Introduction                       

Total hip arthroplasty (THA) is one of the most successful and cost-effective procedures in all of medicine.1 With well-designed prostheses, the vast majority of patients enjoy reliable clinical outcomes and good implant survivorship.2 Nevertheless, advances in technology and improved understanding of the biomechanical nuances of THA have driven manufacturers and surgeons alike to continually strive for better results.3 One area of longstanding focus has been the design of the femoral component. Unresolved issues such as stress-shielding, proximal-distal morphology mismatch, and tissue-sparing surgery have given rise to a generation of conservative femoral prostheses.

Standard length cementless femoral components generally abide by one or a combination of three main philosophies for shape and fixation. Anatomic prostheses are designed to match or so-called “fit-and-fill” the proximal femur. Using a combination of reaming and broaching, the host bone is prepared to accept the stem for not only a tight fit for initial stability, but also maximal fill of the metaphyseal endosteal space. Cylindrical, extensively-coated stems rely not on proximal fixation for initial stability, but rather distal fixation via a “scratch fit” between the prosthesis and a slightly under-reamed femoral canal. Tapered stems rely on proximal fixation, but in comparison to anatomic prostheses, they are wedge-shaped and do not completely fill the metaphyseal space. After compaction broaching of the proximal cancellous bone, the tapered stem allows for self-seating for axial stability and achieves rotational control by virtue of its rectangular cross-sectional shape in an ellipsoid canal.4

Excluding true resurfacing implants such as the Birmingham Hip Resurfacing System (Smith and Nephew, London, UK),5 there are several different design philosophies for conservative femoral components.6,7 The Mayo Conservative Hip (Zimmer, Warsaw, IN) was developed by Morrey in the 1980s8-10 and has a trapezoidal coronal shape. It engages the lateral femoral cortex in order to resist varus-valgus stress and aims to achieve multiple-point contact within the irregularly shaped proximal femoral cavity.11 Short, femoral neck-sparing curved designs, such as the Collum Femoris Preserving prosthesis (CFP, Waldemar Link, Hamburg, Germany), aim for triplanar stability by blocking rotational and varus-valgus movements with an intact cortical cylinder of neck.12 Short, bulky designs that do not spare the neck, such as the Proxima prosthesis (Depuy, Warsaw, IN), engage the lateral trochanteric flare in order to utilize the lateral femoral column to transmit weight-bearing loads.13 Finally, short tapered stems such as the TaperLoc Microplasty (Fig. 19.1), described in detail below, rely on the same philosophy as the tapered standard-length stems.

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Figure 19.1: The TaperLoc Microplasty stem (Biomet, Warsaw, IN) is a collarless, titanium (Ti-6AL-4V) medial-lateral tapered wedge design with proximal, circumferential plasma spray porous coating. Sizes range in length from 95 mm to 130 mm, the neck-shaft angle is 138°, and offset options are standard and lateralized

 

With any innovative technology or novel design, the foremost essential question is “why?” The senior author has extensive experience with the Mallory-Head Porous prosthesis (Biomet), reporting a 98% survivorship rate at ten years with an endpoint of revision for any reason.14 There have been multiple other stems that have performed exceedingly well over many years.2 On initial glance, it would appear that the marginal benefit of a new design is limited, but a closer review reveals a handful of reasons to consider short tapered stems.7,15

First, the optimal length of a tapered femoral component is unknown. Since the goal of a tapered stem, short or standard length, is to preferentially load the proximal femur, the only purpose of a diaphyseal extension would be potentially to prevent varus malalignment. Multiple studies, however, have demonstrated that varus malalignment does not adversely affect outcome for tapered stems.16,17 Furthermore, elimination of the distal stem would, if anything, facilitate more physiologic loading of the proximal femur and reduce stress-shielding.8 Second, short stems may be advantageous in certain populations. Patients under 55 years of age represent 15% of THA and in this more active group with longer mean lifespans, implants fail at a significantly higher rate.18 Aseptic loosening occurs in between 20-70% of patients under 50 years old by 10-year follow-up.19-22 Less violation of femoral bone stock during primary arthroplasty allows for more favorable conditions for future revision. Third, short stems are more versatile in accommodating patients with proximal-distal morphology mismatches and variations in proximal anatomy such as champagne-flute type femurs, significantly bowed diaphyses, or post-traumatic or developmental femoral deformities. Fourth, the surgical technique for short tapered stems is very similar to traditional implants and likely would not incur the steep learning curve and associated complications as with true resurfacing.23-25 Recent concerns related to metal-on-metal articulations have tempered enthusiasm for hip resurfacing. In addition, the premise of bone conservation on the femoral side with resurfacing may be negated by the increased reaming of valuable acetabular bone necessary to accommodate a larger head.26 Fifth, although the distinction of “minimally-invasive surgery” generates controversy amongst surgeons,27 the public is conceptually in favor of procedures that disrupt less soft tissue and bone and that utilize smaller implants. Short-stem components have greater ease of insertion and are more compatible with smaller incisions and tissue-sparing surgery.

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Implant                         

Technique of Short Stem Microplasty

 

There are two cementless Microplasty® options manufactured by Biomet (Warsaw, IN): the Taperloc® (Fig. 19.1) and the Balance® stems. The latter system is based on an anatomic fit-and-fill proximal geometry philosophy. The former system, preferred by the senior author, is based on a collarless, medial-lateral tapered wedge design for self-seating and rotational stability. It is titanium (Ti-6AL-4V) with a proximal, circumferential plasma spray porous coating. In contrast to sintering or diffusion bonding methods, Biomet uses a proprietary technology to apply the porous coating such that the implant itself is not substantially heated, hence retaining high levels of fatigue strength. Stems come in eleven sizes, ranging from 95 mm to 130 mm, growing incrementally laterally while the medial curvature remains constant. The neck-shaft angle is 138° and there are two options of offset, standard and lateralized. Aside from an opening reamer, no reaming is required; it is a broach-only system.

 

Surgical                    Technique                    

The senior author utilizes a less-invasive modification of the direct lateral (LIDL) approach (Fig. 19.2). The patient is placed in the direct lateral decubitus position with the operative side up and the pelvis perpendicular to the table using a rigid peg-board positioner. For stability, long pegs are placed posteriorly at the level of the mid-back and at the mid-sacrum, and anteriorly at the sternum and the pubic symphysis. The contralateral hip is flexed approximately 15° and the proximal fibula is padded to prevent excessive pressure on the common peroneal nerve. In this position, a rough estimation of the leg-length equality is made by assessing the level of the patellae and heels, with the caveat that the operative side will be slightly shorter since it is slightly adducted in the lateral position. After prepping and draping, the tip of the greater trochanter is located by palpation and percutaneous assessment with a 24 gauge spinal needle.

 

 

A postero-proximal to antero-distal oblique incision centered over the tip of the greater trochanter is made approximately 10-12 cm in length for the average 70 kg male. Subcutaneous dissection to identify the underlying fascia is minimized to prevent creation of dead space. The fascia lata and the anterior fibers of gluteus maximus joining the fascia lata proximally

 

Figure 19.2: The preferred surgical exposure of the senior author for primary THA is a less invasive modification of the direct lateral approach. The anterior portion of the gluteus medius is elevated off the greater trochanter at a 45° antero-proximal angle, and in a continuous sleeve the anterior portion of the vastus lateralis is elevated off the vastus ridge and split distally between its middle and anterior one-thirds. (Reproduced with permission, ©Joint Implant Surgeons, Inc., New Albany, Ohio, USA)

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Figure 19.3: Upon exposure of the capsule, the assistant abducts, flexes and externally rotates the limb to dislocate the femoral head

 

are incised with electrocautery in line with the skin incision. The trochanteric bursa is swept posteriorly to help visualize the underlying gluteus medius and vastus lateralis. The anterior portion of the gluteus medius is elevated off the greater trochanter using electrocautery, then bluntly dissected in line with its fibers at a 45° antero-proximal angle at the junction of the middle and anterior one-thirds of the muscle. Dissection is limited to less than 3-4 cm cephalad to the tip of the greater trochanter to avoid injury to the superior gluteal nerve. In a continuous sleeve with the gluteus medius, the anterior portion of the vastus lateralis is elevated off the vastus ridge and split distally between its middle and anterior one-thirds. A blunt Homan is placed in the gluteus medius split to retract the posterior fibers and expose a layer of fat between the gluteal planes. The capsule is incised posterior to the gluteus minimus and along the superior aspect of the femoral neck. An assistant abducts, flexes and externally rotates the limb to dislocate the femoral head (Fig. 19.3).

The femoral neck resection level is templated preoperatively (Fig. 19.4) and aligned intraoperatively with the greater trochanter (Fig. 19.5). Following resection of the femoral neck, acetabular preparation is performed in standard fashion (Fig. 19.6) prior to femoral preparation. The senior author typically prefers to position the cup according to anatomic landmarks to recreate the patient’s natural acetabular orientation and verifies proper version and inclination by palpation and direct visualization (Figs 19.7 to 19.9). Upon seating of the acetabular component and placement of the polyethylene liner, attention is turned to preparation of the femur. The proximal femur is accessed using the offset chisel (Fig. 19.10) and the starting reamer (Fig. 19.11). Care is taken to align the offset chisel and subsequent instruments in line with the patient’s natural anteversion. Although varus alignment has not been shown to be associated with inferior results in tapered stems, adequate lateralization of the femoral canal with even slight valgus bias to avoid varus positioning is ensured by using the broach as a rasp. Starting with the smallest broach (Fig. 19.12), the proximal cancellous bone envelope is progressively enlarged until the broach engages the medial and lateral cortices and cannot be advanced deeper. If a fracture occurs during femoral preparation, it is stabilized using cerclage cables. Typically, broaches and components, one to two sizes larger than the standard length tapered designs are required. A power broach may be utilized to hasten the broaching process (Fig. 19.13). Once the largest possible sized broach is in place (Fig. 19.14) the neck cut is finished with a calcar planar inserted over the broach

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Technique of Short Stem Microplasty

 

Figure 19.4: Preoperative templating for component size and neck resection level is performed digitally on the less affected contralateral hip

 

 

 

Figure 19.5: The femoral neck resection level is based on preoperative templating and aligned intraoperatively with the greater trochanter

 

Figure 19.6: The acetabulum is prepared in standard fashion with sequential reamers

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Figure 19.7: The porous, plasma sprayed titanium shell is inserted into the prepared acetabulum

 

 

 

Figure 19.8: A highly crosslinked, vitamin E enhanced polyethylene liner is inserted into the metal acetabular shell

 

Figure 19.9: The cup is positioned according to anatomic landmarks to recreate the patient’s natural acetabular orientation. Proper version and inclination are verified by palpation and direct visualization

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Technique of Short Stem Microplasty

 

Figure 19.10: The proximal femur is accessed first using the offset chisel

 

 

 

Figure 19.11: A starting reamer is introduced into the femoral canal taking care to align with the patient’s natural anteversion

 

Figure 19.12: Sequential broaches from small to large are used to rasp the femoral canal

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Figure 19.13: A power broach may be utilized to hasten the broaching process

 

Figure 19.14: The largest possible sized broach is in place, after progressively enlarging and adequately lateralizing the proximal cancellous bone envelope, and engaging the medial and lateral cortices

 

(Fig. 19.15). A magnetic neck trunnion corresponding to the offset option selected via preoperative templating is then placed and a trial head with the shortest available neck length is used for initial reduction (Fig. 19.16). Leg length and stability are assessed using a well leg down technique. The well leg is used as a reference, and with medial malleoli aligned the relative difference between leg lengths is felt at the patellar tendon and correlated with preoperative templating and supine leg length difference (Fig. 19.17). During impaction of the actual implant (Fig. 19.18), additional effort is often necessary to fully seat it compared to the same sized broach (Figs 19.19 and 19.20). A trial head component is again placed and the construct reduced to test for leg length equality and stability (Fig. 19.21). A final selection of neck length is then determined (Figs 19.22 and 19.23), the modular head is seated and final reduction accomplished (Fig. 19.24).

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Technique of Short Stem Microplasty

 

Figure 19.15: A calcar planar is inserted over the final broach to finish the neck cut

 

 

 

Figure 19.16: A magnetic neck trunnion corresponding to the selected offset option is placed and a trial head with the shortest available neck length is used for initial reduction

 

Figure 19.17: Leg length and stability are assessed using a well leg down technique. The well leg is used as a reference, and with medial malleoli aligned the relative difference between leg lengths is felt at the patellar tendon and correlated with preoperative templating and supine leg length difference

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Figure 19.18: The TaperLoc Microplasty stem is ready for implantation

 

 

 

Figure 19.19: Additional effort is often necessary to fully seat the final component corresponding to the same sized final broach

 

Figure 19.20: If the final implant sits slightly proud, autograft cancellous bone may be applied at the shoulder of the implant

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Technique of Short Stem Microplasty

 

Figure 19.21: A trial head component is again placed and the construct reduced to test for leg length equality and stability

 

 

 

Figure 19.22: Final determination of neck length appropriate to equalize leg length and provide optimal stability is made based on trial reduction. A corresponding titanium alloy option taper adapter is selected and paired with a Biolox delta ceramic head

 

Figure 19.23: The assembled Biolox delta ceramic head with option taper adapter is ready for insertion

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Figure 19.24: The modular ceramic head is seated and final reduction accomplished

 

 

 

Figure 19.25: Meticulous wound closure is performed in layers, starting with reattachment of the gluteus medius–vastus lateralis sleeve to the proximal femur with nonabsorbable suture

 

Figure 19.26: The abductor complex has been reconstituted, and is tight, strong and stable

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Technique of Short Stem Microplasty

 

Closure of the wound is performed, starting with reattaching the gluteus medius–vastus lateralis sleeve to the proximal femur with nonabsorbable suture (Figs 19.25 and 19.26). The fascial and skin incisions are repaired meticulously in separate layers.

 

Postoperative Protocol

 

The senior author utilizes a multimodal regimen to manage postoperative pain and mobilizes patients with a rapid-recovery protocol28 as soon as possible. Standard hip precautions are maintained for 6-8 weeks although the risk of dislocation is significantly less than with a posterior approach. In patients with standard risk for venous thromboembolic disease, prophylactic aspirin (325 mg) twice daily is prescribed for 6-8 weeks.

 

Results

 

The senior author and his partner (Keith R Berend) performed 1589 THAs with the Biomet Taperloc Microplasty stem from May 2003 through July 2010 via the LIDL (51%), anterior supine intermuscular (44%) or standard direct lateral approach (5%). There have been 15 stems revised (0.9%) for periprosthetic fracture (8), infection (3), subsidence (2), patellar dislocation (1), and intraoperative fracture (1). Conservatively treated complications related to the stem were one periprosthetic fracture and one instance of initial subsidence. In a subset of patients with average follow-up of 25.8 months, the average Harris Hip Score (HHS) improvement was 33.4 (preoperative 49.9, most recent 83.3, 0-100 possible).

 

Conclusion

 

Short stem femoral components have many potential advantages over standard length prostheses. Conservation of existing bone stock, compatibility with soft-tissue sparing surgery, more physiologic loading of the proximal femur, and versatility with varying femoral anatomy make the Taperloc Microplasty stem an attractive implant. The senior author has reported promising short-term clinical results and a low rate of complications. As with any new technology or innovative device, long-term follow-up is necessary to fully evaluate for efficacy and safety. Nevertheless, the biomechanical principles governing the design rationale and implementation technique are well-guided and based on time-tested fundamentals of THA. The tapered wedge short stem represents the natural evolution of joint arthroplasty to a smaller, less-invasive, and more efficient implant.

 

Illustrative Case

 

A 64-year-old female patient presented with severe pain in her right hip, localized in the groin and increasing in severity over the past several months. Preoperative anteroposterior

(A) and lateral (B) radiographs of the right hip demonstrate severe narrowing of the joint space, sclerosis, and formation of osteophytes and cysts, indicative of osteoarthritis. The patient was treated with primary cementless total hip arthroplasty using a 10 × 105 mm TaperLoc Microplasty stem with a 36 mm biolox-delta ceramic head articulated with an ArComXL highly crosslinked polyethylene liner in a 54 mm Mallory-Head RingLoc Radial cup (C, D). Radiographs at 53 months postoperative (E, F) demonstrate well fixed components in satisfactory position and alignment. The patient has a Harris hip score of 96.5 (Figs 19.27A to F).

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Figures 19.27A to F

 

References

 

1. Chang R, Pellisier J, Hazen G. A cost-effectivness analysis of total hip arthroplasty for osteoarthritis of the hip. JAMA 1996;275:858-65.

2. Migliore A, Perrini M, Romanini E, Fella D, Cavallo A, Cerbo M, Jefferson T. Comparison of the performance of hip implants with data from different arthroplasty registers. J Bone Joint Surg Br 2009;91-B:1545-9.

3. Huiskies R, Verdonschot N. Failure scenarios and the innovation cycle. In: Callaghan JJ, ed. The Adult Hip 1:167-83.

4. Mai K, Verioti C, Casey K, Slesarenko Y, Romeo L, Colwell CJ. Cementless femoral fixation in total hip arthroplasty. Am J Orthop 2010;39:126-30.

5. Dabirrahmani D, Hogg M, Kohan L, Gillies M. Primary and long-term stability of a short-stem hip implant. Proc IMechE 2009;224:1109-19.

6. Learmonth I. Conservative stems in total hip replacement. Hip Int 2009;19:195-200.

7. Lombardi AV Jr, Berend KR, Adams JB. A short stem solution: through small portals. Orthopedics 2009;32:1-4.