Technique of Modular Stem: S-ROM
History
The S-ROM stem was developed by Hugh Cameron taking some ideas from the Sivash hip prosthesis. This was developed in Russia in 1956 by Konstantin Sivash; it was the first titanium stem with a cobalt chrome head. It was a constrained metal-on-metal implant with a sleeve locking on the stem. It was available for cemented and cementless use.
In 1971, US Surgical Corporation obtained license to manufacture the Sivash hip in the US. Douglas Noiles was the Chief of R & D. In 1975, an evolution of the Sivash hip called SRN was produced; the changes included the polyethylene liner instead of the metal-on-metal and the longitudinal flutes to prevent rotation. SRN was intended for cementless use only. In 1982, US Surgical sold the Sivash hip to a company called Joint Medical Products in which Hugh Cameron contributed to the modifications of the SRN. Finally, after a few more changes, (the shape of the sleeve, the coronal slot in the distal stem) the S-ROM was finalized in the present version in 1984 and it remarkably stays unchanged on the market for 26 years. The “S” in the name is a tribute of Dr Cameron to Dr Sivash for the ideas derived from the Sivash hip.
In 1996, Johnson and Johnson Orthopaedics acquired Joint Medical Products and included the S-ROM in the hip portfolio.
Many unique ideas of the S-ROM stem could not be copied for 20 years since they were patented.
Description of the Implant
The S-ROM stem is made of one piece of forged titanium. The distal part is cylindrical with 6 longitudinal sharp flutes that engage in the inner cortex to achieve rotational stability. In the proximal part the stem is conical in shape with a Morse taper to lock on the proximal sleeve. There are different neck designs: the standard in the 30 mm, 36 mm and 42 mm lengths, the increased off-set (+4,+8, +12 mm) and the so called calcar replacement for situations where the calcar is generally damaged or missing (Fig. 24.1). This is available for all the lengths of the stems and adds 21 mm in the metaphyseal region, practically moving the sleeve 21 mm more distal. The calcar replacement versions have the option of using a trochanteric washer with a screw fixing the washer to the metaphyseal part of the prosthesis; this is useful in case of trochanteric osteotomy to reattach the trochanter (Fig. 24.2). In spite of the fact that some plastic pegs are inserted among the threads of the screw to prevent it from backing out this can still occur so the head of the screw has some holes to pass a 0.9 mm cerclage wire to lock the screw to the bone or to the prosthesis. This has proven effective in my
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Total Hip Arthroplasty
Figure 24.1: Various neck designs for S-ROM Figure 24.2: Calcar replacement prosthesis with trochanteric washer
Figure 24.3: Surface finish of sleeve has steps to reduce shear stress
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experience and since I have used the cerclage wire, I have never experienced backing out of the screw. Each stem has 10 different sleeves with different sizes and in terms of thickness of the material and shape of the calcar triangle. Apart from the size, the sleeve can be locked on the stem in 360° rotation so as to compensate for any version abnormality in primary and DDH or to fit the best available bone in revision situation (very often in revision situation the lesser trochanter is the best place where to place the triangle of the sleeve).
The surface finish of the sleeve has steps to reduce the shear stress and a porous coating of a single layer of sintered beads (Fig. 24.3).
The S-ROM was originally developed as a primary stem and therefore it was available only in the standard length ranging from 115 mm for the diameter of 6 mm to the 175 mm of the 19 mm diameter (21 mm diameter is available on request). Once the good results in primary situation were seen the S-ROM stem was then also used in revision situation. This necessitated expansion of the range of sizes to longer stems. A long, Xlong and XXlong stem were developed. The long stems are available in straight and bowed version, Xlong and XXlong stems in bowed version only.
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Indications
The S-ROM stem can be used in primary THR with the advantage of excellent proximal fixation avoiding distal fixation (that makes revision surgery difficult) and the ability to optimize the version to achieve the best stability and range of motion. It can be easily removed for any reason and has an outstanding track record after 26 year follow-up.
Technique of Modular Stem: S-ROM
The above mentioned characteristics are even more valuable in complex primary, DDH and revision situations.
Surgical Technique: Primary
The surgical technique in primary hips is made of four steps. After performing the neck osteotomy which is generally made in an “L” shaped fashion (Figs 24.4 and 24.5); a classic oblique cut can also be made without any functional consequence but leaving the tip of the calcar uncovered; this can be found cosmetically not ideal. The femoral canal is then opened with a round box chisel and progressively reamed until the reamer is cutting into the inner cortex. Since most of the femurs are oval in shape referencing only to the AP X-ray generally leads to undersizing. Reamers come in 0.5 mm increments; generally 1 mm increments are used while reaming unless the bone is particularly hard. In this case 0.5 mm increments are used to avoid overheating of the bone (Fig. 24.6).
Once the distal diameter is established ( remember to over ream the proximal half of the femur by 0.5 in normal bone and 1.0 mm in hard bone to avoid an excessively tight fit in the mid-stem region), the second step is the conical reaming of the metaphysis (Fig. 24.7). For historical reasons the conical reamers are named B, D, F from the smaller to the larger (originally there were A, B, C, D, E, F cones with 1 mm difference from one another. This was then practically found unnecessary so the A, C, E cones were discontinued). The B reamer is always used first. Then the surgeon must check the anterior part of the reamed metaphysis to see if all the cancellous bone has been removed. In most primary hips, this is the case. If there is still cancellous bone in that area the D reamer and, very rarely in primary situations, the F reamer can be used. Once the proximal size is determined with the conical reaming, the third step is to prepare the triangle in the calcar. This is made with a specific instrument called calcar miller (Fig. 24.8). When the appropriate reaming is made, the surgeon can read
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Figure 24.4: Neck osteotomy—Intraoperative L-shaped Figure 24.5: Model: Neck osteotomy for S-ROM
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Figure 24.6: Canal reaming
Figure 24.7: Conical reaming of the metaphysis
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Figure 24.8: Calcar milling
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Technique of Modular Stem: S-ROM
Figure 24.9: Impaction of trial sleeve
Figure 24.10: Trial prosthesis in place
on the instrument the size of the sleeve to be used (remember that there are two different calcar shapes available for the B and D sleeves named small and large and three sizes available for the F sleeve named small, large and extralarge). At this stage, the fourth and final step is made, i.e. the trial reduction. The trial sleeve of the selected size is placed in the bone and gently tapped (Fig. 24.9). Then the trial prosthesis is assembled using the distal pilot of the size the canal has been reamed, the body trial and the neck of the selected length (generally a 36 mm neck is selected in most cases) (Fig. 24.10). The version can be selected on the trial implant before introducing it into the sleeve or once already implanted. During the trial reduction the surgeon must check the stability, (Fig. 24.11: for posterolateral approach with hip in flexion, adduction and internal rotation) the soft tissue tension, the range of motion and the absence of impingement. At this stage all the desired adjustments can be made change in the version, off set change, changing neck length or head length, or changes in the vertical drop, changing the sleeve size (bigger sleeve for less vertical drop and vice versa). If the smallest sleeve available is the one used and a more distal implant position is desired, the conical reaming has to be redone. If the large or extralarge triangles were used one can
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Figure 24.11: Confirming anteversion of the trial implant
Figure 24.12: Impaction of final sleeve
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Figure 24.13: Implantation of final stem with same anteversion as that of trial
Figure 24.14: Confirmation of final anteversion
use a smaller triangle without rereaming the calcar. If the small triangle was used then the calcar has to be reamed again.
When the optimal selection for the given case is made, the final sleeve is impacted in place (Fig. 24.12) and the stem is introduced into the sleeve (Fig. 24.13). Care should be taken so that the final stem is implanted with the same anteversion as that of the trial. There are laser marks on the upper part of the sleeve (one mark for every 20°) and a line on the medial part of the stem to refer to. Another option is to mark with the cautery on the calcar the position where the final stem has to rest. My favorite method, since I use the posterolateral approach, is to use the alignment jig of the S-ROM instrumentation introduced into the body of the prosthesis and to reference with the leg (generally between 15° and 25° are correct) (Fig. 24.14).
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Surgical Technique: DDH
Technique of Modular Stem: S-ROM
It is established that the proximal femur has different degrees of deformity in developmental dysplasia of the hip (DDH). The most common abnormalities include excessive valgus and anteversion of the femoral neck together with small femoral diameters. Since in the past many osteotomies have been performed to correct DDH and these hips later on have developed arthritis, the surgeon has to face virgin osteoarthritic DDH cases as well as DDH that have previously undergone an osteotomy. I will deal with these two conditions separately.
NONOPERATED DDH FEMURS
In femurs that have not been operated on different options for managing the anteverted femoral neck are available. The first option is to use a distally fixed implant with a narrow proximal part (such as the Wagner Conus) so that the deformed metaphysis can be ignored. This surgical technique is simple, and a relatively small inventory is needed. However, stress relief osteoporosis in the proximal femur will occur. Stress relief osteoporosis coupled with the difficult removal of the implant caused by distal fixation are a less desirable option than others, especially in younger patients who are more likely to undergo revision surgery during their lifetime. Different types of modular stems provide other options. In some types, the neck and the metaphysis are monolithic and are coupled with different distal parts. These stems cannot alleviate the complication of excessive anteversion because the placement of the metaphysis in the existing bone causes the prosthetic neck to replicate the pre-existing situation. If one of these stems is used, then a surgeon must perform a derotating osteotomy distal to the junction of the two components to reduce the anteversion and reconstruct a physiologic anatomy. Other stems have the ability to independently place the metaphyseal component and the neck. This can be achieved with interchangeable necks, with different versions fixed to the metaphysis, with an oval or with a round Morse taper that can be locked in different degrees of anteversion; sometimes an extra screw is used to lock the morse taper. It is unfortunate that both of these stems tend to be bulky and cannot be made small. If the stems were made small, the strength of the prosthesis would be compromised. In patients with DDH, the proximal femur is small and sometimes shows a reverse mismatch between the metaphysis and the diaphysis, with the metaphysis being smaller. For this reason, the ideal prosthesis for DDH must be small in the proximal part. The last option is a proximally modular prosthesis in which the neck and the distal part are one single piece and the metaphyseal component is locked on the stem with a Morse taper. The Morse taper can be impacted in the position to best fill the bone, allowing a surgeon to place the neck in the correct version. This system has many advantages. Because the load is transferred from the neck to the distal part, which is a monoblock, the metaphyseal sleeve is loaded only in compression and can be made thin to fit small femurs. (The S-ROM stem is available down to 6 mm distal diameter). With this type of prosthesis, an osteotomy is not needed to correct the anteversion unless a surgeon wants to reposition the greater trochanter (which might be too posterior) to the correct lateral position to regain a more physiological lever arm of the glutei muscles. Although most surgeons agree that this type of osteotomy is sometimes necessary, debate exists about how often they may be needed. In my experience, osteotomies performed to reposition the greater trochanter alone are rare.
The angular deformity that occurs in DDH is excessive valgus neck shaft angle (Fig. 24.15). An option for managing excessive valgus is a thin metaphysis distally fixed stem. Stems with a physiological shape at the calcar are unsuitable because the large metaphysis prevents the stem from seating properly and results in distal undersizing. A stem with modular sleeves of different shapes is a good option because a surgeon can select the most suitable shape for the sleeve and place the triangle in the best position (Figs 24.16 and 24.17). (In some patients the triangle can be inserted in the calcar anteriorly, or in the greater trochanter [Figs 24.18 and 24.19]). In the rare cases in which the triangle of the sleeve is placed in the greater
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Figure 24.15: Excessive valgus and reduced offset in a severely dysplastic hip
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Figure 24.17: Lateral view of the patient shown in figures 24.15 and 24.16. Modular stem takes care of metaphysio-diaphyseal mismatch
Figure 24.16: Anteroposterior radiograps of S-ROM used in dysplastic hip. Suitable shape of sleeve can be selected and seated in best position
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Figure 24.18: Deformed proximal femur with an arthritic hip joint
Figure 24.19: Patient in figure 24.18 treated with S-ROM. The triangle of the sleeve is placed in the greater trochanter
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Figure 24.20: Previously (valgus) osteotomized femur leading to excessive valgus
Figure 24.21: Patient in figure 24.20 treated with S-ROM. Sleeve was rotated to optimally fill the metaphysis
Technique of Modular Stem: S-ROM
trochanter the surgeon cannot use the calcar miller instrument because this would destroy the upper part of the trochanter and the muscular insertions at its apex. The technique to be used in these cases is, after the conical reaming, to create space for the triangle in the greater trochanter using a bone curette and then to use the sleeve introducer instrument without the pilot attached to it so that a “round the corner” movement can be done to achieve the correct alignment of the sleeve after the triangle has bypassed the apex of the greater trochanter.
PREVIOUSLY OSTEOTOMIZED DDH FEMURS
In patients with femurs that have been osteotomized, the two most common situations are the valgus osteotomy and the varus osteotomy. The valgus osteotomy creates an excessive valgus proximal femur (Fig. 24.20). It is possible to redo the osteotomy and place the proximal femur in the original position. However, this option is not usually chosen because it is not necessary and adds complexity to the procedure. The options chosen most often are straight stems with distal fixations or stems with modular sleeves. I prefer the latter for the possibility of rotating the sleeve to optimally fill the metaphysis (Fig. 24.21). Femurs that have had varus osteotomies are rare but more complex to manage (Fig. 24.22). After a varus osteotomy, the greater trochanter is moved more medially, which makes it difficult to implant a stem without damaging the greater trochanter and the insertion of the glutei muscles with a high probability of limping. Unless a surgeon is willing to accept a varus position of the stem (which is known to be a mistake) both in cemented and uncemented situations, he should use a cemented stem pushed laterally during the insertion after passing the greater trochanter to avoid varus positioning. It is unfortunate that dysplastic femurs are generally small and that cemented stems are often unsuitable. This is because leaving an adequate space for the cement mantle (2 mm circumferentially) causes the stem to become too thin to resist the stresses, resulting in too much flexibility. The thinness and flexibility of the stem puts the cement–implant interface at risk, increasing the likelihood of aseptic loosening. Another contraindication to the use of a cemented stem is that the holes of the plate and the screws generally prevent an adequate pressurization of the cement. The other option for femurs with varus osteotomies is to perform the osteotomy again and place the proximal femur in the original position. This automatically moves the greater trochanter laterally, enabling a surgeon to gain access to the femoral canal without damaging the greater trochanter and
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Figure 24.22: Femurs with previous varus osteotomies are more complex to deal with
Figure 24.23: Osteotomy and use of S-ROM stem restores biomechanics without damaging the greater trochanter
muscular insertion (Fig. 24.23). Performing this osteotomy usually requires cutting the femur slightly distal to the lesser trochanter and preparing the distal part in an antegrade fashion and the proximal part in a retrograde fashion after removal of the hardware (if this has not been performed in a previous surgery). Another option is preparing the metaphysis from above after dislocating the hip and cutting the femoral neck. Surgeons should remember to ream in a varus direction, cut the femur, and progress to preparing the distal femoral canal. I have tried both options using a cementless stem to perform a valgus osteotomy in femurs with previous varus osteotomy. My preference is in favour of antegrade reaming before performing the osteotomy because it is easier to handle the metaphysis when it is still attached to the distal femur. This also reduces the risk of fracturing the proximal fragment while reaming it retrogradely. This risk is likely to occur because the bone is weakened by the previous osteotomy and may be fractured by the pressure of the bone clamps. To obtain perfect congruency at the site of the osteotomy, a laterally based wedge should be removed. Unfortunately, this will shorten the femur that most of the times has already been shortened by the removal of a wedge during the first osteotomy. Thus, I prefer not to remove the wedge and leave a medial gap to be filled by bone formation. After having had one delayed union, I have used bone graft from the femoral head to enhance the healing of the osteotomy. I have not had delayed unions since then.
Shortening Osteotomy
Crowe Type III and IV hips are characterized by a high hip center. Femoral shortening is required to minimize the risk of nerve palsies (28% if you lengthen more than 4 centimeters) and to overcome the issue of soft-tissue contracture which will make difficult or impossible to reduce the prosthetic femoral head into the acetabular component placed in the true acetabulum (Fig. 24.24). It should also be understood that patients who have had a short leg for all their life are not going to be happy if the legs are made equal because they will feel the lengthened leg too long. If for example the pre-op leg length discrepancy was 6 centimetres leaving postoperatively the leg 2 centimetres short will probably be ideal allowing the patient to get rid of the heel raise and feeling the leg just right. It is very important to understand that the main structure preventing the femur from moving distally are the adductor muscles and not the glutei because in dysplastic hips the proximal femur is moved proximally but also posteriorly so that moving the femoral head to the true acetabulum (i.e. inferiorly and
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Technique of Modular Stem: S-ROM
Figure 24.24: Crowe Type IV hip. Femoral shortening is mandatory in this case
Figure 24.25: Femoral osteotomy is performed 1 cm distal to the distal end of sleeve, the preparation for which is done with the standard technique. Trial reduction of the proximal femur alone is done after putting trial prosthesisi without the pilot, ignoring the distal femur
anteriorly) will not imply considerable lengthening of the glutei. The modular S-ROM stem provides advantages when performing a shortening subtrochanteric osteotomy. Cemented stems are not suitable because cement will leak into the osteotomy during pressurization preventing the osteotomy from healing. Distally fixed stems by transferring load into the distal fragment and not loading the osteotomy in compression, have a reduced chance of rapid healing and will lead to proximal stress shielding in the long term. The S-ROM stem achieves rotational stability in the distal femur allowing compressive forces from the sleeve to force the proximal fragment against the distal fragment at the osteotomy site. The operation needs a complete capsulotomy to be done (the capsule is generally extremely hypertrophic) and preparation of the femur as in the standard S-ROM technique. Then you should perform the osteotomy 1 cm distal to the distal end of the sleeve, put the trial prosthesis in place without the pilot and do a trial reduction (Fig. 24.25) of the proximal femur alone ignoring the distal femur. At this stage, observing the amount of overlapping of the proximal and distal fragments (with your assistant applying a gentle traction on the distal femur) you
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Total Hip Arthroplasty
Figure 24.26: Amount of bony resection is decided by the extent of overlap between the proximal and the distal fragments
Figure 24.27: Prophylactic cerclage wires should be used when the bone is soft
decide how much bone to remove (if in doubt remove a smaller amount because you can always remove more later on (Fig. 24.26). Remember to re ream the distal fragment because the tip of the stem will be more distal than the first reaming you did from the top. It would be possible to ream 3 to 4 centimetres longer in the beginning but since there is always some degree of deformity in the femur it is preferable to do the final distal reaming after the osteotomy. Remember also to ream the distal femur at the exact diameter of the prosthesis you are implanting and not 0.5 mm more because you need perfect rotational stability. If the bone is soft a prophylactic cerclage wire both on the proximal fragment and at the proximal end of the distal one can be a safe option (Fig. 24.27). During the final implantation the surgeon should concentrate on the position of the neck in relation with the distal femur to give the correct anteversion until the flutes are engaged in the distal femur. At this stage, care should be taken to place the proximal fragment in the correct position (I suggest drawing a longitudinal line on the femur before performing the osteotomy as a reference or better using the saw with a thin blade to mark the bone) before the sleeve and the morse taper lock together.
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Surgical Technique: Revision
Although proximal fixation is not adequate for all revision situations in my experience (23 years and over 500 hip revision surgeries), more than 90% of the stem revisions can be managed with a proximally modular stem without distal fixation so as to avoid its drawbacks (difficult retrievability and stress shielding).
Technique of Modular Stem: S-ROM
The surgical technique in revision is identical to the primary if you are using a standard length stem. In case a calcar replacement neck is needed due to loss of the calcar region, the triangle is generally placed in the lesser trochanter (Fig. 24.28) with the sleeve placed in retroversion, not affecting the final anteversion of the implant. In cases where a long, X-long or XX-long stem is needed (the two latter are generally used only in periprosthetic fractures— Figs 24.29 and 24.30) the femur should be reamed using a flexible reamer (Fig. 24.31). In case a periprosthetic fracture occurs in a femur where an S-ROM stem was implanted, the surgeon does not have to remove the ingrown sleeve but he can remove the standard length stem and change it to a longer one to nail the fracture. Since the bow of the femur and the bow of the implant cannot be always identical it is advisable to overream the distal femur of 1 mm if the bone is soft and of 2 mm if the bone is hard to reduce the risk of cracking the femur or having premature locking of the stem during implantation. Fortunately, the coronal slot of the S-ROM stem helps in accomplishing small differences in the bow between the femur and the implant (Figs 24.32 and 24.33).
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Figure 24.28: When a calcar replacement prosthesis is used, the sleeve should be placed in lesser trochanter
Figure 24.29: Periprosthetic fracture with standard S-ROM stem in situ
Figure 24.30: Periprosthetic fracture of Figure 24.29 treated with an X long stem leaving the sleeve unchanged
Figure 24.31: Flexible reamers should be used when long, X-long or XX long stem is needed
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Figure 24.32: Coronal slot in S-ROM is able to accomplish differences in the bow between the femur and the implant
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Figure 24.33: Coronal slot in S-ROM Figure 24.34: Failed fixation forces the stem in varus and retroversion
Surgeons should have an understanding of the proximal femur deformation that occurs as a result of failed implant fixation, regardless of the cause of failure of the previous stem (cemented or uncemented). The load on the femoral head and the lever arm of the femoral neck force the stem in varus and retroversion (Fig. 24.34). Until bone implant fixation is effective, the proximal femur maintains its original shape and resists stresses. When the fixation fails and micromotion occurs, a variable degree of progressive bone deformation follows the migration of the prosthesis. In revision surgery, the difficulty of implant removal and bone loss become more challenging as a result of the abnormal shape of the proximal femur.
Both the varus and retroversion deformities influence the strategy that will be adopted during revision surgery. The varus deformity prevents a surgeon from using a long stem that reaches or bypasses the isthmus. Using a long stem is often mandatory because proximal bone loss dictates purchasing fixation more distally. Even if proximal bone loss does not require distal fixation, a surgeon should resist the temptation of using a short stem to prevent the varus deformity from progressing after the implantation of the new stem. A surgeon has two options for overcoming the varus deformity. The first option is to undersize the implant. Undersizing is suitable if the deformity is mild and the surgeon is willing to accept a non canal filling implant, for example when using fluted stems that can obtain rotational stability with a three-point fixation. If a cylindrical fully coated stem is used, then scratch fit and full canal fill at the isthmus is mandatory to obtain rotational stability; in this situation undersizing should be discouraged because it would endanger the stability of the implant. The amount of undersizing can be reduced by taking advantage of the flexibility of the femur, especially in
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Figure 24.35: Failed implant with varus deformation of the femur
Figure 24.36: Underestimation of varus deformation of the femur can result in fracture or perforation at the time of implantation
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Technique of Modular Stem: S-ROM
Figure 24.37: Varus deformity with failed implant Figure 24.38: Corrective osteotomy with S-ROM stem
the proximal part where the cortical bone is thinned by the failure of the previous implant. Two risks may occur as a result of this technique. The first risk is that the tip of the stem can cause too much pressure on the lateral cortex when the new stem is inserted, especially where the tip of the failed stem formerly rested and where the bone is weakened; this may result in a femoral fracture or a perforation. The second risk is that the stress caused by excessive deformation can lead to subsequent femoral fracture when the patient starts to load the operated limb. This may occur even if the implantation has been completed successfully (Figs 24.35 and 24.36). A second option for overcoming the varus deformity is to perform a straightening osteotomy (Figs 24.37 and 24.38). A straightening osteotomy should be performed at the apex of the deformity, sometimes 2 cm or 3 cm more proximal, to preserve a longer distal fragment. This technique relies on the axial and rotational stability guaranteed by the long stem into the distal femur. Some degree of rotational stability can be
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Figure 24.39: Transverse femoral osteotomy
Figure 24.40: Cement removal after osteotomy
obtained with an oblique or a step-cut osteotomy. Unfortunately these techniques make the reduction more difficult at final stem implantation. For these reasons, I suggest performing a transverse osteotomy and using an S-ROM stem that gives excellent rotational stability due to the flutes on the distal part of the stem. Osteotomies can be performed for reasons other than straightening the femur. They can also help to remove the cement or enable trephining the distal part of an ingrown stems after having cut it. If the cement removal is the purpose of the osteotomy, then the stem should be removed first. Then the surgeon should remove the proximal cement using a chisel, a high-speed burr, an ultrasonic instrument, or a combination of these. After the osteotomy is performed, the remaining cement can be removed in a retrograde fashion in the proximal fragment (Fig. 24.39) and in an antegrade fashion in the distal femur (Fig. 24.40), facilitating this part of the surgery and reducing the likelihood of femoral perforation. Another advantage of the transverse osteotomy is the preservation of vascularity of the femur, which accelerates the healing of the osteotomy. Vascularity is preserved because the muscles are not stripped away from a large part of the femur. When a transverse osteotomy is performed, the stem length is of paramount
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Technique of Modular Stem: S-ROM
Figure 24.41: Prophylactic wiring on either side of osteotomy
importance. Although, it is said that the stem should advance beyond the osteotomy at least 2 femoral diameters, I prefer to advance more than that. Even if the distal part of the stem enters the femoral flare without cortical contact, a longer stem guarantees a lower risk of fracture when a patient loads the femur excessively before the osteotomy has healed.
In terms of cerclage wires at the site of the osteotomy, a surgeon can place a prophylactic wire on one or both sides of the osteotomy, (Fig. 24.41) the distal one being more important due to the high-stress concentration, or place the wires after implanting the stem in case of fractures. Cerclage wiring after the fracture has occurred may not be as effective as prophylactic wiring and sometimes requires removal of the implant to obtain a satisfactory reduction of the fracture. The challenge of managing the deformation in retroversion of the metaphysis is generally not as important as managing the varus deformity. The deformation of the metaphysis is often not too evident due to the proximal bone loss but has an important role in determining the implant a surgeon will use. If a short stem is used, then the options are a distally fixed implant with a narrow proximal part (such as the Wagner Conus), which will allow for the deformed metaphysis to be ignored, or a proximally modular implant in which the neck of the prosthesis can be inserted in the correct anteversion regardless of the shape of the bone. If proximal bone loss dictates the use of a long stem that bypasses the isthmus, then multiple options are available. If a straight stem is used, then the canal cannot be filled unless an osteotomy is performed. However, performing an osteotomy will cause the loss of the natural bow of the femur, and the knee will have a recurvatum of 5° to 7°. Although these complications are well-tolerated, they can be avoided by using a bowed stem. Even if the S-ROM stem in the long version is available in both straight and bowed versions I have always used the bowed version only and I suggest doing so to avoid causing recurvatum at the knee. The potential advantage of a straight stem is that it can be rotated in any degree of anteversion because no fixed relationship exists between the neck and the bow of the distal part of the stem; this advantage is more theoretical than real since the prosthetic neck anteversion should always be in the 15° to 25° range which is obtainable also with a bowed stem. To overcome this challenge, modular stems were developed in which the neck, metaphysis, and distal part are independently adjustable from each other. It is unfortunate that the locking mechanism of these devices are Morse tapers, which work not only in compression which is highly desirable but also in flexion and cannot rely on positive locking. To alleviate this complication, a screw was added to force the two Morse tapers in compression. The addition of the screw causes the proximal part of the prosthesis to become
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bulky (this is generally not an issue in revision surgery because the missing bone provides enough space for the prosthesis due to bone loss) and bone fragility, as indicated by the failures of devices such as the ZMR Hip System-XL (Zimmer Inc, Warsaw, Ind). Straight stems rely on flutes for rotational stability and are cylindrical in the proximal part to ignore proximal deformity. If a bowed monoblock stem is used, then it has a fixed relationship between the plane of the bow and neck anteversion (generally 15°). If these stems are designed to have a degree of proximal fixation, then the metaphysis cannot fill the proximal femur unless the stem is put in retroversion. Retroversion will force the distal bow in a different plane from the bow of the femur, causing premature locking and making fractures more likely to occur. If the stem used is proximally modular (I consider this option to be an advantage in this situation), then the metaphyseal component should be free to rotate independently from the neck and the distal part so that it can be placed in the most favorable position to fill the metaphysis. In practice, when a long S-ROM stem is used the sleeve is not implanted in the bone before the stem as when a short stem is used but it is loosely introduced on the femur and both are advanced into the femoral canal, the sleeve is kept in the correct orientation and finally driven against the bone by the stem once the Morse taper between the two is locked. The metaphyseal component should also be free so that a seal can be provided to prevent particulate debris from migrating distally into the femur and causing osteolysis. Unfortunately, in case of severe and asymmetrical bone loss this is not always possible. Stems in which the neck and the metaphysis are made of one piece and not free to be adjusted independently are unsuitable in proximally deformed femurs because they will cause the neck to be placed in retroversion.
In conclusion, understanding of bone deformation that occurred during the failure of the old implant, as well as accurate preoperative planning is important in performing faster, more effective revision surgery that leads to fewer complications. The surgical approach, implant and cement removal, implant selection, management of bone loss, bone preparation and implantation as well as soft tissue repair require similar knowledge and preparation.
Bibliography
-
Bono JV, McCarthy JC, Lee JA, Boston RN, Carangelo RJ, Turner RH. Fixation with a modular stem in revision total hip arthroplasty. J Bone Joint Surg Am 1999;81:1336.
-
Cameron HU. Failure of a long stem press fit hip with no proximal load transfer. Contemp Orthop 1991;22:437.
-
Cameron HU. The two- to- six-year results with a proximally modular noncemented total hip replacement used in hip revisions. Clin Orthop 1994;298:47.
-
Cameron HU, Bhimji S. Early clinical trials with a proximally fixed uncemented hip stem. Contemp Orthop 1988;17:344.
-
Cameron HU, Jung Y, Noiles DG, McTighe T. Design features and early clinical results with a modular proximally fixed low bending stiffness uncemented total hip replacement. Scientific exhibit. AAOS Annual Meeting, Atlanta, Georgia, Feb. 4-9, 1988.
-
Chandler HP, Ayres DK, Tan RC, Anderson LC, Varma AK. Revision total hip replacement using the S-ROM femoral component. Clin Orthop 1995;319:130.
-
Christie MJ, DeBoer DK, Tingstadt EM, Capps M, Brinson MF, Trick LW. Clinical experience with a modular noncemented femoral component in revision total hip arthroplasty: 4 to 7 year results. J Arthroplasty 2000;15:840.
-
Clark JM, Freeman MA, Witham D. The Relationship of Neck Orientation to the Shape of the Proximal Femur. J Arthroplasty 1987;2(2):99-109.
-
Gruen TA, McNeice GM, Amstutz HC: “Modes of failure” of cemented stem-type femoral components: a radiographic analysis of loosening. Clin Orthop 1979;141:17.
-
Marega L, Ferrero G, Feroldi G. Clinical experience with the non-cemented modular S-ROM prosthesis for primary and revision surgery; results at 2-6 years. Italian J Orthop Trauma: 1998;14 (issue 1).
-
Ohl MD, Whiteside LA, McCarthy DS, White SE. Torsional fixation of a modular femoral hip component. Clin Orthop 1993;287:135.