Surgical Techniques: Extensively Porous-Coated, Cylindrical Stems




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Chapter Synopsis


This chapter reviews the indications and techniques for successful implantation of an extensively porous-coated stem for management of the femoral component in revision hip arthroplasty.


Important Points





  • Preoperative templating, including on lateral radiographs, is used to determine whether a curved stem is required and to establish the level of an extended osteotomy.



  • Extended trochanteric osteotomy should be used with little hesitation if needed to aid in implant extraction and safe, accurate femoral preparation.



Clinical/Surgical Pearls





  • A cerclage wire or cable used distal to the osteotomy before implantation can minimize fracture risk.



  • Ensuring 4 to 5 cm of scratch-fit helps to optimize the fixation and outcome.



Clinical/Surgical Pitfalls





  • Failure to adequately prepare the femur (e.g., leaving too much scratch-fit) can increase the risk of fracture during implantation of the prosthesis.





Introduction


Total hip arthroplasty (THA) is one of the most successful interventions in the medical profession. There are many clinical scenarios in which revision arthroplasty is necessary. We review the use of extensively porous-coated, cylindrical stems for management of the femoral component in THA.




Indications and Contraindications


Indications for femoral revision include aseptic loosening, progressive osteolysis, periprosthetic fracture, a malpositioned component leading to instability, infection, and the need for enhanced exposure when performing acetabular revision.


The specific indications for use of an extensively porous-coated stem depend on the remaining bone stock. The most widely used system, the Paprosky classification, divides bone defects into four types. Type I describes intact or minimal loss of metaphyseal bone and intact diaphyseal bone. Although these patients are candidates for an extensively porous-coated stem, most would also do well with a stem using metaphyseal fixation. Type II is a loss of metaphyseal bone with an intact diaphysis. Type III describes partial loss of diaphyseal support, with type IIIA having more than 5 cm of diaphysis or isthmus remaining and type IIIB having less than 5 cm of diaphysis or isthmus remaining. A type IV defect has no remaining normal diaphysis for fixation. The ideal bone types for revision with an extensively porous-coated, cylindrical stem are Paprosky types II and IIIA. Type I bone usually can be managed with any stem design, including an extensively porous-coated, cylindrical stem. Types IIIB and IV are best managed with other methods, such as modular, tapered, fluted stems; impaction grafting; or allograft–prosthetic composite reconstruction.




Equipment


In addition to the usual hip arthroplasty exposure instruments, the tools that should be available to handle these revision cases include an oscillating saw, high-speed bur with a metal-cutting tip, Gigli saw, cement extraction devices (e.g., trephines, back-biting curettes), full array of osteotomes and curettes, cerclage wires or cables with appropriate tensioning devices, Mersilene tape, and medium and large vessel clips. Trephines may be needed to remove the diaphyseal portion of well-fixed, cementless stems, and multiple trephines of each diameter should be available.




Surgical Techniques


Anatomy and Approaches


Preoperative planning is paramount to a successful and efficient operation. All information should be obtained about the current implant, including the type of coating used. The posterior approach is uncomplicated for revision procedures due to its extensile nature and ability to fully expose the femoral and acetabular side. Many surgeons also achieve excellent revision exposure through an anterior or anterolateral approach.


Prior skin incisions should be used if they can provide adequate exposure, and scars can be excised if there is concern about their appearance or vascularity. An extended trochanteric osteotomy (ETO) frequently is used to improve exposure and facilitate removal of previous implants, especially when components are well-fixed, contain large amounts of cement, or have an associated femoral deformity.


Examination and Imaging


Preoperative examination of the patient should focus on the actual or perceived limb length inequality. In addition to measurements made from the umbilicus and anterior superior iliac spine to the medial malleolus, we recommend block testing to assess the patients’ perception of their leg lengths. For perceived limb length inequalities, patients should be assessed for pelvic obliquity and counseled that surgery may not correct the inequality. Patients should understand that the goal of revision surgery is a stable, well-fixed hip and that a desire to achieve equal limb lengths should not compromise this goal.


The preoperative examination evaluates the status of the abductors by assessing the patient’s gait and Trendelenburg sign. Patients should understand that if they have a limp because of previous damage to the abductors, revision surgery is not likely to address the issue.


Preoperative imaging should include full-length femur radiographs, including dedicated views of the pelvis and affected hip. The x-ray films should be scrutinized for any deficiencies in the femoral bone stock and the size of the remaining intact diaphysis. The shape of the femur should be evaluated on the lateral view, and templates should be used to determine whether a bowed stem is required to avoid anterior cortical perforation. The femur should be templated to choose the shortest option that allows approximately 5 cm of cortical fit in the femoral isthmus.


The ETO is planned to allow adequate exposure and cement removal without compromising the cortical purchase of the revision implant. The osteotomy should be measured from the tip of the greater trochanter on the templated films because this is an easily measurable reference point intraoperatively.


Procedure


The patient should be secured in the lateral decubitus position with a Capello peg board (i.e., hip positioner). Ideally, the posterior approach is used for the reasons discussed previously. Prior skin incisions should be incorporated when possible and excised if necessary. The incision should be carried down to the level of the iliotibial fascia and a small portion cleared to define the plane for later repair. We mark the fascia to ensure that it is evenly apposed at the end of the case.


At this point, the leg is abducted and placed onto a sterile, padded Mayo stand. The iliotibial fascia is incised over the greater trochanter and extended distally with scissors in line with its fibers. It is then extended proximally in line with the gluteus maximus fibers. The iliotibial band is dissected free of the vastus lateralis fascia to define this plane, and the fibers of the gluteus maximus are bluntly dissected using cautery when necessary due to scarring. A Charnley retractor is then placed to optimize exposure.


The greater trochanter should be inspected for damage to the abductor tendon, and the anterior and posterior borders of the gluteus medius should be identified and protected during the case. The bursal tissue should then be dissected free of the posterior edge of the trochanter and a Cobb elevator used to define the short external rotators if they were previously repaired. The interval between the abductors and the piriformis should be dissected and protected with a Hohmann retractor along the superior aspect of the femoral neck area. The short external rotators and posterior capsule can then be dissected free of the greater trochanter with Bovie cautery to expose the joint, and the proximal femur should be skeletonized. After this is accomplished, the hip can be dislocated, and the acetabulum can be inspected for its integrity.


At this point, we recommend defining the posterior edge of the vastus lateralis and retracting it anteriorly off of the femur up to the vastus ridge. The length of the ETO is then marked as determined by preoperative planning, taking care not to compromise the distal fixation of the implant. The ETO improves visualization and has a very low rate of complications.


An oscillating saw is used to make the osteotomy, which is ideally one third of the circumference of the proximal femur. A pencil-tip bur is used to make the distal lateral corticotomy in a beveled and rounded fashion to prevent creation of a stress riser in the bone. The anterior cut can be made with a saw, with the vastus lateralis retracted further anteriorly up to the vastus ridge, and an osteotome can be slid proximally from the end of the saw cut beneath the vastus still attached to the trochanter. Osteotomes are placed along the posterior aspect of the osteotomy and used to gently lift the fragment, hinging it open at the anterior cortex. This exposes the femoral component to aid in its extraction. If there is significant varus remodeling of the proximal femur, the osteotomy can be completed on the medial side to create a transfemoral osteotomy, which aids in realignment of the femur. This occasionally happens on its own as the final cerclage cables or wires are tightened to repair the ETO.


After removal of the component, the femur is prepared for the appropriate length and shape of stem that was chosen during preoperative planning. Reaming can be done with rigid (for a straight stem) or flexible (for a bowed stem) reamers, and it can be done by hand or with power, depending on the surgeon’s preference and the patient’s bone quality. The femoral canal is reamed sequentially, first by 1-cm increments and then by 0.5-cm increments, to fit the templated stem, allowing 4 to 5 cm of scratch-fit of the long, cylindrical, fully porous-coated stem. To achieve this, underreaming by 0.5 cm is recommended. For straight stems, a reamer of the same size as the proposed stem is inserted on a T-handle (by hand), and it is pushed into the canal using good force with a few twists. The surgeon should verify that 4 to 5 cm of the reamer is still outside the canal. This roughly equates to the scratch-fit. If this reamer is too easy to insert, proceeding to the next size is recommended. Bowed stems usually require line-to-line reaming with a flexible reamer, and overreaming by 0.5 cm is common.


The proximal bone is broached (particularly when no ETO has been done) with broaches starting one or two sizes smaller than the proposed stem size and progressing up to the size of the stem if needed. Some designs have a larger metaphyseal option for better metaphyseal fit, the use of which can be determined at this point by using appropriate broaches. Typically, the surgeon broaches to the standard size first, and if room remains to contact cortical bone, the larger metaphyseal option is tried. Broaching is usually skipped with an ETO.


If an ETO was used, a cerclage wire or cable is placed around the femur diaphysis at least 1 cm distal to the osteotomy site and tightened. This may be done before reaming (our preference), broaching, or trialing, but it should be done before implant insertion. This dissipates the hoop stresses and decreases the risk of fracture distal to the ETO. Trial implants are then placed, and the limb is assessed for length, soft tissue tension, and stability. The final implant is then inserted. If the stem cannot be placed by hand to within 5 cm of its final position, which estimates the amount of scratch-fit, we recommend reaming the difference line to line to allow placement to this position. A trial head is then placed, and the hip is reduced to check for stability and length before impacting the final head.


The osteotomy fragment is placed over the final implant, contouring the endosteal surface with a bur to optimize its fit. The osteotomy is secured with a minimum of two cables that are evenly spaced across the osteotomy. The cables are carefully passed with a cable passer, and care is taken when dissecting the posterior femur to avoid the perforating vessels of the profunda femoris artery. If there is concern about the integrity of the osteotomy site or fixation, we recommend supplementation with a tibial cortical strut spanning the osteotomy site.


The short external rotators and posterior capsule can be repaired to the posterior edge of the trochanter or to the tendinous portion of the gluteus medius if there is concern about the integrity of the trochanter. The iliotibial band is closed with a running, looped, 0 polydioxanone (PDS) suture. The deep fat layers are closed with interrupted, 0 Vicryl pop-off sutures, and the deep dermal layer is closed with 2-0 Monocryl suture. Our revision skin closure is done with 2-0 Prolene vertical mattress sutures, which can be left in for a prolonged period if there are concerns about slow wound healing. On larger patients or for particularly worrisome wounds, we often apply a vacuum-assisted closure (VAC) strip over the incision.

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May 29, 2019 | Posted by in ORTHOPEDIC | Comments Off on Surgical Techniques: Extensively Porous-Coated, Cylindrical Stems
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