The Proxima—A Metaphyseal, Stemless, Proximally Loading Implant
Introduction
Excellent long-term results have been achieved with both conventional cemented and cementless implants. However, the patient profile has changed in the twenty first century, and surgical decision making is now often guided by quality of life issues. Thus ever younger cohorts of patients are presenting for total hip replacement. The potential increased risk of revision surgery focuses the need for bone conservation at total hip replacement.
Any implant that makes contact with the diaphyseal cortex or osseointegrates distally will offload distally—with consequent proximal stress protection bone resorption. Bone conservation is achieved both by taking less bone at the time of surgery, and by preserving bone in the longer term with more physiological loading.
The Proxima is a metaphyseal stemless anatomic implant which is ZTT coated except for the polished bullet shaped tip. It has a pronounced lateral flare. Fetto1 noted that the ilio-tibial band acts as a tension band laterally, which accounts for the fairly symmetrical bone morphology in the subtrochanteric region of the femur.
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The proximal femur is characterized by a pronounced medial and lateral flare. Leali and Fetto2 have shown that a proximally loading implant with a pronounced lateral flare preserves bone and increases periprosthetic bone mass. Walker et al3 also observed that changes in the pattern of proximal loading stimulated the formation of new trabeculae which stream up to buttress the lateral flare (Fig. 18.1).
Figure 18.1: Postoperative radiograph at one year showing trabecular buttressing of both the medial and lateral flare
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Total Hip Arthroplasty
Santori et al3 reported on periprosthetic bone densitometry studies comparing a customized short stemmed implant with a customized stemless implant. The proximal femoral BMD of the stemless implant was superior to that of the short stemmed implant and was indeed almost similar to the nonoperated contralateral side. Albanese et al4 compared the periprosthetic bone mineral density of five conventional/conservative implants with that of a customized stemless implant. The short implant was shown to produce an “homogeneous and more physiological redistribution of bone density, allowing maintenance of periprosthetic bone stock”.
In an editorial in 2005, Berry noted “It remains difficult to escape the common sense logic that less invasive operative methods can provide benefits to patients...” 5 We believe that this philosophy applies not only to soft tissue sparing surgery, but also to bone preservation at total hip arthroplasty.
Surgical Technique
The surgical technique for the insertion of the Proxima femoral stem is different but not difficult. The pronounced lateral flare precludes “in line” insertion of the prosthesis, as this would damage the soft tissues and the greater trochanter. A “round the corner” technique has been developed which protects the soft tissues and the greater trochanter and optimizes loading of the lateral flare.
A high, almost horizontal neck resection preserves as much of the medial neck as possible to provide support to the short stem (Fig. 18.2). A slightly curved canal finder awl is inserted into the posterolateral quadrant of the resected neck. The top of the awl is initially directed laterally, until the lateral flare is identified. It is then turned through 180° and advanced down the femoral canal. Moving the canal finder mediolaterally compresses the proximal metaphyseal cancellous bone. The box osteotome should only be used to remove very hard cancellous bone encountered laterally—but care should be taken never to invade the greater trochanter or the glutei.
Broaching is directed at creating an anatomic cavity of the required size and shape to provide immediate total press-fit stability to the selected implant. There are four starter broaches—three non-anatomic and one anatomic—that are used prior to the nine size-matched definitive broaches. All anatomic broaches are sided (left or right).
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Broaching is sequential. The broach is inserted into the cavity prepared by the canal finder at an angle of approximately 20°. Once the prominent lateral edge of the broach is below the resection level at the greater trochanter, it is progressively aligned to the axis of the femur. This is achieved by applying a gentle lateral pressure to the broach handle, and hammering on the oblique portion of the strike platform (Figs 18.3 and 18.4).
Figure 18.2: Diagram showing high, almost horizontal neck cut used for proxima compared with conventional neck cut. Inset shows resected head
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The Proxima—A Metaphyseal, Stemless, Proximally Loading Implant
Figure 18.3: Illustration showing the insertion of the initiator broach at 20° to the long axis of the femur. The broach handle is rotated laterally once the prominent lateral corner has cleared the base of the trochanter, and is then impacted coaxially
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Figure 18.4: Illustration showing progressive, round-the-corner seating of the broach
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Total Hip Arthroplasty
Figure 18.5: The alignment guide is used to assess alignment in both planes. Accurate alignment is achieved when the axis of the rod is parallel to the femoral axis in two perpendicular planes
Figure 18.6: Pictorial representation and intraoperative photograph showing use of alignment rod in vertical position and pointing at the medial condyle
The alignment guide is used to ensure that correct positioning of the implant has been achieved in both the sagittal and coronal planes (Fig. 18.5).
Overcorrecting the smaller broaches into a valgus position assists with avoiding varus and achieving neutral alignment with the final implant (Figs 18.6 and 18.7). Alignment can be checked both with the rod vertical and when centered over the tip of the greater trochanter.
The broach should be allowed to follow the natural geometry of the proximal femur in both the anteroposterior and mediolateral planes. Sequential anatomic broaching is continued until absolute torsional stability can be demonstrated by torque on the broach handle (Fig. 18.8).
Removal of the broaches is achieved with the same “round the corner” movement. If sclerotic bone is encountered anywhere within the broach envelope, it may have to be removed with a rasp. Excess bone in the region of the resected neck can be removed with a calcar reamer.
The definitive implant is inserted in a similar manner, taking care that the prominent lateral corner has cleared the greater trochanter before rotating it into a more neutral position.
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The Proxima—A Metaphyseal, Stemless, Proximally Loading Implant
Figure 18.7: Pictorial representation and intraoperative photograph showing use of alignment rod when centered over the tip of the greater trochanter. It then lies in the long axis of the femur and points at the center of the knee
Figure 18.8: The final broach must exhibit absolute stability when challenged with robust torsion of the broach handle
It should be possible to insert the prosthesis into the broach envelope to within approximately one centimeter of full seating without undue force. The implant is then further rotated and seated with gentle hammering. The alignment is controlled with the guide. When it has been confirmed that the stem is in neutral alignment, final seating is achieved by impaction in line with the femur.
Postoperative Rehabilitation
As noted it is essential that absolute rotational stability of the prosthesis is obtained at the time of implantation. Following surgery the patient is mobilized on two crutches as soon as possible. If the postoperative X-rays are satisfactory, there is no restriction on weightbearing, and rehabilitation progresses through two sticks/one stick to walking without support as rapidly as comfortably tolerated by the patient.
If there is any cause for concern on the postoperative X-ray-such as incomplete seating of the prosthesis, radiological under sizing of the implant, or varus mal alignment of the implant—the patient would be kept protected weightbearing on two crutches for six to eight weeks.
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Total Hip Arthroplasty
Figure 18.9: The Santori customized (left hip) appears virtually identical to the proxima implant (right hip). The customized stem provides a longer term clinical heritage for the proxima
Results
Renkawitz et al6 reviewed the design rationale of the Proxima and considered the study design of the prospective international clinical trial. It was perhaps unfortunate that data collection for the trial commenced at a stage when the smallest available prosthesis was the equivalent of a contemporary size 4, and the essential sequence of starter broaches were not available. These limitations—which were subsequently addressed—undoubtedly contributed to some of the early failures encountered with this prosthesis.
Ghera and Pavan7 reviewed 65 consecutive THAs using the Proxima stem followed-up for a mean of 1.7 years. There was one intra-operative fracture. No prosthesis was revised, and the authors report excellent clinical results.
Tóth et al8 reviewed 41 consecutive THAs with Proxima stems followed-up for a mean of 26 months. They reported excellent clinical results with no revisions, no migration and no evidence of radiological loosening.
In the mid 1990s, Santori et al developed an ultrashort proximally loading custom-made component with a prominent lateral flare. Santori and Santori9 have recently published the midterm results of this prosthesis. They reviewed 129 patients with an average age of 51 years (21-71 years) and a mean follow-up of 8 years (4.9-14.1 years). No femoral component had been revised, and there were no radiological features of concern.
Figure 18.9 shows the Santori customized stem to be virtually identical to the Proxima stem. It is therefore proposed that the Santori customized provides a clinical heritage to predict the longer term behavior of the Proxima stem.
Complications
The wedge shape of the Proxima may occasionally be associated with a vertical crack fracture in the proximal femur—as with any other wedge shaped uncemented femoral prosthesis.
These are readily stabilized with cerclage fixation, and there has been no reported case culminating in implant failure.
The absence of a stem may result in the Proxima drifting into slight varus. This is almost always compatible with a well fixed implant and a good clinical result.
Lack of initial press-fit stability with consequent absence of osseointegration culminating in loosening of the implant is the commonest cause of failure of the Proxima (Figs 18.10A and B) necessitating revision (Fig. 18.10C).
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The Proxima—A Metaphyseal, Stemless, Proximally Loading Implant
Figure 18.10A: Bilateral proxima THAs: the right stem is in slight varus, the left is an early postoperative view
Figure 18.10B: Marked subsidence of the left stem at 3 months
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Figure 18.10C: Revised to a primary S-ROM stem (other primary stems can be used): radiological appearances reminiscent of a primary THA
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It is important to achieve immediate press-fit stability with appropriate sequential broaching, followed by ‘in-line’ impaction of the final broaches, and of the definitive implant. It is also necessary to robustly test the rotational stability of the final broach.
Total Hip Arthroplasty
All reported failures have occurred early (within the first 3 to 6 months). While longterm results are required to determine the durability of the implant, it is believed that the Santori customized results9 suggest that once fixation is achieved it is durable. The results of Tóth et al8 and Ghera and Pavan7 indicate that secure early biological fixation can be achieved in virtually all cases, suggesting that uniformly good long-term results will be achieved.
More than 10,500 proxima stems have been inserted with a reported survivorship in the post-market surveillance of 99.58%. While the failure rate in this cohort is likely to be somewhat underreported, it is nevertheless interesting to note that 24 of these failures occurred in the first 1,000 implants when the prosthetic inventory and instrumentation were incomplete. Only 20 failures have been reported in the subsequent 9,000 implantations.
Discussion
A more conservative approach dominates surgical practice today: it seems logical that this ethos should extend to arthroplasty surgery. Conservative stems can provide one extra operative intervention in the surgical management of patients with osteoarthritis.
Femoral neck and metaphyseal implants not only conserve bone at surgery, but also preserve bone in the proximal femur with more physiological loading. Implants with diaphyseal stems will, to a greater or lesser extent, offload distally with proximal stress protection osteopenia (Fig. 18.11). Therefore when short stems which invade the proximal diaphysis fail, the associated proximal loss of bone will usually necessitate the use of the isthmus for fixation at revision.
Is the proxima effective? Yes—in the short to medium term, but longer term follow-up is required to establish durable survivorship.
Can a conservative implant ‘buy time’? Yes particularly in the younger patient. Both femoral neck and metaphyseal implants—such as the proxima—can be revised to a primary total hip replacement (Fig. 18.10C). However, femoral neck prostheses—such as the BMHR and the Silent—are not suitable for the hip with significant morphological and biomechanical abnormalities—such as coxa breva, extensive avascular necrosis, etc. In contradistinction, the anatomic shape and the prominent lateral flare of the proxima enables it to accommodate most an anatomic abnormalities in the proximal femur (Figs 18.12A and B).
The proxima can also be used to facilitate potentially difficult revisions in high risk patients, as reported by Sangüesa Nebot et al10 (Figs 18.13A and B).
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Figure 18.11: Anteroposterior radiograph of the pelvis showing a proxima on the right and a short stemmed prosthesis on the left. Note the bone ingrowth in zone 5 and the loss of bone in zones 6 and 7 around the short stemmed implant
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Figure 18.12A: Anteroposterior radiograph of the femur showing a dysplastic left hip with a coxa breva and a markedly flattened femoral head. Note the coxa vara on the right
Figure 18.12B: Six months following successful reconstruction of the left hip with a proxima stem
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The Proxima—A Metaphyseal, Stemless, Proximally Loading Implant
Figure 18.13A: Fractured Furlong stem in an elderly lady with significant comorbidities
Figure 18.13B: A successful outcome at 8 months after a simple, quick revision with a proxima stem
Conclusion
In seeking conservative prosthetic options at total hip replacement it is important not to compromise on the long term results of conventional implants. Early results with the proxima are encouraging, and suggest it will meet this prerequisite.
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Total Hip Arthroplasty
Illustrative Case (Figs 18.14A and B)
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Figure 18.14A: Preoperative X-ray of a 34-year-old male with stage 4 avascular necrosis of the femoral head (AVN)
Figure 18.14B: Postoperative X-ray taken 4 years and 10 months postoperatively. Patient is asymptomatic and maintains a full profile of professional and recreational activities
References
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Fetto JF, Austin KS. A missing link in the evolution of THR: ”discovery” of the lateral femur. Orthopaedics 1994;17:347-51.
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Leali A, Fetto JF, Insler H, Elfenbein D. The effect of a lateral flare feature on implant stability. Int Orthop 2002;26:166-9.
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Santori N, Albanese CV, Learmonth ID, Santori FS. Bone preservation with a conservative metaphyseal loading implant. Hip Int 16:i(Suppl3) 2006:16-21.
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Albanese CV, Rendine M, de Palma F, Impagliazzo A, Falez F, Postacchini F, Villani C, Passariello R, Santori FS. Bone remodeling in THA: A comparative DXA scan study between conventional implants and a new stemless femoral implant. A preliminary report. Hip Int 2006;16(1)(Suppl 3):9-15.
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Berry DJ. ”Minimally invasive” Total Hip Arthroplasty. J Bone Joint Surg 2005;87:699-700.
The Proxima—A Metaphyseal, Stemless, Proximally Loading Implant
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Renkawitz T Santori FS, Grifka J, Valverdi C, Morlock MM, Learmonth ID. A new short uncemented proximally fixed anatomic implant with a proximal lateral flare: design rationale and study design of an international clinical trial. BMC Musculoskeletal Disorders 2008:4;9:147.
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Ghera S, Pavan. The De Puy Proxima Hip: a short stem for total hip arthroplasty. Early experience and technical considerations. Hip Int 2009;19(3):215-20.
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Toth K, Mecs L, Kellerman P. Early experience with the De Puy Proxima short stem in total hip arthroplasty. Acta Orthop Belg 2010;76(5):613-8.
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Santori FS, Santori N. Midterm results of a custom-made short proximal loading femoral component. J Bone Joint Surg(Br) 2010:92(9):1231-7.
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Sangüesa Nebot MJ, Soriano FC, Gabarda RF, Valverde Mordt C. Revision hip arthroplasty with a short femoral component for a fractured hydroxyapatite fully-coated femoral stem. J Arthroplasty 2010:7:25. 1168 e13-16.