 8mm with the head 22mm (6mm without the shell and 2mm within the shell).
The friction coefficient of the porous tantalum on the bone is about twice that of other biomaterials with a porous surface1,6. This fact in conjunction with the hard and relatively "rough" external surface of the shell which has the property to adhere strongly to soft tissues, as well as the fact that the shell with the polyethylene is embodied in a unique fragment not offering the possibility of viewing and estimation of the complete contact of the shell with the bottom of the acetabular in the introduction stage of the protheses, lead mainly in the initial period of the implantation to its ideal placement and specifically to the incomplete contact of the implant with the bone of the acetabular in all its range, resulting in the existence of bone spaces, mainly at the bottom and sideways which were recognized in the postoperative radiography[19,20].

METHOD
All the operations were carried out by posterior access. The placement of the porous tantalum cementless acetabular was performed by the press-fit technique. Special care was taken not to insert soft tissues between the implant and the bone of the acetabular during the stage of the implantation. The stability of the implant was checked directly by the operation and in all the cases it was considered absolutely satisfactory.
The postoperative mobilization of the patients was carried out on the second post operative day with partial loading for three weeks and then the full loading was permitted. All patients had undergone radiology control before their hospital exit which included:
1) Anteroposterior x-ray of the pelvis-hips
2) Anteroposterior and profile x-rays of the operated joint.
All the x-rays were taken with the patient lying down on the radiological table and the x-ray lamp vertical and at a distance of one meter from this.
The x-rays of the operative joint were studied in order to determine if there was a space between the implant and the acetabular and at what point. In order to make possible the exact description of deficit topography, as well as, the expected bone response, the acetabular was divided into 5 zones A.B.C.D.E as is shown in shape 1.The separation of the acetabulars in 5 zones was described by Bobyn & Ass at an experimental model and was favoured in our study versus the classic separation in three zones by Charnley and De Lee, because it offers the possibility of more detailed and specific topographic estimation of the space between the protheses and acetabular bone.
In the direct postoperative x-rays of the operative joint the following were measured.
1) The number of cases where complete contact of the implant with the bone of the acetabular existed
2) In how many cases there was a space
3) The width of the space
4) In which of the five zones there was a space
In those cases that a deficit was determined the x-rays were repeated on the 6, 12 and 24 weeks, a similar separation of zones was made and the behaviour of the space was studied which in turn was noted in the immediate post operative x-rays.
All the x-rays of the counter-check were carried out in the same laboratory and an attempt was made so that the procedure was done by the exact same way so that comparisons could be made between them. In order for the deficit between the implant and the acetabular to be measured and its alteration during our study, the following methodology was followed. A digital scanning was made of all the x-rays with a high resolution scanner and in turn processing and analysis of the data using the software Optimate and EBRA was performed[16-18].
This software offers the possibility of measuring the spaces in the range of 0.1mm. In this way, it was made possible to evaluate the space in the five above mentioned zones with a referral to the numbers which were found in the scale 1/ 1,15 so that the numbers corresponded to the real existing patient deficit.

RESULTS
Direct postoperative X-rays study showed that in 155 acetabulars full contact between acetabular bone and implant was established and there was no deficit except in eight cases where in zone C a deficit from 0.2 to 0,4mm was observed, obviously due to incomplete depth scalpeling and thus resulting in a small gap in fossa (figure 2).
In 25 acetabulars, a gap existed between the implant and the bone, ranging form 1 to 4mm (figure 3). All these 25 acetabulars were placed during the first six months of 1998, when our surgical team was trying to adjust itself to the new material. In Table 1 range deficit is noted in five above mentioned zones, directly after the operation.
In Table 2 the change in bone and implant is shown 6 and 12 weeks after the operation. Measurements in 24 weeks proved that all deficits were completely covered (figure 7).
The result from data analysis with EBRA method16-18 (figures 4-6) was that all 25 acetabular implants remained virtually stable in x and y axis, given that mean x axis displacement was 0.01mm and y axis displacement was 0.02 mm (practically unconsidered values) and there was no alteration in acetabular implant deviation angle, thus resulting in no implant immigration.
The deficit presence between bone and prostheses did not influence postoperative protocol that was the same in all patients. None of the 25 acetabulars has shown any clinical symptom of implant instability from the implantation day till now when all cases have completed two years of postoperative follow-up.
From 152 patients who had undergone 155 acetabular implantations with no deficit between bone and acetabular directly after the operation, nine (ten acetabulars) were lost during postoperative follow-up and thus our study includes data from 145 acetabulars. None of these 145 acetabulars has shown a radiological lucid line during the last radiology control through all prothesesΥ circumference, minimum acetabular bed deficit of 0.2 to 0.4mm was filled in the eight acetabulars and we could observe a picture of complete acetabular protheses integration from pelvis bone. However, the most narrow and uniform implant-bone contact was seen in zones A and E, where bone density was larger (cortical bone) thus resulting in a denser bone implant penetration. Clinically, all patients presented with complete hip mobilization in six months, they had no pain at all and fully recovered in their previous activities.

DISCUSSION
It clearly results from our study that there was no immigration of the implant. The speedy completion of newly formed bone in the space that existed between the prothesis and the bone of the acetabular is impressive and absolutely compatible with the experimental studies in dogs and rats[3-5]. Till today all the clinical studies for other implants form titanium alloys have proven the coverage of smaller spaces[12-15]. The filling of spaces up to 4mm and the apparent thicker bone deposit and as a result, the penetration in the prothesis circumference (zones A and E) where cortical bone exists, directs us to the following three interesting conclusions.
1) The porous tantalum obviously due to its great porous surface (70-80% of its total volume) and its similar texture and elasticity with the physiological bone seems that it acts like a osteoconductive material and in conjunction with the existence osteoinductive material proceeding from the carving of the acetabular, facilitates in the completion of the bone deficits and consequently, in the reduction to the minimum of the "effective joint space" and therefore, the minimization of the possibility of entrance of corrosion products in the acetabularΥs calva, where as it is known in the long term leads to the mechanical prothesisΥ relaxation.
This fact is reinforced by the monoblock construction of the acetabular which does not allow micromovements and production of corrosion products between the polyethylene and the metal, also the absence of spaces in the shell, as well as the importantly larger thickness of polyethylene in relation to all the other acetabulars which is well known that it is a very important factor in the behavior and survival of the protheses.
2) The great volume of porous surface on the prothesis shell provides the possibility of grater bone penetration inside these pores and mainly in zones A and E where cortical bone exists offering greater stability than the spongy one[1-3,5,8,9]. This means that the biological stability of the implant is strong and secure.
3) The high friction coefficient of the porous tantalum toward the bone and the fact of the increasing adhesion toward this and the soft tissues, provides very good initial stabilization giving this way the possibility of secondary bone penetration on the tantalum metal shell, and the full implant integration[19,20].

CONCLUSION
The use of the porous tantalum in Orthopaedics seems that it can give a solution to the problem of the primary stabilization of the acetabular implants, their premature immigration and possibly the corrosion of the polyethylene and the osteolysis associated with it. The bone correspondence to this material seems to be very good. Further clinical and radiological estimation of the behaviour of these protheses will show to what degree these premature observations are right or not.

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