Original articles

Eur J Orthop Surg Traumatol (2000) 10:27-33

European Joumal of Orthopaedic Surgery &

Traumatology �9 Springer-Verlag 2000

Preliminary experience with isoelastic total hip arthroplasty using impaction bone grafting

O.N. Nagi, M.S. Dhillon, J.R. Bapuraj and V.G. Goni

Department of Orthopaedic Surgery & Radiology, P.G.I.M.E.R., Chandigarh (India)

Summary: Isoelastic THA using an uncemented acetabular cup fixed with pegs and screws along with a femoral stem containing a core of metal inside polyacetyl resin was used in 51 hips with an average fol low up of 4 years lO months. All cases were immobilized in abduction for 6 weeks post operative with deferred full weight bearing for another 6 weeks. One case failed due to deep infec t ion , and one had extra medullary placement of the distal pros- thesis. All others had improvement in the pain and function score ; 2 patients were unsatisfied with the result. Excessi- ve femoral reaming was avoided as we preferred to leave behind the "living" cancellous bone to promote ingrowth along the prosthesis. This procedure may be a good al ternative in young adults where cemented THA is not the procedure of choice. We attribute the good short term results in this series to the rigid post operative immobilisation (which we have labelled as the "critical period"), filling up of the medullary canal with our technique of bone graft paste prior to prosthesis insertion, and leaving behind the endosteal cancellous bone inside the medullary cavity which

Code M~ary: 4232.2

Correspondence to: O.N. Nagi

possibly hastens bony incorporation of the femoral stem. A unique point in this study was our attempt to study bone for- mation around the prosthesis by axial CT scans, the results of which are dis- cussed.

Key words: Isoelastic hip - - Bone graf- ting -Allograft - - Bone paste

With better designs and availability of newer materials, the indications for total hip arthroplasty, one of the most signifi- cant o r t h o p a e d i c advances of the modern age, have been widened. The major area of concern is the fixation of the components to the bone ; to date cemented hips have been the gold stan- dard. As THA is being done increasingly in younger patients, and a longer follow up of previously implanted hips is avai- lable, problems like bone resorption and prosthetic loosening have come to light. The cement bone interface has been implicated as the site of complication [22] and the inherent potential compli- cations of bone cement are numerous [12]. Bone cement has been shown to have mechanical weakness both in ten- sion and shear, and the mismatched modulus of elasticity between implant- cement and cement-bone has the poten- tial for loosening at a relatively early sta- ge. Over the years , n u m e r o u s uncemented hip prosthesis have been designed with claims ranging f rom

improved biocompatibility, better stabi- lity and better force distribution bet- ween prosthesis and bone. None have been universally accepted and all have certain limitations.

Thirty years ago, Rob Mathys esta- blished that bone could grow into surfa- ce pores and recesses of a plastic pros- thesis, provided that the bone was being loaded and the prosthesis was stable [1, 2, 13, 14]. Polyacetyl resin was selected for the femoral component, due prima- rily to its durability and tensile strength, but also due to the fact that it had a modulus of elasticity which was nearer to that of bone [9, 13]. It also had good tissue tolerance ; however, due to the fact that this material deteriorated with a concave surface configuration, it was considered unfit for implantation as an acetabular cup. The newest model of the femoral stem has a central metal core inside the polyacetyl resin, with better shape for proximal fit [1]. The upper third of the stem has circumferential cross ha tch ing while the lower two thirds is longitudinally grooved to per- mit better grip and bone in-growth res- pectively. The acetabular cup, made from UHMW Polyethylene (UHMW- PE) has an outer coating of pure tita- nium ; it has a relatively rigid dome with two plastic anchoring pegs, along with fixation holes for screws at the periphery [13]. The acetabulum is much more elas- tic in the region of the transverse liga- ment as compared to the dome [1] and

28 O.N. Nagi, et al.: Preliminary experience with isoelastic total hip arthroplasty using impaction bone grafting

the component has been similarly desi- gned to deliver compressive forces at the dome by the plastic pegs, while riding clear of the more elastic lower half of the bony acetabulum. We have been routi- nely using this prosthesis in young adults for indicat ions ranging f rom Juvenile Rheumatoid arthritis, ankylo- sing spondylitis, to post traumatic and post avascular necrosis (AVN) arthritis. We present our results in 51 hips done since over a six year period (Jan 1990- Jan z996).

Materials and methods

51 hips in 48 patients (3 bilateral) were replaced using the uncemented Isoelas- tic hip prosthesis (Rob Mathys Co., Switzerland) over a six year period. Most cases were young adults with the age ranging from 19-63 years (average age 35.3 years), and there were 40 male and 8 female patients. The p r imary indicat ion for a r throplas ty in these cases was debilitating pain accompa- nied by crippling deformity, or bilateral severe hip disease not allowing inde- pendent mobility. All cases were opera- ted through an anterolateral approach, and no t rochanter ic os t eo tomy was done. The femoral component size used was either 8 or lO mm stems. The aceta- bular component was inclined 5 ~ to lO ~ more to the horizontal than advocated by the manufacturers of this prosthesis, to achieve maximum vertical compres- sion with weight bearing. It was stabili- sed by the two pegs and a minimum of three cancellous screws were passed into the ischium, ilium and pubis res- pectively. Femoral reaming was done till the size No. lO only, and care was taken to ensure that a lining of "living" can- cellous bone was left on the medullary surface of the femur if possible. The lon- gitudinal slots of the stem were filled with bone paste made from a mixture of allograft and autograft. Formalin pre- served allograft and autograft taken from the excised femoral head was mil- led in the bone mill and mixed with rea- med marrow material to form a paste. This paste was manually inserted into the femoral canal after a distal plug was inserted, and repeatedly compressed by

gently hammering it in by using a trial prosthesis one size smaller than the size of femoral component to be used. The tapered shape of femoral component forces the bone graft paste into all the recesses of the medullary canal ; at the time of prosthesis insertion a specially made slotted plate was placed at the proximal cut end of the femoral canal to minimize extrusion of bone paste and to ensure a press fit inside the canal. Due to unavailablity of laminar air flow system in the operat ion theatres, all cases were given postoperative intrave- nous antibiotics for a period of five days to minimise the infection risk. All cases were immobilized in abduction for a period of six weeks using foam gutter splints. They were then allowed free activity in bed and non weight bearing crutch walking was started at 9 weeks post operative. Weight bearing was gra- dually increased till the patient was independently mobile ; crutches were discarded at the end of three months and the patient subsequently used a stick. Unrestricted full weight bearing was on ly al lowed at the end of six months. All patients were encouraged to perform quadriceps exercises from the first day and hip range of motion exercises and abduction exercises from 6 weeks post operative.

Serial clinical assesments were done using Charnley's modificat ion of D- Aubigne and Postel's evaluation system [18]. The radiological criteria which were previously employed in cemented THA were modified for analysing these cases. Routine AP and lateral radiographs were taken at three weeks, 3 mon ths , 6 months, 9 months and then every six month until last follow up. Additional use of 3-D CT imaging was done in an attempt to evaluate the bone incorpora- tion and to see if there was new bone formation around the prosthesis. CT scans were done on somatom HIQ (Sie- mens) scanner. Only cases with a follow up period of 12 months or more, who were willing to participate in the study, could be evaluated by CT scans. Two mm contiguous sections were obtained at the subtrochanteric level and at the tip of the prosthesis ; lO-14 sections were obtained at each site. These regions were

selected due to certain factors: (i) these areas bear the maximum stress and (ii) at the trochanteric levels, the presence of the two screws made metallic artifact reduction more problematic. All sec- tions were obtained using a metal arti- fact reduction protocol. An attempt was made to see if we could demonstrate whether new bone formation was actual- ly induced, and if bone paste filled the entire reamed cavity or not. The criteria for bone format ion was an irregular hyperdense area which was present cir- cumfrentially around the prosthesis. A note was made of sclerosis around the prosthesis tip, which was labelled as "Cap formation".

Results

There was no per-operative mortality ; 46 cases were available for follow up on request in the per iod Jan-March 1998. One failure was excluded, and one case was lost to follow up after 1.7 years and could not be reviewed. In the available 46, follow up ranged from 2.1 year to 8 years 3 months (average 4 years lO months) . Preopera t ive hip score for function averaged z as per Charnley's criteria, and this improved to 5.5 postoperatively, which was consi- dered significant. The grade for pain improved from an average of 2.5 pre- operatively to 5.5 post operatively, whi- le the range of motion improved from a grade of three to an average of 5-3 at follow up.

Forty four cases felt that improve- ment after surgery was significant whi- le two patients felt that they were only moderately improved. Six of our cases had varying degrees of mid thigh dis- c o m f o r t ; this usua l ly s t a r t ed 2 to 11 months after full weight bearing and reso lved wi thin one year of onset .

No case showed radiological loose- ning of the acetabular cup at the follow up available (Fig. la, d). By the end of six months increasing sclerosis was noted around the cup in the weight bearing area and around the two pegs. The scle- rosis became more dense over the next six months and did not change after one year in any case (Fig. 2a, b). All femoral stems used by us were third generation

O.N. Nagi, et al.: Preliminary experience with isoelastic total hip arthroplasty using impaction bone grafting 29

predominantly along the medial side of the femoral stem.The "stellate contour" of the prosthesis was seen to be filled up at this level.

Intra-operative complications inclu- ded inaccurate placement of the aceta- bular screw in three cases and screw breakage in one case. There was perfora- tion of the femoral cortex in one case and the prosthesis was inadvertently placed outside the medullary canal in its distal part. This was discovered on post- operative lateral films and the patient was told about this and advised to have a strict follow up every three months and to report if there was any pain or signifi- cant discomfort. He has been advised about the possibility of revision of the femoral stem. Twenty seven months post operatively the patient is painlessly mobile. There were two cases of superfi- cial wound infection which responded to wound debridement and appropriate antibiotics. The single case (not included in this analysis) of deep infection did not resolve with any measures and the pros- thesis was removed seven months after implantation. This was the only failure in our series. Another case fell from a motor scooter two years post operative and developed an undisplaced fracture of the greater trochanter with a fracture of a trochanteric screw. He was treated by immobilization in abduction for six weeks and then mobilized.

Fig. la-d a, b X-ray photograph (AP and Lateral view) 3 months after Isoelastic THA. c, d Same case 2.5 years post operative. Note the new bone formation and the bony cap at the tip of the prosthesis

prostheses and in spite of the narrower diameter components, none showed any varus tilt or subsidence at average 4 years lO months. In cases where CT was done, all except two showed formation of bone around the prosthesis(Fig. 3a). The density of the outer hyperdense allograft was nearly the same as the polyacetyl resin (Fig. 3b) ; the change was clearly defined at the prosthesis tip and beyond.

3D reconstructions showed a distinct pattern (Fig. 4a, b). Concentric areas of new bone/allograft paste were seen at the trochanteric level. This layering was more prominent in compact bone on the outer side, compared to the trabecu- lated bone near the marrow cavity. At the tip of the prosthesis, a distinct cap of bone/allograft was seen (Fig. 5a, b). In all prostheses inserted with a valgus orien- tation, bone formation was observed

Discussion

Even today we cannot pinpoint a single total hip prosthesis which has been uni- versally accepted, and research is on- going to produce the most suitable pros- thesis. The Isoelastic hip has been used at var ious cent res in Europe and a review of the published literature [1, 3-8,

10-17, 19-21] has revealed that numerous such prosthesis have been implanted and reported upon (Table 1). The short term results are comparable to cemen- ted THA using conventional techniques, and also to other uncemented arthro- plasties. More and more of these pros- theses are being used for revision THA [11]. The most c o m m o n p rob l em encountered in primary THA with this

30 O.N. Nagi, et al.: Preliminary experience with isoelastic total hip arthroplasty using impaction bone grafting

Fig. 2a, b X-ray photograph (AP & Lat. View) of a case 2.1 years post operative showing good incorporation of the femoral stem (arrows)

implant was intra-operative fracture due to excessive femoral reaming which was previously considered an important part of the operat ive procedure. Femoral fractures are a more important problem when the available implants and instru- mentation (which have been developed in the western world), are used in the Asian population, who have relatively

narrower femoral canals. We have thus modified the operative procedure and employ only No. 8 or No. lo reamers for the femur. We feel that excessive rea- ming of the inner cortices of the femur may cause micro fractures and unneces- sarily removes the endosteal bone of the femur. This space is to be fdled up using cancellous bone paste as described in the

original procedure. Minimal reaming (to the size of the stem) leaves intact the endosteal bone ; this further hastens the bony in-growth into the grooves of the prosthesis. Bone paste, however, is still liberally used while inserting the pros- thesis. We insert the bone paste like bone cement , and fill up the whole medu l l a ry canal p r io r to p ros thes i s insertion. Proximal oozing out of the paste is minimized by using a slotted plate a round the prosthesis over the proximal end of the medullary canal to ensure compression of the graft.

To allow bony incorporation of this prosthesis, tension and torsional stresses have to be neutralized. This is done with lag screws inserted in the proximal part of the femur and by our regimen of rigid post opera t ive immobi l i za t ion for 6 weeks, which we feel is critical for the incorporation of the stem and for good bony ingrowth. We have labelled this the "critical period". Inspite of the two lag screws, complete immobility at the graft site is not ensured if early weight bearing is allowed ; even micro motion will lead to fibrous tissue formation, and resorp- tion rather than graft incorporat ion. Other authors[5] using allograft and cement combinations for revision THA also advocate delayed weight bearing ; immobil izat ion in bed is not a major p r o b l e m in the younge r popu la t i on groups.

The use of the compound prosthesis (Polyacetyl resin with a metallic core) has been shown to transfer the strain to

Fig. 3a, b a Pilot film showing the 2 levels at which section were taken for coronal 3D and reconstruction, b Coronal reconstruction of the proximal and distal areas of the isoelastic stem (M = metallic core, P = Polyacetyl resin and N = new bone formation). There is good bone formation all around the prosthe- sis which appears as a continuous hyperdense layer over the polyacetyl resin. The lucent line between the polyacetyl resin and new bone formation is arti- factual

O.N. Nagi, et al.: Preliminary experience with isoelastic total hip arthroplasty using impaction bone grafting 31

Fig. 4a, b a 3 D CT reconstruction at distal part of femoral stem in the early stage (M = Metal, P -- Polyacetyl resin, N=new bone). Bone formation occurs in the interstices of the stellate polyacetyl resin and circumferentiaUy, b 3D reconstruction at proximal end of femoral stem after 2.1 years sho- wing good new bone formation

the p r o x i m a l f e m u r in c adave r s [9]. Wi th to ta l ly metal l ic THAs, whe the r c e m e n t e d or uncemen ted , m a x i m u m strains occur at the distal tip with signi- ficant decrease at the calcar level. With the c o m p o u n d p r o s t h e s i s the m a i n strain directions, both medially and late- rally, are also longi tudinal ly directed, but are max imum in the subtrochanteric region [9]. When a tension band device is added to the lateral cortex, in the form of lag screws, the load transfer occurs even more proximal . This reduces the problem of calcar resorption.

The ace tabula r c o m p o n e n t in our series was inserted at five degrees more

horizontal inclination than advocated in c o n v e n t i o n a l THA. Th i s m a y l i m i t abduct ion at the hip by a few degrees, but the longitudinal forces t ransmit ted by weight bearing appear to aid signifi- cantly in the early incorporat ion of the cup. The design of the cup is such that the inclination of the two pegs of the cup is slightly different f rom the two holes drilled in the acetabulum, thus ensuring a press fit. However the insertion of per- iphera l screws is m a n d a t o r y and we have found that three screws provide sufficient stability.

In an at tempt to demonstrate suffi- cient fdling up of the medullary canal by

the bone graft paste, as well as to try and see if new bone could be visualised, 3-D CT scans were done in a selected group of patients who could afford it and were willing to participate in the study. Due to logistic const ra in ts , serial sect ions were done in 4 cases only ; bone was definitely observed all a round the isoe- las t ic s t ems , a n d we cou ld say wi th confidence that there were no residual "empty spaces". The bone was seen as a hyper dense area all around the stems ; however, it would be ideal to do a radio isotope tracer study to better define this "new bone". Unfortunately this investi- gat ion is p resen t ly unavai lab le to us.

32 O.N. Nagi, et al.: Preliminary experience with isoelastic total hip arthroplasty using impaction bone grafting

Table i. Review of literature

Authors No. Av. age Mortality Follow up Femoral Screw Dislocation Infection Loosening/ Others (year) of cases (years) (Nos.) (years) Fracture Fractures (Deep) subsidence

Bombelli & 214 68.5 6 4.9 7 (Post 4 2 Nil 3 DVT Mathys, 1982) approach)

Morscher & 250 Acetabular 5 5.5 5 o Dick, 1983 cups, 40 stems

(221 cases had good or excellent results)

Bertin et 0.1., 788 Acetabular 2 Nil 1985 cups

Andrew et al., 400 26 9 shaft, femoral (3) m 6 2 Nerve 1986 19 trochnter, 8 femoral palsy

83 calcar 2 Acetabulum

Horne et al., 88 1987

(Plus 28 Hemirathroplastics)

Jakim et al., 34 1988

Rosso, 93 1988

Myseru et al., 24 1988

Decker, 71 revision 1987

Gerundini et al., 43 revision 1987

Trager et al., 6o 1985

Matricali et al., 19 revision 1993

Miinimaki et al., 114 1994

59.6 Minimum 5 fissure, 1 3 Femoral 6 months 5 fractures Subsidence

53

3.5

Minimum 5 years

results

Maximum 1 None 5 years

3 years 1 1 acetabular lO months loosening

3-4 years 5

3 years lO months

8.z years 2t loose stems

n revised 69% poor

59% good results

2 ankylosis 28 good results

5o% good results

5o% good results

i revision

11 revision

Nevertheless, it could be def ini te ly shown that the prosthesis was well enca- sed in bone ; as we have used our allo- graft in the form of a paste, it seems logi- cal to assume that at the end of 2 years, if it is not r e so rbed and rep laced by fibrous tissue, then it is either incorpora- ted or new bone has formed. Either way the purpose of the impaction bone graf- ting has been served adequately.

Conclusion

In the present series, we found that short term results were comparable to cemen- ted arthroplasties done at our institute. Home et al [7] noted minor gait distur-

bances in some cases undergoing Isoe- lastic THA which they felt may be due to the high neck shaft angle (144") of the femora l c o m p o n e n t ; we have not encountered this problem in any of our cases. The previously recommended aggressive approach to medullary rea- ming was not employed in our cases. In our series, there was adequate radiogra- phic evidence of bone covering the pros~ thesis even distally, where the prosthesis was not fitting snugly. Thin (z mm) axial CT sections of the prosthesis stem sho- wed good bone coveting of the prosthe- sis. Previous reports in the literature[I, 3, 13] have been able to his tological ly demonstrate the excellent incorporation

of the prosthesis in the host, with new bone growing into its indentations. We are thus of the opinion that this prosthe- sis, with certain modifications in the ori- ginal technique of implantation, could become well incorporated biologically, and should be considered as an option in young adults.

References

Andrew TA, Flanagan ]P, Gerundini H, Bom- belli R (1986) The Isoelastic, noncemented total hip arthroplasty. Preliminary experience with 4oo cases. Clin Or thop 206:127-138

O.N. Nagi, et al.: Preliminary experience with isoelastic total hip arthroplasty using impaction bone grafting 33

2. Bombelli B, Mathys R (1989) Cementless RM Isoelastic total hip prosthesis J R Soc Med 75: 588-597

3. Decker S (1987) The special problem of the replacement operation in cement -free pros- thesis. Langenbecks Arch Chit 372:477-480

4. Gerundini M, Fusco U, Avai A, Maistretl G (1987) Cementless RM Isoelastic total hip prosthesis in revision surgery for loose pros- thesis.J Orthop Traumatol 13(2): 159-165

5. Gie GA, Linder L, Ling RSM, Simon JP, Sloof TJJH, Timperley AJ (1993) Impacted cancel- lous allografts and cement for revision total hip arthroplasty. J Bone Joint Surg 75-B (I): 14- 21

6. Heitmyer U, Hierbolzer G, Haines J (1987) The importance of Trochanteric lag screw to achieve primary stability in cementless ftxa- tion of the RM hip prothesis. Arch Orthop Trauma Surg lo6:12o-122

7. Horne G, Berry N, Collis D (1987) Isoelastic uncemented hip arthroplasty - early experien- ce. Aust NZ J Surg 57:461-466

8. Jakim I, Barlin C., Sweet MB, RM (1988) Isoe- lastic total hip arthroplalsty. A review of 34 cases. J Arthroplasty 3(3): 191-199

9. Mathys R, Jr Claes L, Mathys R Sr (1985) Strain distribution in the proximal femur after cementless implantation of hip replacments.

In SM Perren & E Schneider (eds). Biomecha- nics Current inter disciplinary Research. Mar- tinus Nijhoff, Publishers,pp 371-376

lO. Matricali GA, Thibaut H, Hendrickx M, Thia- baut R (1993) Revision of total hip arthroplas- ty using the RM isoelastic prosthesis. Acta Orthop, Belg 59 (Suppl 1): 374-376

u. Minimaki T, Puraneun J, Jalovaara P (1994) Total hip a r th rop las ty using isoelastic femoral stems. A seven to nine year follow up in lo8 patients, l Bone Joint Surg 76(3): 413-418

12. Morscher EW, Bombelli R, Schenk R, Mathys R (1981) The treatment of femoral neck frac- tures with an Isoelastic endoprosthesis implant without bone cement. Arch Orthop Trauma Surg 98:93

13. Morscher EW, Dick W (1983) Cementless fixa- tion of isoelastic hip endo-prosthesis manu- factured from plastic materials. Clin Orthop 176:77-87

14. Morscher EW, Mathys R (1985) Operative technique of insertion of cementless RM poly- ethylene hip socket. J Orthopaedic Surgical Techniques 1(2)

15. Museru LM, Tay BK, Balachandran N (1988) Isoelastic cemenfless total hip replacement. Preliminary results of 24 replacements. Singa- pore. Med J 29(4): 361-366

16. Nagi ON (199o) Cemented low friction total hip arthroplasty: present status. Bull PGI 24(4): 186-189

17. Nagi ON, Dhillon MS, Nagi B (1989) Roentge- nographic assessment of lOO consecutive cases of total hip replacement using the Charnley's prosthesis. Ind J Rad & Imag 43(2): 269-277

18. Nagi ON, Dhillon MS (1988) Charnley's total hip arthroplasty. (5 to 7.9 year follow up stu- dy), Delhi. Orth Journal 4(2): 25-34

19. Rosso R (1988) Five year review of the isoelas- tic RM total hip endoprosthesis. Arch Orthop Trauma Surg lo7(2): 86-88

2o. Trager D (1989) Results of 5-7 years follow up of implantation of RM isoelastic hip endo- prosthesis. Unfallchirurg 92(6): 3Ol-3O4

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22. Wilier HG, Ludwing J, Semlitsch M (1974) Reaction of bone to methacrylate after hip arthroplasty. J Bone Joint Surg 56-A: 1368- 1382

Received June I0, 1999 / Accepted in final form November 15, 1999

Etude pMliminaire d'arthroplastie totale de hanche avec une proth~se iso~lastique

Une s&ie de 51 arthroplasties totales de hanche isodastiques avec cotyle non ciment~, utilisant une cupule fix& par t&ons et vis et avec une tige f6morale m&aUique entour& de r6sine en polyac&yle, est &udi& avec un recul de 4 ans et lO mois. Tousles patients ont 6t6 immobilis~s en abduction pendant 6 semaines, puis interdits d'appui complet pendant encore six semaines. I1 y eut un &hec par sepsis profond et un autre par positionnement extra-m6dullaire de la tige f~morale. Tousles autres op6r6s eurent une amdioration des douleurs et de la fonction ; seule- ment 2 patients ont ~t~ insatisfaits. L'al~sage du canal m~dullaire f~moral fut r~alis~ de faqon &onome afin de conserver du tissu osseux spongieux apte ~t se r~g~n~rer le long de la tige f~mo- rale. Cette faqon de faire fut particuli~rement utile chez des adultes jeunes, chez lesquels le cimentage n'est habituellement pas la r~gle. Nous avons estim~ que les bons r~sultats pr&oces ~taient li& ~ l'immobilisation rigide post-op~ratoire, p&iode que nous avons r~pertori& com- me <~ p~riode critique >>, au remplissage de la cavit~ mCdullaire avec des greffons osseux avant l'introduction de la fige et en laissant en place le produit d'al~sage, ce qui accd~re certainement l'incorporation osseuse de la tige. Une ~tude scanographique de la repousse osseuse autour de la tige f~morale n'a pas pu &re exploit~e et fera l'objet de discussion ult~rieure.

Mots-d~s : Proth~se totale de hanche - - Greffe osseuse - - Allogreffe - - Isodastique