Revision Total Hip replacement- Stryker custom acetabulum and SERF Dual mobility Hip (De Puy)

Once a CT scan has been performed according to proprietary protocols, a 3D model of the hemi pelvis can be printed and the acetabular deficiency appreciated.
Segmental losses are challenging but can usually be dealt with by conventional revision cups. Combined deficiencies and discontinuities pose a greater problem and it is in these circumstances that a custom cup excels.
The 3D model of the hemipelvis is supplied by the manufacturer in advance of definitive implant production; this allows the surgeon to determine the extent of bony loss and provide the engineeers with feedback on optimum areas for fixation.
This particular model shows loss of the anterior column as well as cavitary and segmental deficiciencies.

Once the 3D model has been printed the manufacturing engineers design an initial implant. The fixation features take account of the bony deficit, bone quality and optimum residual bone for fixation.

Screw fixation orientation and screw lengths can be predetermined from the model. There is no requirement to position the cup in the exact preferred anatomical version and closure as this can be fine tuned when cementing in the DM cup after the shell is securely fixed.

The printed cup and hemipelvis can be supplied in sterile form and this helpful intraoperatively to determine extent of preparatory reaming and screw orientation.

For the vast majority of my primary and revision cases, I use the posterior approach.
The patient is placed in the lateral decubitus position with a pillow placed between the knees; the hips are flexed at approximately 30 degrees and the postion secured with anterior and posterior props.

The posterior support is positioned above the natal cleft. The lower border is in line with the posterior superior iliac spine.

The anterior support is positioned between pubic symphysis and anterior superior iliac spine. Postioning too low can restrict femoral exposure.

A regular posterior incision based on the greater trochanter is performed. In a primary situation this incision can be quite short but in a revision case I tend to perform an eliptical incision removing the previous scar and subcutaneous tissues.

It is important to incise the fascia lata sufficiently distal so that it can slide over the lesser trochanter; if incomplete, it is difficult to elevate the femoral head into the wound. Failure to do so also results in excessive femoral torque in achieving exposure which can result in a spiral fracture.

A Charnley type retractor is placed in the centre of the wound; increased tension does not equate to increased exposure so the retractor is set a little ‘loose.
A Judd nail is placed in the ilium between the piriformis and gluteus minimus tendons to delineate the proximal extent of capsular exposure.
The trochanteric bursa is swept back protecting the sciatic nerve inferiorly. In revision cases it is important to identify the nerve through its course from the sciatic notch to beyond the Gluteus Maximus insertion as it it can be trapped in scar tissue. The course of the nerve can vary in dyspastic cases with a typically high division around the piriformis tendon.

A useful but not infallible aid to help with leg length restoration a skin marker suture which is used to define a point around the Greater Trochanter with a diathermy mark. At the end of the procedure, pre and postop positions can be compared giving an indication of leg length alteration.

The assistant internally rotates the extended femur to tension the external rotators. A cutting diathermy cuts the external rotators from the nail marker proximally to include the gluteus maximus tendon distally. It is helpful to keep tensiion on the external rotators with a clean swab to facilitate high vision of the rotators from the femur.
The assistant should keep a hand on the foot during this manoeuvre to dtermine excitation and hence proximity of the sciatic nerve.

In revision cases I tend to perform an elliptical incision to remove as much of the pre-existing scar as possible

I use a Charnley bow retractor and routinely make a diathermy mark over the greater trochanter for an indication of leg length.

In a revision case I tend to aspirate the joint prior to division of the fascia lata ensuring more reliable sterility.

When taking an aspirate for aerobic and anaerobic cultures in blood culture bottles, it is not advisable to add more than 2-3mls to the bottles. Saturation of the culture bottles can lead to a false negative result.

The external rotators and posterior scar tissue is best divided with a cutting diathermy; this keeps the operative field dry but also gives an early alert of the proximity of the sciatic nerve caught up in any scar tissue.
Only a cutting diathermy discriminates between capsule and rotators; we believe that it is increasingly important to be able to perform a capsular repair on closure to optimise function.
The femur is gradually internally rotated to tension the capsule and the femoral head becomes exposed.
Once the femoral head is exposed, three critical releases are performed;
the first is to make a linear cut in the fascia lata/ vastus down to periosteum just distal to lesser trochanter to prevent these tissues from restricting the femur in the wound.
The second manoeuvre is the inferior capsular release.
The assistant extends the leg and internally rotates whilst the operator passes the capsular scissors close to psoas tendon inferiorly cutting the capsule as far as the anteroinferior acetabular wall. This can be performed under direct visualisation.
The rotators are most efficiently divided with the hip slightly extended and the leg internally rotated.
The tissues should only ever be divided longitudinally/horizontally and never vertically.( Caveat Sciatic nerve)

The femoral head is dislocated by adducting and internally rotating the femur.
Capsular scissors are then used to release the antero-inferior and antero-superior capsule in much the same manner as described in the Birmingham Hip Resurfacing exposure.
This release is the most critical and harm can be done !
The assistant sweeps the leg across so that it is flexed at the hip to almost 90 degrees and fully internally rotated with the tibia perpendicular to the floor.
The surgeon needs to stand directly in line with the femur and place the capsular scissors with tines either side of the anterosuperior capsule aiming towards the medial epicondyle- away from the femoral vessels. The assistant will feel the tension change as the capsule is cut.
This manoeuvre must be undertaken with great care and allows for critical anterior retractor placement and displacement of the femoral head when preparing the acetabulum. Poor execution of this step is the commonest cause of failure to achieve an adequate exposure.

Stability of the femoral component is assessed and the femoral head and necked are swept forward anteriorly and held in a retracted position by a Hohman retractor positioned at 10 o’clock in the acetabulum for a left hip (2 o,clock for right)
A Judd nail is then placed superiorly in the ilium at the 12 o’clock position to reveal the acetabular component.

A further Judd nail is positioned in the ischium and a broad retractor placed inferiorly under the ‘transverse acetabular ligament’ if present or inferior margin of the acetabulum close to the obturator foramen.; this gives circumferential exposure of the loose cup.
Rongeurs are used clear soft tissue debris from around the cup and delineate its bony fixation.

In revision cases cup fixation is often feeble and simple leverage with a curved instrument such as a curved Moreland osteotome is sufficient to remove the cup. Fibrous ingrowth however can have an extremely tenacious hold and care should be taken in over exuberant force in removing a cup where there is medial wall deficiency. Traction and tearing of the pelvic vessels is a very serious complication.
In cases where bony ingrowth is more impressive a specialist device such as an Explant blade is helpful and safe.

In such cases the cup is usually loose and often has migrated medially; extreme care should be given in mobilising the cup as vessels may be adherent to its medial wall. Occasionally the cup is partially ingrown and a curved Moreland osteotome can be used to separate bone from implant. More usually the there is fibrous ingrowth and the cup should be separated with judicious traction and sharp dissection.
In cases of cemented revision cups, when the medial wall has been breached, there is usually a fibrous layer adherent to the cement. Similarly, judicious traction should be applied to the cup and the fibrous tissue gently dissected off the cement mantle.

A soft tissue debridement is performed; samples can be sent for microbilogy and histology to exclude infection. The extent of the pelvic deficit can finally be appreciated.

Fibrous tissue debridement of the interfaces is undertaken. Cognisant of the the implant diameter, a reamer 3-4 mm smaller is placed into the deficit and used to remove fibrous tissue and define the bed for the custom shell. The cavity is reamed 1mm less than the designated implant and the sterile trial can be used to assess primary fix and orientation.
Often the deficit is irregular and morcelised allograft or bone subsitute can be used a filler.
It cannot be overemphasised that excessive reaming medially is dangerous and that over reaming peripherally may jeopardise implant fixation and stability.

Once the acetabulum has been prepared, the quality, postion and quantitity of residual bone can be appreciated and at this point it is helpful to compare to the 3D printed model.

The custom cup is mounted on its specific introducer via a polar screw thread and orientated with the iliac flange superiorly.

The cup is introduced up to the prepared acetabulum. It is helpful to approximate the iliac flange first with a rather open or vertical approach angle and then by lowereing the introducer to engage the pubic and ischial flanges. It is rare for this to be a perfect fit first time and small ostomes and rongers can be used to fine tune the recipient bony beds.

The cup is then offered up to the deficit and once a satisfactory orientation has been achieved, it is hammered into place; usually there is a secure pressfit but occasionally, downward pressure with the introducer is required until there is secure screw fixation.

Screws are inserted as for any regular cup but in this case, the model has indicated ideal screw length and orientation.
The screw lengths tend to be longer than for regular cups. The superior screws can be 60-80mm in length, however when using longer screws one has to be certain that the threads remain in bone and do not breach pelvis or encroach upon the scaiatic nerve posterosuperiorly.
The pubic and ischial screws sometimes have a less convincing fix but the flanges alone prevent medial migration and rotation.
These screws tend to be 25-40mm in length.

Whilst screw lengths may be predetermined by the model, it is sage practice to measure screw depths individually.

The cup is usually secure with three or for robust screws but in these cases maximal fixation is desirable so whilst it is not necessary to use all available screw holes, multiple fixation points offer greater security in the medium term.
The stability of the implant can be assessed by reattachment of of the cup introducer and gentle toggling.

In this particular case it can be seen that the posterior column is intact and the posterior rim is providing a snug fit with the implant giving good primary stability.
At this stage an appreciation of cup orientation can be made. The main purpose of the cup is to provide fixation; the dual mobility component provides fine tuning of orientation and stability in a way that a fixed bearing insert may prove deficient.

The bearing surface orientation issue is solved by utilising a Dual Mobility (DM) Cup cemented into the reconstruction shell. DM cups offer significant stability and allow for some margin of error in terms of cup orientation which is sometimes compromised in complex cases.
The DM cup size selection should allow for a cement mantle of not less than 4mm. The cup size will then determine the interposing highly cross linked polyethylene (HXLPE) head size in a similar manner to bipolar hemiarthroplasty.

The SERF Cup has a lever arm introducer whereby closure of the lever compresses and expands a plastic insert which grips the inner surface of the metal shell of the SERF Cup. This provides good control when offering the component up to the cement filled reconstruction cup.

Medium viscosity cement is placed into the custom cup in doughy form and pressurised using a conventional acetabular pressuriser.

The Serf Cup is then offered up to the cement on the introducer and pressurised in a position of approximately 40 degrees closure and 2o degrees of anteversion.

The DM outer cup shell is cemented in situ with medium viscosity cement ensuring and even cement mantle. The cup introducer affords good control and allows perfect component orientation.

Hitherto, no explanation of the femoral component has been given. In the majority of cases performed to date, the femoral component has been stable and no revision has been required.
The SERF system can accommodate 28 and 32mm heads. It is usual to revise the head and exchange for a compatible virgin 28/32 head.
The HXLPE ball is compressed onto the modular femoral head; it is essential that a definite ‘end-click’ is felt and that the two components rotate freely.

The construct of virgin head and poly ball is inserted onto the femoral trunnion and the poly ball is reduced into the dual mobility shell and an assessment of stability, range of motion and leg length is made.
This is most easily done with a femoral head pusher. Assessment of leg length is made by tension, leg marker suture and equality of flexed knees. Prior to definitive reduction, trials are available and leg length can be adjusted by altering the head length..

Once the hip is reduced, a conscious effort is made to identify the sciatic nerve through its course so that a secure capsular closure can be safely performed. The nerve should already have been identified taking the steps as described at the start of the procedure. No suture should be placed in the exterbal rotators/posterior capsule/scar tissue unless there is confidence that the nerve is well clear.

Closure is in layers; I initially close the rotators/capsule with a running locked looped PDS suture; I perform this in two layer for extra security, initially suturing from cephalad to caudal in the foirst layer and then in the reverse direction for the second layer.

Routine closure of the wound in multiple layers is performed.
The fascia lata is closed with a loop Ethilon to avoid fascial hernias, then the fat is closed with two layers.

Skin clips and dressings are finally applied.

The preoperative image shows osteolysis around the left acetabular cup with failure of fixation, a breached medial wall and cavitary lesions.

The post-operative xray is taken and the patient mobilised with appropriate protective weight bearing.
The custom cup has good bony contact and numerous point of secure screw fixation. The hip centre is correct and leg length preserved.