Following on from my first article on the rational behind the Proxima Stem I now would like to talk about the results that have been obtained in the last 11 years in which I have been using this stem. This is in order to reply to questions that have already been asked and to the natural doubts that surge to mind when one is confronted with a new technology. What I have included below is the contents of a paper that has also been published on Hip International.
Our experience with this type of system began in 1995 and we have therefore a maximum follow-up of 13,8 years. In the first years, 1995 to 2004, Proxima, have been customized for every single patient with CAD-CAM technology. Clinical and radiological results have been so satisfactory that, the custom made stem, has evolved into the Proxima standard (a commercial version developed with DePuy). In the present study, we illustrate the clinical results obtained from the Author on a series 429 Proxima implants applied in 361 patients of variable age between the 30 and 91 years. The results obtained are extremely encouraging and, at the present time, we have had no cases of stem revision and no cases of thigh pain. The radiographical appearance has demonstrated constant absence of stress shielding and the appearance, already a year after implantation, of a progressive reinforce of the trabecular bone around the implant.
Materials & methods
The cases reported in this work relate to the patients that received both a Proxima Commercial stem and a Proxima custom made stem. This study describes the clinical results of 429 Proxima implants executed in 361 patients aged between 30 and 91 years with average age 66.4. In 68 patients, 20 for the period Proxima custom (1995-2004), and 48 during the period Proxima Commercial (2004-2007), was executed a bilateral hip implant.
The average height of the patients was 167 cm (154-195 cm). The average weight 76 KG (48-136 kg). The diagnosis pre-op has been primary arthrosis in 278 patients, aseptic necrosis of the head of the femur in 46, secondary arthrosis asw a result of past operations in 32 patients and finally in 5 patients neck fractures of the proximal femur.
In 111 patients, for a total of 131 hips were implanted using the Proxima Custom-Made Stem (1995-2004). The remaining 250 patients were implanted with 298 Proxima standard stems (2004-2007). In 26% cases, an anterolateral (Watson-Jones) access was used, in 15% a lateral access (Hardinge) and finally in 59% a postero-lateral access. 71% Of patients were operated with a mini invasive technique.
Proxima is made from a titanium alloy (Ti-6al-V), and is available both with a standard and lateralized neck. Its main features are, the absence of the stem, the presence of a lateral support that allows you to distribute the load, also, on the lateral side of the femur and the preservation of the neck with a high neck cut. The implant has a surface coating of type ZTT (Staircase pattern) to reduce the effects of shear forces and is covered with a thin layer of hidrossapite.
Fig. 1. Proxima custom at 13 years of follow-up. Note the presence of the short stem this in the first series of Proxima custom (1995-1998) and the modest depletion of proxima femoral bone quality (stress shielding).
Fig. 2. Proxima custom in 8 years of follow-up. This implant belongs to the series of II type of the period 1999-2004. Excellent radiographic bone quality and peri-prosthetic remodeling.
In all cases full weight bearing was granted immediately with the help of two walking sticks and one with one stick after 2 weeks. The clinical evaluation has been obtained with the Harris hip score and the radiological results with continual check ups.
The average follow-up in this series of 429 patients was 5.8 years (minimum 6 months maximum of 13.8 years). The evaluation with Harris hip score gave an average score of 96/100, self-evaluation card with an womac average score of 92/100. We have had cases of infection in this group of patients.
The incidence of complications was low. The initial series of 131 Proxima custom implants had 7 femoral “cracks” intra-op all resolved with the application of wire straps. In none of these cases were we forced to change the prosthesis intra-op.
The high incidence of femoral cracks is explained by the availability a single femoral broach for the implantation of the custom made Proxima implant. Therefore the gradual broaching of the femoral canal, performed in a normal operation was not possible in this case.
In the series of 298 commercial Proxima implants we have had no case of femoral crack during surgery. Other complications there were 4 late dislocations, 5 skin conditions, 2 sciatic nerve complications that spontaneously corrected themselves.
In 29 cases we had heterotrophic ossifications, only 12 cases of grade III and IV (Brooker). In 3 cases we had to replace the insert of polyethylene for peri-prosthetic osteolisi caused by wear. In these 3 cases the implant appeared stable during the operation and it was not considered necessary to proceed with its replacement. In no case was the presence of medial Thigh pain detected.
In about 5% of cases we have had intermittent pain resolved spontaneously after the first 6 months.
The alignment of the stem was neutral in 89% of cases, varus in 7.5% and valgus in 3.5%. In no case was varus placement more than 5°. The bone resorption of 5mm in the area 7 of Gruen was observed in 6 cases of first model of Proxima custom (1995-1998) was always associated with an oversizing of the distal portion of the stem.
There has been no case of migration distal or secondary varus positioning of the stem. In no case, it was observed the appearance of Peri prosthetic radiolucent lines. In all cases with more than a year follow-up (94% of cases reinforcement of peri-prostethic bone (buttressing) was noted.
A radiographic landmark common to all the implants was the presence of “spot welds” in the areas 2 and 6 of Gruen. In 42% of the implants the appearance of “spot welds” in both sides were noticed. In other cases the thickening of the peri-prosthetic cancellous bone has been seen only in the region of lateral flare. Moderate hypertrophy of the distal cortical was detected in only two cases with oversizing of the implant.
The dismetria of the lower limbs was, on average, of 0.9 cm before and 0.2 cm in the post-operative.
Our experience with Proxima to began in June 1995. This prosthesis has therefore now a maximum follow-up of 13 years. Until 2004 the implant was implanted with a cutom made technique and then, from February 2005, the same prosthesis became a standard prosthesis.
The stem Proxima presents itself as an extremely conservative implant and allows the saving of all the femoral neck. The reasons for its success lie in the respect of biomechanics of the proximal femur and confirm the validity of work of Fetto in revising the mechanism of load transmission in this region. In our experience we believe that the role of lateral support on the proximal femur obtained with the addition of lateral flare has been particularly important.
The possibility to fill the metafisis area with a prosthesis that had 360 degrees of contact (circumferential contact) guarantees the maximum torsional stability as assessed by Westphal et al. (11). These data are confirmed by both the long term radiographic results and, above all, from the DEXA studies which demonstrated, unequivocally, the best bone remodelling and the lowest stress shielding of this stem with respect to all the other implants tested.
In particular, the most recent study of Albanian et al. (12-13) has shown that the best quality peri-prosthetic proximal bone is obtained with the elimination of all of the implant stem. In fact, the comparison between the implants Proxima custom Type I (1995-1998), with a short stem, and that of Type II (1999-2003), stemless, showed that even a short stem will cause much worse bone remodelling and the risk of stress proximal shielding.
Fig. 3. Proxima custom at 4.5 years of follow-up. It is possible to observe how, in the last period, the Proxima custom stem is reduced in size while maintaining the biomechanical fundementals. Spot welds are visible in the region of lateral flare.
In elderly patients or those suffering from osteoporosis, in case you opt for a not cemented implant, it is however necessary, for a good primary fixation to use implants of a larger size than normal i.e. to fill the femor as much as possible. This is because the cancellous bone is less resistant to the stresses especially torsional forces. Of course, even for the Proxima stem, as with any conventional prostheses, in the presence of osteoporosis, it is necessary that the implant is of a large dimension in order to obtain a primary stability that will provide a good result.
In our opinion, the use of Proxima in the osteoporotic patient, i.e. a stemless implant, has clear advantages over conventional implants which in these patients, have well-known drawbacks. For example:
1. The rigidity of a component that provokes femoral atrophy of the calcar and thinning of cortical, the so-called stress shielding. 2. The appearance of Thigh-Pain that, especially in the elderly, is related to the increased the size of the stem.
3. The risk of a point contact created from tip of the stem also related to thigh pain.
In our experience in patients older than 70 years, even with the presence of mild osteoporosis, we have achieved always satisfactory results upon using this type of implant. The incidence of proximal femoral (7/429) is similar, if not lower, than that observed with conventional prostheses.
In the continuous radiographical check ups, the improvement of the quality of proximal femoral bone has been good. The maintenance of the bone stock over time, without the phenomena of stress shielding, is a confirmation of anatomic transmission of the loads and is particularly important in the elderly where sometimes this (stress shielding) may cause significant problems.
In patients with aseptic necrosis of the femoral head, the use of Proxima appears particularly indicated given their young age, and the frequent co-existence of a relative osteoporosis due to the prolonged use of cortisone (14).
The X-ray data, associated to the absence of pain of thigh, confirms the hypothesis of Fetto, Walker and Whiteside (2, 5, 8) that a prosthesis with circumferential support and preservation of the femoral neck allows an immediate and excellent stability that allows a quick Osteo-integration even when it is given immediate weight bearing.
The clinical results obtained and presented in this study confirm that Proxima with its lateral support and circumferential fixation, gives satisfactory results not only in young subjects with good bone stocks but also in elderly subjects, and that it can be used with results as good as if not better those of conventional implants (15).
The initial clinical results after the launch of the implant further confirm this fact. In fact of almost 5000 Proxima implants used since it’s launched only 27 have been revised. The large majority of those 27 revisions occurred during the launch of the product which allows us to attribute, to a certain degree, the obvious learning cure as the reason for failure.
The geometry of this implant has been shown to ensure a good initial stability, obtaining the transmission of the load in physiological manner within the proximal femur. The long-term results obtained with the custom made Proxima and those in the medium term obtained with the commercial Proxima, also in subjects over seventy years, show that, with this stem created according to the modern conceptions of biomechanics, is possible to obtain excellent results.
The prosthesis Proxima is therefore not a niche prosthesis, as are resurfacing implants as well as almost all of the prosthesis that conserve the femoral neck. Instead Proxima has much wider indications and it may be used with the same indications as a conventional non-cemented prosthesis.
Fig. 4. Bilateral case. The comparison with a conventional prosthesis exalts the mini-invasiveness of Proxima.
Fig. 5. Follow-up X-ray of Proxima commercial prostheses at 4 years. Please note the identical form in figure 3 that shows a custom made Proxima of the last 5 years of the period custom made (1999-2004). Spot welds are visible in regions 2 and 6 of Gruen.
1 Fetto JF, Bettinger P, Austin KS. Re-examination of hip biomechanics during unilateral stance. Am J Orthopedics 1995;8:605-12. 2 Fetto JF, Austin KS. A missing link in the evolution of THR: “discovery” of the lateral femur. Orthopedics 1994;17:347-51. 3 Koch J. The laws of bone architecture. Am J Anatomy 1917;21:177-201. 4 Gallinaro P, Massè G, Elloy M, et al. Variable geometry for proximal femoral fixation. Coombs editor 113 Joint replacement state of the art; 1994, p.116. 5 Walker P. The effect of the lateral flare feature on uncemented hip stems.Hip International 1999;9:71-80. 6 Jasty M, Krushell R, Zalenski E, et al. The contribution of the nonporous distal stem to the stability of proximally porous-coated canine femoral components. J Arthroplasty1993;8:33-41. 7 Kim YH, Kim JS, Cho SH. Primary total hip arthroplasty with a cementless porous-coated anatomic total hip prosthesis: 10- to 12-year results of prospective and consecutive series. J Arthroplasty 1999;14:538-48. 8 Whiteside LA, White SE, McCarthy DS. Effect of neck resection on torsional stability of cementless total hip replacement. Am J Orthopedics 1995;24:766-70. 9 Pipino F., Calderale C. Biodynamic total hip prosthesis. Ital J Orthop Traumatol 1987;13:289-97. 10 Santori FS, Manili M, Fredella N, et al. Ultra short stems with proximal load transfer: Clinical and radiographic results at five year follow-up. Hip Int 2006;16(Suppl. 3):S31-39. 11 Westphal FM, Bishop N, Püschel K, et al. Biomechanics of a new shortstemmed uncemented hip prothesis: An in-vitrio study in human bone. Hip Int 2006;16(Suppl. 3):S22-30, 12 Albanese CV, Rendine M, De Palma F, et al. Bone remodeling in THA: a comparative DXA scan study conventional implants and a new stemless femoral component. A preliminary report. Hip International 2006;16 (Suppl. 3): S9-S1. 13 Santori N, Albanese C, Learmonth, ID, et al. Bone Preservation with a conservative metaphyseal loading implant. Hip International 2006;16:3;16-21. 14 Learmonth ID. The place of conservative femoral prostheses. Simposium surgical treatment of a-vascular necrosis of the hip 8° EFFORT congress 2007; 9.2.
15 Santori FS. A stemless neck preserving implant: the Proxima. Hip Simposium Why a short stem 8° EFFORT congress 2007; 7.3.