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    The Role of Extensively Porous-Coated Cylindrical Stems

    The authors discuss preoperative planning for a femoral revision, review their surgical technique, and present a case in which they used an extensively porous-coated cylindrical stem to manage significant proximal femoral bone loss.

    Authors

    Nirav Shah, MD, and Nitin Goyal, MD

    Introduction

    Femoral-sided revision of total hip arthroplasty in patients with significant proximal femoral defects can be quite challenging. Several techniques have been utilized to reconstruct these challenging cases, including:

    • Impaction grafting
    • Allograft prosthetic composite (APC)
    • Reconstruction with cemented stems
    • Cementless cylindrical and tapered stems

    For the past four decades, cementless cylindrical extensively porous-coated stems have been used at our institution in femoral revision arthroplasty.

    These stems are designed to achieve initial fixation in a 5- to 7-cm section of femoral diaphysis that is selected during preoperative planning. Distal-fixation stem designs include:

    • Tapered stems
    • Fluted stems
    • Cylindrical stems

    The design used at our institution consists of a straight, cylindrical, non-tapered distal geometry with a porous surface consisting of cobalt-chrome beads sintered to a cobalt-chrome substrate.

    The coating was initially designed to extend distally from the shoulder of the prosthesis and cover the entirety of the stem (Anatomic Medullary Locking [AML]; DePuy Orthopaedics, Warsaw, Indiana). In subsequent designs, 80% of the length of the stem was coated (AML), with an uncoated bullet-shaped tip. These stems are available with (Prodigy; Depuy Orthopaedics, Warsaw, Indiana) or without (Solution; DePuy Orthopaedics, Warsaw, Indiana) 10° of anteversion built into the neck design.

    Surgical Indications

    The need for femoral component revision has become less common due to improved technique and materials for primary implants. When revision is necessary, it can pose a significant challenge. Although other techniques of femoral revision may spare the proximal femoral bone from stress shielding, the reliability and ease of technique for implantation of distally fixed stems makes this a choice procedure for a wide variety of femoral revisions.

    The most frequent indications for femoral revision are:

    •  Infection
    •  Aseptic loosening
    •  Component failure [4]

    Despite the ability of the extensively porous-coated stems to be used for any of the above indications, certain patient-specific characteristics should be considered.

    • First, a relative contraindication for implantation of this device is the lack of 5 to 7 cm of intact diaphyseal bone that can be prepared to accept the stem and serve as the site of fixation.
    • In addition, cases of severe bone loss, classified as Paprosky 3B or 4, have had poor results reported when this stem design is used.
    • Finally, if the femoral component is well fixed, the surgeon should be wary of planning to exchange the component in a patient who has complaints of thigh pain without a definitive cause.

    Preoperative Planning

     Prior to radiographic evaluation, a clinical evaluation should be completed to document:

    •  Pain, including quality and location of the pain
    •  Existing leg length discrepancy
    •  Abductor dysfunction
    •  Gait abnormalities

    The radiographic evaluation should include an anteroposterior pelvis and femur, as well as a frog-leg lateral of the involved femur. All patients with existing arthroplasties should have a cross-table lateral of the hip performed as well to assess acetabular component positioning.

    A 5-step process of preoperative planning has been previously described [1] and is summarized here.

    • Step 1 uses the AP radiograph to identify the area of diaphyseal bone that will be the site of fixation of the distal portion of the stem. This segment of bone should be 5 to 7 cm in length. Once this segment of bone has been identified, the axial alignment and diameter of the stem that will be needed can be determined.
    • Step 2 involves identifying the acetabular center, which will be the existing center if the acetabular component is to be retained.
    • Step 3 focuses on determining the appropriate offset, stem seating position, and modular head length. This is accomplished by determining the preoperative leg length discrepancy and then seating the femoral component in a position that will correct the leg length the desired amount. Once the seating level of the component has been determined, the distance between landmarks identifiable intraoperatively (ie, the tip of the greater trochanter and lateral aspect of the prosthesis). This is measured and recorded.
    • Step 4 involves ensuring that the proximal girth of the chosen prosthesis will not be too large for the existing femoral metaphysis.
    • Step 5 uses the lateral radiograph to ensure that the chosen implant, seated at the predetermined depth, will match the native bow of the femur (Figure 1). If a straight stem will not fit appropriately, then a bowed stem should be the implant utilized.

    Figure 1. Lateral radiograph demonstrating anterior cortical impingement of a straight femoral stem

    Surgical Technique

    Obtain appropriate exposure of the femoral canal, either with standard techniques or by using an extended trochanteric osteotomy at a level just distal to the most distal of all cortical defects.

    The most important step in preparing the femur is appropriate reaming of the diaphyseal bone that will be used for distal fixation of the stem. If an osteotomy is used, we recommend placing a prophylactic cable around the femur just distal to the osteotomy site to decrease hoop stresses as the femur is reamed and the final component is impacted into place. This cable is ideally placed prior to completion of the osteotomy to minimize fracture propagation beyond the osteotomy site.

     Remove all cement plugs, cement remnants, and bone pedestals from within the femoral canal to ensure that the reamer is being directed by the cortical bone and not by any other debris within the canal. If in doubt about the direction of the reamers, obtain an intraoperative image with a reamer within the canal.

    Start reaming with the smallest straight, rigid reamer on power and progress in 0.5-mm increments until a size 1.5 to 3 mm smaller than the templated implant is reached. Place a trial implant of equivalent length to, but smaller diameter than, the templated implant size into the femoral canal to determine the distal extent of the reaming. Mark the trial implant and remove it from the femur.

    Hold together the trial component and the next size reamer. Using a blank label sticker or marking pen, mark on the shaft of the reamer the depth that the reamer needs to be sunk into the femur. Continue reaming in 0.5-mm increments until a reamer 0.5 mm smaller than the intended final implant size is reached.

    Place a reamer the size of the final intended implant on a T-handle and insert it into the femoral canal, but do not turn on the reamer. Measure the distance from the calcar to the top of the reaming threads or from the distal extent of the osteotomy to the tip of the reamer; it should be at least 4 cm (Figure 2).

    Figure 2. Final reamer placed within femoral canal being used to measure the amount of interference fit.

    Although the implant size was templated preoperatively, it is important to obtain good cortical chatter with the reamers prior to stopping to trial a component.

    Once the femur has been prepared, implant trial components, reduce the hip, and take the hip through a range of motion. At this point, set the modular neck length and femoral component version to achieve maximum stability. We recommend using the tip of the greater trochanter as a landmark for the desired center of the femoral head during the reconstruction.

    Remove all trial components and impact the femoral component into the prepared femur. The back end of a reamer is inserted into the hole in the shoulder of the prosthesis and held by an assistant to maintain the version of the stem as it is impacted (Figure 3).

    Figure 3. Final component placement. Ensure appropriate “scratch fit” and use of tools to control version as the component is impacted.

    As the component is impacted, ensure that it is progressing 2 to 3 mm with each blow. For the final 2 cm, the component should progress 1 mm with each blow. Thirty to 40 blows are often required for complete seating of the component.

    If, at any point, the component is no longer progressing with each blow, remove the component and use a hole gauge to measure the actual diameter of the final prosthesis. Ream the canal once again, ensuring that the area of distal fixation is reamed to a diameter 0.5 mm smaller than the diameter of the final prosthesis, as measured by the hole gauge (Figures 4-5). A femoral extractor system should be available for all cases to guarantee that this step will be possible.

    Figure 4. Hole gauge used to assess diameter of reamer.

    Figure 5. Using a hole gauge to measure the precise diameter of the distal aspect of the final component.

    Postoperative Care

    All patients are permitted protected weight-bearing with a walker for a minimum of 6 weeks. If metaphyseal and diaphyseal fixation are deemed adequate, then the patient is permitted full weight-bearing with the walker.

    The patient is permitted 10% to 50% weight-bearing for a minimum of 6 weeks if diaphyseal fixation is good but metaphyseal fixation is inadequate, if there is a femoral perforation, or if an extended trochanteric osteotomy was utilized, then. The patient is then advanced to full weight-bearing progressively after 6 weeks.

    If there is severe comminution of the osteotomized trochanter or severe proximal bone deficiency, then the patient is permitted 10% weight-bearing for 6 weeks, followed by 50% weight-bearing for 6 weeks, and then progressed to full weight-bearing at 3 months.

    Complications

    The most common complications after this procedure are:

    • Postoperative dislocation
    • Infection
    • Distal femoral fracture
    • Femoral perforation [3]

    We believe the incidence of these complications is directly related to intraoperative attention to detail and the surgeon’s comfort with the surgical technique.

    Postoperative dislocation is most commonly caused by component malposition and/or leg length inequality. Therefore, we advocate the use of trial components and multiple measurements of leg lengths and final seating position of the stem. Ensuring appropriate restoration of leg length, and thus abductor tension, will most reliably result in a stable reconstruction.

    If stability cannot be achieved due to a malpositioned but well-fixed acetabular component, then the component should be revised. If the femoral and acetabular components are well-positioned but appropriate stability cannot be attained, then trochanteric advancement or a more constrained construct must be considered.

    Distal femoral fracture usually occurs during impaction of the final prosthesis, especially in cases in which the prosthesis is too long or too large in diameter. The surgeon should be concerned about this complication when, during impaction, the prosthesis begins to advance greater than 3 mm with each blow, rather than 1 mm with each blow.

    The femur should be imaged with an AP and lateral radiograph when there is concern for fracture.

    • If the fracture is non-displaced and the femur and prosthesis move as a single unit, then no additional fixation is necessary; however, the patient’s weight-bearing status should be limited for a minimum of 6 weeks.
    • If the fracture has displaced, fixation should involve cables and a strut allograft, followed by 3 months of protected weight-bearing.
    • If there is significant concern, the surgeon may consider utilizing a longer stem prosthesis to bypass the fracture site.

    Anterior perforations most commonly occur from drilling too far down a bowed femur with a straight drill or reamer. The advantage of a straight reamer is the creation of a perfect cylinder for the stem; however, the disadvantages include the difficulty of measuring the length of the cortical interference fit and the ease of perforation, especially if the patient has a femoral deformity or significant femoral bow. A flexible reamer allows measurement of the length of the interference fit and potentially decreases the risk of causing perforation, but it does not truly create the straight cylinder for the femoral stem (Figure 6).

    Figure 6. Flexible reamers to be used in cases of impending perforation.

    Lateral femoral perforations can be avoided by taking care to direct the reamer down the canal of the femur and not allow any retained cement or bone pedestal to redirect the reamer. If there is any doubt, intraoperative imaging should be obtained with a thin reamer or a drill in place.

    If there is significant concern over reamer misdirection, thin-shafted reamers with a bulbous tip can be used. These reamers will flex when they hit an obstruction within the canal. In addition, the resistance of these short-tipped reamers will change significantly once they have bypassed the diaphysis and can give a good gauge of the length of diaphyseal fixation available.

    If there is significant concern of femoral deformity or femoral bow that may result in a perforation, strong and early consideration should be given to performing an extended trochanteric osteotomy to avoid this significant complication.

    Case Example

    A 67-year-old male presented 3 years and 1 month after a primary left total hip arthroplasty using an extensively coated cylindrical stem. He complained of persistent thigh pain refractory to all conservative therapies, including:

    •  Anti-inflammatories
    •  A trial of limited weight bearing
    •  Physical therapy

    Serial radiographs showed a lack of ingrowth into the femoral component (Figures 7-8).

    Figure 7. Preoperative AP with a pedestal seen at the tip of the femoral stem indicating a loose stem.

    Figure 8. Preoperative lateral with radiographic signs of loosening.

    Because of the failure of ingrowth and persistent symptoms, the patient was scheduled for revision of the femoral stem. Figures 9 and 10 show x-rays with preoperative templates. The level of the osteotomy and the distance from the tip of the greater trochanter to the osteotomy were templated on the AP femur x-ray. The potential revision implant was templated on the AP and lateral femoral x-rays to ensure appropriate congruency with the bow of the femur.

    Figure 9. Preoperative AP x-ray with osteotomy and final implant templated.

    Figure 10. Preoperative lateral x-ray with final implant needing to be bowed to avoid impingement.

    Intraoperatively, we were not able to remove the stem from above, despite the stem being loose and completely clearing the shoulder of the prosthesis with osteotomes and a burr. Therefore, we proceeded with an extended trochanteric osteotomy to expose the proximal portion of the stem.

    A metal-cutting burr was then used to transect the stem at a point distal to the metaphyseal flare of the proximal stem. Once the proximal stem was removed, trephines are used to disrupt any areas of spot-welding along the length of the distal stem. It is important to note that, along with a change in resistance of the trephine, an audible pop or crack will be encountered once the trephine has bypassed the distal aspect of the stem.

    The distal stem was removed, as it was usually well-fixed within the final trephine. It is not uncommon to need five to 10 trephines to complete this portion of the procedure, and therefore these should be ordered and available from the beginning of surgery. Due to the thermal necrosis that often occurs to the bone with this technique of stem removal, we believe it is best to achieve at least 4- to 5-cm of interference fit distal to the tip of the removed stem to ensure fit within healthy and viable bone.

    The femur was then reamed and the final stem implanted. Trial heads were used to ensure appropriate stability and leg length.

    Figures 11 and 12 show the 1-year follow up radiographs with cables (Dall-Miles; Stryker, Mahwah, New Jersey) used to fix the trochanteric osteotomy and achieve appropriate ingrowth of the component.

    Figure 11. One-year postoperative AP x-ray.

    Figure 12. One-year postoperative lateral x-ray.

    References

    1. Engh Jr CA, Hopper Jr RH, Engh Sr CA: Distal Ingrowth Components. Clin Orthop 2004;420:135–141
    2. Engh CA, Hopper Jr RH: The odyssey of porous-coated fixation. J Arthroplasty. 2002;17(4 Suppl 1):102–107.
    3. Egan EJ, DiCesare PE. Intraoperative complications of revision hip arthroplasty using a fully porous-coated straight cobalt-chrome femoral stem. J Arthroplasty. 1995;10(Suppl):S45–S51.
    4. Surdam JW, Archibeck MJ, Schultz SC Jr, Junick DW, White RE Jr: A second-generation cementless total hip arthroplasty mean 9-year results. J Arthroplasty. 2007 Feb;22(2):204-9.

    Figures reprinted courtesy of the Anderson Orthopaedic Research Institute (AORI), Alexandria, Virginia.