Component Design

The design phase of the triflange cup is the most critical step in the surgical technique. Because the implant is custom made, surgeon input on bone removal, flange size and location, head center location, and cup face orientation is required before the implant is made. Because these decisions are made preoperatively, the operative time is reduced by eliminating the intraoperative improvisation and fabrication necessary when using cages and structural allografts. Careful consideration and contemplation of the various design decisions are necessary during the design phase, however, because no alterations are possible during surgery with the exception of the modular insert choices and the amount of bone removed.

The design process begins with a computed tomographic (CT) scan done on a modeling protocol. Having the radiology technician review the protocol with a design engineer from the manufacturer before the scan is often helpful. The CT scan should capture both hemipelvises. It should begin one slice above the ilium and end one slice below the ischium to ensure that the entire pelvis is captured. A standard CT artifact algorithm should be used to avoid enhancing metallic scatter in the scan region. To produce an adequately detailed model, slice thickness should not exceed 3 mm.

Once the scan is complete, it is sent to the manufacturer and a three-dimensional model is created. This complex task first requires the removal from the scan of the failed acetabular component, bone cement, screws, plates, and old grafts. Despite this complexity, the results are surprisingly accurate. The surgeon can then review the reconstructed hemipelvis on the computer (see Fig. 41-1, B ), or a 1:1 model can be constructed by stereolithography. The model is sent to the surgeon, who marks the thin, fragile periacetabular bone to be removed at surgery and draws the outline of the flange geometry on the model (see Fig. 41-1, C ). Remember, cup stability and fixation no longer come from the deficient rim of remaining acetabular bone but from the better bone of the ilium, ischium and pubis. The implant bridges the defect.

The iliac flange is the most important. The iliac bone is the best, and this flange has the largest surface available for contact. The authors generally plan for five to six screws in excellent bone through this flange. The ischial flange is the next most important flange. When using a posterior approach, this flange sits under the sciatic nerve. Consideration of the flange geometry is important. The ischial flange should extend far enough and be broad enough to accept three to four screws. The edges should be rounded and of low profile to avoid the possibility of sciatic nerve irritation. The pubic flange generally is small, roughly triangular in shape, and without a screw hole. It provides the “third leg on the stool” for stability.

The surgeon-marked pelvic model is returned to the manufacturer for the first pass at the construction of an implant model. At this point, the engineer should be notified if the surgical plan is to reestablish a near anatomic position of the head center or if head center will be raised (cephalad) or lowered (caudal). This will depend more on the planned retention or revision of the femoral component than on the acetabular bone loss.

The engineer returns the pelvic model with any areas of bone removed or machined (with acetabular reamers) marked in bright red for surgeon reference. The pelvic model is later sterilized and referenced intraoperatively. The model of the proposed triflange cup is sent along with the pelvic model. The surgeon reviews the size and location of the three flanges and changes are marked on the model for engineer referencing. Special attention should be made to the contour of the flanges and how the flanges will interact with the soft tissues. Smooth transitions and rounded edges are important where the sciatic nerve may pass. To maximize initial implant stability, the iliac flange should fit over the two planes formed by the gluteal ridge and the iliac screw holes should begin at the inferior edge of the remaining structurally sound ilium.

Over the past 2 years the authors have added locking screws to enhance the stability of the triflange cup/pelvic construct. The location of any locking screw holes is indicated on the model. One nonlocking screw hole should always be present in both the ischial and iliac flanges. This nonlocking lag screw will be inserted first and will pull the implant down into intimate contact with the bone surface before the locking screws are inserted.

Cup stability on the pelvic model is assessed. The flanges should fit intimately against the pelvic bone. The construct should feel stable to superior/inferior and anterior/posterior loading. Increasing the diameter of the central hemispherical portion often will greatly improve overall stability. Although the central hemispherical portion does not have to intimately contact the remaining acetabulum, the component should have inherent stability when placed on the model. The authors’ original designs attempted to fill the acetabular volume and have circumferential rim contact. The attempt to attain simultaneous rim and flange contact results in a component that is nearly impossible to insert without injury to the abductor muscles and superior gluteal nerve. The hemispherical portion of the cup should be of a large enough diameter that the cup resists any movement of the implant on the hemipelvis. Implants produced more recently have been designed with smaller central portions and have done as well clinically as the larger original designs. The cup likely derives its stability primarily from the flanges. The authors’ current designs are created without dome screw holes although a central, threaded dome hole is present to allow attachment of an impactor. The impactor handle can be helpful in manipulating the cup as it is positioned on the pelvis.

Head center location and the cup face orientation are critically important. The head center is chosen on the basis of patient-specific considerations, including leg-length discrepancy and planned retention or revision of the femoral component. The vertical head center is first selected by approximating the anatomic position of the head center with the superior aspect of the obturator foramen as a reference point. The remnants of the anterior and posterior columns help center the head in the coronal plane. The head center often is lateralized, which reduces possible femoral component impingement with the flanges. Lateralization of the head center has not affected long-term cup fixation. Cup face orientation is targeted at 35 to 40 degrees from the inter-teardrop line. Anteversion and abduction initially are established by using the plane of the obturator foramen as a guide. Throughout the design process, a full pelvic sawbones model is used as a reference.

In cases of pelvic discontinuity, the obturator foramen is not an accurate reference point because the inferior segment of the pelvis rotates away from the anatomic position around the pubic symphysis. In these cases the referenced sawbones model, the radiograph, and the iliac wing are used to establish cup face orientation. Establishing cup face orientation remains the most difficult part of the design process, precisely because the reference points often are so ambiguous.

Once the design is finalized, the pelvic and implant models are returned to the engineer to begin manufacture. The design process may go through several iterations before the design is satisfactory. The surgeon must take the time necessary to ensure that the component design is satisfactory before approving it.

Manufacture generally takes approximately 6 weeks after approval of a design. The central cup face and modular locking mechanism are created first. The flanges are then machined out of a titanium block. The screw holes are added. The pelvic side of the component is plasma sprayed and the hydroxyapatite coating is applied. The cup is then sterilized and shipped.

Component Implantation

Whenever the pelvis must be exposed beyond the immediate acetabular rim, a more extensile approach must be used. The incision tends to be longer because the surgeon must be able to see enough of the bone beyond the acetabular deficiency to ensure good contact with healthy bone and see that the implant is seated at the planned position. Although the imposing size of the triflange cup may suggest that implantation will be difficult, the necessary exposure is identical to that used to place an antiprotrusio cage or a structural allograft. A standard posterior approach is used most often. This can be combined with an extended trochanteric osteotomy if necessary for femoral component revision.

Surgical Approach

After the incision is made, the soft tissue planes are reestablished. The tensor fascia is separated from the underlying gluteus medius and vastus lateralis fascia. The sciatic nerve must be identified and protected. It is located as it crosses over the ischium and then traced distally beneath the gluteus maximus insertion and into the hamstrings. Proximally it is traced into the sciatic notch. It must be identified and mobilized because the ischial flange fits intimately on the “sciatic” surface of the ischium. Because bone contact is critical for implant stability, the nerve must be mobilized enough that the flange can be correctly placed without traction or compression injury. This usually is easily accomplished, but the surgical planes may be confused and the nerve may be enveloped in scar. In this situation, identification of the nerve more distally (deep to the gluteus maximus insertion or directly over the ischium) and tracing it proximally are important. An external neurolysis occasionally is necessary.

Once the sciatic nerve is mobilized, a posterior arthrotomy is made. The periarticular scar is removed and the hip is mobilized. The hip is dislocated and the femoral component is inspected. If the femoral component is to be revised, it is removed at this stage. The authors often use an extended trochanteric osteotomy to facilitate femoral component revision. If the component is retained, it is mobilized and retracted anteriorly under the gluteus minimus and anterior to the acetabulum. The gluteus minimus and gluteus maximus are elevated off the ilium. Care is taken to avoid significant traction on the superior gluteal nerve contained in the neurovascular bundle exiting the superior aspect of the sciatic notch. Enough mobilization must occur to allow the iliac flange to slide under the muscle. A small pocket is then created anterior to the pubis for the pubic flange. From the lateral decubitus position the pubis usually is straight down toward the operating table.

The ischium must be exposed to allow placement of the ischial flange. The soft tissue is elevated off the remaining posterior column extending from the inferior edge of the sciatic notch past the ischium. This tissue must be taken as a sleeve. The dissection off the ischium includes part of the hamstring origin so that the ischial bone beneath is exposed.

The pelvic model is repeatedly referenced. It is sterilized preoperatively and kept in the sterile field for review during the procedure. The areas of the model marked in red denote bone that has been planned for removal or machining. The bone must be removed to match the pelvic model so that the implant will fit as planned.

Component position on the model should be reviewed before implantation. The iliac flange is inserted first. The leg is abducted and translated cephalad, relaxing the hip abductors. The iliac flange slides between the ilium and the gluteus minimus. Care should be taken to avoid undue traction on the superior gluteal nerve posteriorly. The component is then rotated so that the pubic flange slides into the pocket created anteriorly. The hip is then extended and the knee flexed to relax the posterior soft tissues. The ischial flange is then pushed into position. Care should be taken to avoid trapping the sciatic nerve under the flange. The nerve has been mobilized with the surrounding sleeve of soft tissue. Releasing the soft tissue at the inferior corner of the acetabulum often will create additional mobilization of the posterior sleeve.

The triflange cup is then gently impacted into its seated position. The model is again referenced to ensure that the flanges are at the design position. If the component does not seat as expected, it is most likely malrotated, the planned bone removal (areas marked in red on model) is not complete, or morselized bone graft placed in the medial defect is preventing the cup from seating. Because of the potential interference from the morselized graft, the authors place the graft after the cup is fixed into position. Large gaps usually are present posteriorly between the component and the pelvis where the acetabulum is deficient, which allows backfilling with graft without difficulty.

Once the cup is seated, the screws are inserted. The ischial screws are always inserted first. The bone of the ischium is most affected by osteolysis. A large cavitary defect in the ischium often is present and screw fixation is compromised. If necessary to improve fixation, a cavitary lesion in the ischium can be filled with bone cement. Screws placed through the hardened cement attain excellent fixation. The authors’ most recent modification to the triflange design is the use of locking screws in the ischial and iliac flanges. One screw hole in the ischium remains for a lag screw. The 6.5-mm lag screw is inserted first to pull the flange down. The locking screws are then inserted. Once all the ischial screws are inserted, the iliac screws are inserted.

During iliac screw insertion, care must be taken to avoid traction injury to the superior gluteal nerve. For the iliac screws above the most distal row, drilling and insertion can occur directly through the gluteus minimus and medius. A total of four to five iliac screws usually are placed. The lag screws are inserted first to pull the iliac flange into intimate contact with the ilium. Because the iliac bone usually is well preserved, these screws attain excellent purchase. Because the edges of the flange are obscured by the gluteus maximus and minimus, palpation is the most effective way to determine if the flange is well seated. Once the flange is well reduced, the locking screws are inserted. Morselized graft is then packed behind the cup. This graft does not contribute to cup support or fixation but may aid in remodeling of the acetabular defect over time.

The modular insert is then inserted. In patients with poor abductor function (denervation, detachment, or absent greater trochanter) a constrained liner is used. In other cases, large head ball diameters and alternative bearings are considered.

Component Implantation in Cases of Pelvic Discontinuity

In patients with pelvic discontinuity, the upper and lower halves of the hemipelvis may be rotated through the discontinuity. Because the pelvis was scanned and the model made with the patient supine, the relation between the two halves of the hemipelvis often changes when the patient is in the lateral decubitus position.

Although the sequence of implant insertion is unchanged because of this rotation, the flanges often do not initially seat at their design positions. The ischial screws must be inserted first and the ischial flange placed at its design position. With the ischial screws in place, the iliac flange often is rotated off the ilium, but as the first iliac lag screw is inserted the screw taking purchase into the ilium will rotate the flange with the connected inferior half of the hemipelvis into the correct orientation relative to superior half. The remaining iliac screws are then inserted.