Introduction                       

Short stem designs offer a conservative option for total hip arthroplasty which is bone and soft tissue preserving, in young and active patients with disabling hip arthritis.

The concept of neck preserving total hip arthroplasty is credited to Michael Freeman. Similar concept was proposed by Leo Whiteside and Townley who advocated neck preservation with conventional stems. Whiteside et al in a cadaver study showed that preservation of minimum 50% or more of the neck helps in resisting torsional forces in a proximally fixed stem.1 Jasty et al in a canine study of proximal fitting stems with polished distal stem have shown the diaphyseal component of the stem provides initial stability but is not necessary for the long-term implant stability once the proximal ingrowth is achieved.2 This lead to the development of short metaphyseal anchoring stems.

Short neck preserving stems have been around since 1985 with the Mayo hip (Bernard F Morrey) having excellent long-term track record.3,4 Not all short stems are similar. Five designs are currently available. These include stems inspired by the Mayo hip (METHA, ESKA, NANOS), short, bulky but not neck preserving stems like Proxima, neck preserving curved stems (CFP: Pipino), shortened tapered stems (Taperlock Microplasty, Trilock) and the so called neck pods like the BMHR and Silent hip.

The role of short stems in total hip arthroplasty has been questioned. The obvious comparisons are with hip resurfacing and modern cementless tapered femoral stems with proximal load transfer. While hip resurfacing is bone preserving it can hardly be said as soft tissue preserving, usually the incision and extent of soft tissue dissection needed for exposure is more than that seen in conventional THA. Furthermore resurfacing is restricted to metal on metal articulation which has its own set of problems like hypersensitivity, ALVAL, problems of use in young women of child bearing age and concerns about elevated serum metal ions. Successful results have been documented with tapered stems with metaphyseal loading implants.5. The short metaphyseal fixing implants are bone preserving, they can be implanted with a minimally invasive approach and can be used with a variety of tribological couplings. As compared to the traditional femoral stems the short stem implants are more physiological,

i.e. proximal loading, do not give rise to thigh pain and are easy to revise to traditional hip replacement. Hence, the short metaphyseal loading implants serve as a bridge between hip resurfacing and standard cementless stems and are suitable in young arthritic patients with good quality bone.

METHA is a modular short stem which is based on the highly successful Mayo hip. The implant combines three important features namely, modular construction, metaphyseal anchoring and proximal loading with minimal stem size and circumferential coating leading

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to biologic fixation. It has all the features desired from a modern femoral implant namely conservation of bone, physiological loading of the neck and proximal femoral metaphysis, ability to restore hip biomechanics without lengthening the limb and amenable to less invasive surgical approach.

 

Design                         

METHA is a metaphyseally anchored, modular, uncemented hip implant. Similar to the Mayo hip METHA has a proximal wide trapezoid crosssection which enables the stem to achieve initial stability with preservation of cancellous bone. The implant consists of a conical double tapered wedge titanium stem, circumferentially coated with microporous pure titanium coating (plasmapore) with a thin layer of calcium phosphatedihydrate coating. The plasmapore is given a 20 μm thin calcium phosphate coating to enhance the osteoconduction and lead to early osteointegration of the stem thereby providing secondary stability.6 The distal tip of the stem is uncoated and is essentially to guide the implant along the dorsolateral cortex thereby enhancing the high initial stability. Biomechanical testing has shown that the predominant load transfer is in the femoral neck and proximal metaphysis. The stem tip does not have significant load transfer and serves just to enhance the axial stability of the implant.

The head and neck are modular. Nine neck adaptors made of cobalt chrome are available with neck shaft angles of 130, 135 and 140 degree and version angles of 0, 7.5 anteversion and 7.5 degrees retroversion for each angle. The stems are available in seven sizes from 0-6. The modular adaptors uncouple the head and neck from the stem position and help in optimizing the hip biomechanics to the individual patient.

 

Indications                        

The METHA stem is an attractive option in young and active patients with degenerative arthritis of the hip, avascular necrosis of the femoral head and as a conservative revision option in patients with failed hip resurfacing. The prerequisite is presence of good quality bone.

METHA is contraindicated in patients with poor bone quality. Short varus neck, excessive femoral anteversion and dysplastic hips are relative contraindications. Patients with failed osteosynthesis for fracture neck of femur with screws in situ, those with abnormality at the meta diaphyseal junction are at risk of subtrochanteric fracture and are best managed with standard implants.

 

Preoperative                   Planning                   

A standard anteroposterior radiograph of the pelvis with both hips and lateral radiograph of the affected hip with 15% magnification are needed to assess the center of femoral head, leg length, offset, level of femoral neck osteotomy and stem size. While planning for the neck osteotomy one must take into account the center of femoral head, leg length and also preservation of 5-10 mm of lateral neck cortex. The osteotomy must be at an angle of 50 degree to the long axis of the shaft. Once the osteotomy level is marked it is useful to measure the distance of the medial cut from the lesser trochanter to serve as an intraoperative guide. Templating will help in determining the size of the implant, in the AP plane in addition to neck filling the stem should achieve distal contact with the lateral cortex. In the lateral view the aim is to position the stem centrally with the tip touching the dorsal cortex.

The neck shaft angle is measured on the unaffected side when possible and this allows the surgeon to choose the corresponding neck adaptor for intraoperative trial reduction.

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Surgical                    Technique                    

ANESTHESIA AND ANTIBIOTIC PROPHYLAXIS

METHA: Short Stem THA

 

We perform the surgery usually under combined spinal and epidural anesthesia with epidural catheter for postoperative pain relief. One shot of first generation cephalosporin usually cefazolin are administered at least 30 minutes prior to the skin incision.

 

APPROACH

Any surgical approach can be used for metha implantation. The proximal neck cut and medial entry point make it amenable to minimally invasive anterior, anterolateral and posterior approaches. It is our practice to perform the surgery through an appropriate sized incision and posterior approach with the patient in lateral position.

 

PATIENT POSITIONING

The patient is positioned in the lateral position with anterior and posterior supports to maintain neutral pelvic position. The anterior post should be above the level of the ASIS to prevent hip impingement during trial reduction. Adequate padding under the opposite leg must be done to prevent peroneal nerve palsy.

Standard posterolateral skin incision is used. Gluteus maximus is split in the line of its fibers, usually it is necessary to release its insertion from the femur to facilitate anterior translation and also prevent excessive stretch on the sciatic nerve. Trochanteric bursa is incised and the short external rotators are identified. The plane between the piriformis and gluteus minimus is developed, adhesions between gluteus minimus and the hip capsule released and an anterosuperior pin is placed to retract the abductors. Short external rotators and hip capsule are released in a single flap close to the insertion on the femur. Radial cuts are made along the line of the piriformis and posterioinferiorly to retract the flap posteriorly to expose the posterior half of the acetabulum. Superior and anterior capsule are divided under vision.

The hip is dislocated and the vertical and horizontal offsets are measured. Provisional femoral neck cut is made and the acetabulum is exposed and prepared in the standard fashion. The femur is prepared after acetabular preparation.

The femoral osteotomy line is marked at 50 degree to the long axis of the femoral shaft and care is taken to retain at least 5 to 10 mm of cortical rim on the lateral side. The medial aspect of the cut is marked up from lesser trochanter as templated preoperatively. After the cut is made one should see an intact ring of the femoral neck circumferentially; this is critical for the implant stability (Figs 20.1A to C). Four common errors are encountered especially at the beginning of the learning curve. If the cut is too proximal it leads to varus positioning

 

 

 

 

Figures 20.1A to C: Intact circumferential cortical ring after femoral osteotomy and preparation. Note 5 mm of retained lateral neck cortex

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Figures 20.2A and B: Steep neck cut with exposed medial part of the prosthesis, this does not compromize stability

 

Figure 20.2C: METHA instrument set, note nine neck adaptors 130, 135 and 140 degree with

+7.5, 0 and –7.5 anteversion

 

of the stem and also increases the offset. The cut can be too distal in which case the intact ring is lost, especially on the lateral side and the stability of the implant may be compromised. The cut may be too steep (i.e. >50o causing medial aspect of the implant to be exposed in which case one can still safely proceed with metha implantation (Figs 20.2A and B). There can be accidental resection of the lateral cortical rim in which case conversion to a stemmed implant is advisable.

The instruments are shown in Figure 20.2C.

The first step includes entering the medullary canal using a canal finder (Fig. 20.3). The entry point is in the middle of the neck cut. The canal finder is advanced initially in a varus position till the lateral cortex is contacted after which the handle is straightened and advanced along the lateral cortex. It is important not to hammer the canal finder but to introduce it with hand using gentle twisting movements to avoid perforation of the cortex. Also care must be taken not to damage the lateral cortical rim while straightening the canal finder. A second canal finder with wider anteroposterior diameter is also available which can be used to aid in the neck preparation in patients with hard bone. There are dots marked from 1 to 6 on the canal finder which help in assessment of the approximate size of the stem to be used.

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METHA: Short Stem THA

 

Figure 20.3: Entry point is in the middle of the neck cut with the instrument introduced in varus unlike conventional femoral stem. Note the dots marked to indicate the size of the stem

 

The femoral preparation is done in a staged manner starting with the smallest sized rasp. The smallest available size is size zero which has been introduced recently. The real size zero stem is non modular and is available with a neck shaft angle of 135 degree and anteversion of zero degree. The rasp is introduced centrally into the femoral neck till it contacts the lateral cortex and the rasp handle is then gradually straightened to allow the tip to slide along the lateral cortex. Generally, curved short-stem rasps react differently than straight rasps. With increasing rasp size, valgus positioning may occur because the tip of the stem is guided along the dorsolateral inner side of the cortex and may shift with increasing size. Therefore, care should be taken not to lateralize the rasp during insertion. During the rasping process, the body of the rasp should never be inserted lower than the level of the osteotomy, which would promote valgus positioning of the rasp as well. Under load, the stem could then sink below the level of the lateral resection. To avoid this problem, several extra millimeters of the rasp should be left during rasping (Figs 20.4A and B). A routine check by fluoroscopy is recommended for rasp position with a trial head in situ (Fig. 20.5).

 

 

 

Figures 20.4A and B: Few millimeters of the rasp is left proud to prevent accidental valgus positioning of the stem

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Figure 20.5: Fluoroscopy picture for confirmation of stem positioning

 

Progressively larger rasps are used to compact the cancellous bone into the closed femoral neck. Final size is achieved when the rasp completely fills the femoral neck and the tip contacts the lateral cortex. Rotational stability is assessed at this stage and it is usually possible to rotate the leg with rotation of the rasp.

Trial reduction is initially done with previously templated neck shaft angle and neutral version. Nine different neck adaptors with three different neck shaft angles viz. 130, 135 and 140 degree and three version angles +7.5, 0 and –7.5 anteversion angles are available. The trial reduction is done to achieve optimal stability, leg length, range of motion and soft tissue tension.

The size of the real implant is decided based on the size of the final rasp. It is inserted manually and tapped into place. The stem normally follows the course of the rasp preparation. At the completion of insertion it is desirable that the implant is circumferentially covered by the intact femoral neck ring (Figs 20.6A and B). Occasionally the medial part of the implant may be exposed most commonly when the cut is too steep. In these situations if the lateral part of the neck is covered by the lateral cortex the implantation can be continued. The weight bearing needs to be protected for the first 6 weeks in these patients to allow the stem to achieve secondary stability. Sinking of the lateral part of the implant below the level of the lateral cortical ring is undesirable and indicates undersized prosthesis or excessive valgus position (Fig. 20.7). These stems are at risk of sinking postoperatively and this situation is best avoided.

 

 

 

Figures 20.6A and B: Ideal implant position with circumferential cortical ring cover

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METHA: Short Stem THA

 

Figure 20.7: Undesirable rasp position; note the lateral part of the rasp in below the lateral cortical rim, this indicates undersized stem or valgus position

 

 

 

Figures 20.8A and B: Modular cobalt chrome neck adaptor

 

 

The selected modular taper is then inserted in place with the marker arrow facing medially (Figs 20.8A and B). It is important to clean and dry the inner socket of the stem before insertion of the modular adaptor.

The bearing options available with this stem include metal on polyethylene, ceramic on polyethylene and ceramic on ceramic. Ceramic on ceramic is our preferred bearing in young patients.

 

Rehabilitation

 

Patients should use crutches for 4 to 6 weeks. We allow partial weight bearing as tolerated from day 1. Patients can return to work after 4 to 6 weeks and return to full activities, including sports, after 3 months. During the first 3 months, patients must avoid extensive joint motion so that, all muscles return to normal function. After 3 months, patients can walk, hike, and play sports such as tennis or golf.

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Figure 20.9: METHA stem; note conical shape, circumferential plasmapore with calcium phosphate dihydrate coating and distal uncoated stem

 

Discussion                        

The METHA prosthesis (Fig. 20.9) is modelled on the highly successful Mayo hip design and was developed following good mid and long-term results of this prosthesis. A straight and short geometry with a trapezoidal cross section is common to both designs. There are four differentiating features of METHA from the Mayo hip, the design is essentially neck preserving with more proximal neck osteotomy, it is more curved facilitating easier insertion, is a modular stem which allows optimization of offset without increasing leg length in each case and has a circumferential latest generation osteoinductive coating (plasmapore with calcium phosphate dihydrate) which leads to early osteointegration and prevents distal migration of wear debris which may be a concern with the Mayo hip. METHA represents an overall reduction in length and cross sectional area compared to the Mayo hip making it more bone conserving, however, requiring a very accurate reproduction of the neck osteotomy to a level which allows proper cancellous bone impaction and adequate torsional stability (Fig. 20.10).

Three main concerns with METHA stem are related to its modularity; risk of fracture at the conical taper stem junction, risk of fretting corrosion between the cobalt chrome taper and titanium stem and risk of pull out of the taper from the stem.

 

 

 

Figure 20.10: Well compacted cancellous bone, note the trapezoid cross section and retained neck which provide initial torsional and axial stability

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A few breakages have occurred in the first 4,000 stems implanted, due to failure of the neck-stem adapter made of titanium alloy.7 The breakages occurred between 13 and 18 months after implantation. The taper of the modular cone adapter is manufactured with a precision of some microns, which means that when it is fitted together, there is a process of cold welding. Laboratory testing has shown the strength of this modular connection goes far beyond the recommended standard loads, but this assumes a clean connection. Chemical and electron microscopy investigation of this junction in the failed cases showed the presence of biological material, from both bone and soft tissue debris, which had resulted in micromotion, accelerated wear, and fretting corrosion. Because of this finding, we now use the rasp, rather than the stem to check the head position. This allows clean assembly of the definitive stem and neck adapter on the OT table before implant insertion, with less risk of contamination of the connection. For increased safety, the adapter is now made of cobalt-chrome alloy with higher stiffness (less micromotion) and higher wear resistance.The new modular taper has been examined in dynamic testing to determine the fatigue resistance in accordance with ISO 14242 on servohydraulic test equipment in 0.9% NaCl solution.8 The fretting inside the junction was examined in dynamic conditions with constant high force of 3 Newtons over 10 million load cycles and pull out strength between the stem and taper were evaluated. These tests revealed very high fatigue strength, fretting corrosion similar to that between a femoral head and trunion in traditional THA and very high pull out resistance.8

 

METHA: Short Stem THA

 

Conclusion

 

METHA (Metaphyseal total hip arthroplasty) is an attractive option for young arthritic patients undergoing total hip arthroplasty. It combines all the features of a modern cementless implant namely bone conservation, modularity to allow optimal reconstruction of hip biomechanics without undue lengthening, loading of the proximal femoral bone in a more physiologic way and ease of revision. Meticulous attention to the surgical technique however is critical for successful outcome.

 

Illustrative                     Cases                     

CASE 1 (Figs 20.11A to C)

 

 

• A 29-year female patient, a known case of SLE on steroids presented to us with pain both hips, more on the right side for the last one year.

   

  Figure 20.11A: AP radiograph of the pelvis with both hips showing neck cut marked based on preoperative templating

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  Total Hip Arthroplasty

   

  Figure 20.11B: Postoperative AP radiograph of pelvis with both hips showing restoration of horizontal and vertical offset

   

  Figure 20.11C: AP and lateral radiographs postoperatively showing good implant position

   

  • Core decompression right hip was done 8 months ago following which she had symptomatic relief for 6 months. She developed sudden onset right hip pain and difficulty in walking two months ago.

  • Radiographs revealed collapse of the femoral head more than 50% of weight bearing area (Fig. 20.11A).

  • She was taken up for right total hip arthroplasty with plasma cup, METHA short stem and delta ceramic on ceramic articulation with 36 mm femoral head.

     

    CASE 2 (Figs 20.12A to E)

  • A 24-year-old female patient presented to us with severe hip pain for the past 2 years and difficulty in walking

  • She had past history of tuberculosis right hip 6 years ago for which she took antitubercular therapy for 15 months

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    METHA: Short Stem THA

     

    Figure 20.12A: Preoperative AP radiograph of pelvis with both hips showing adduction and external rotation deformity, the neck shaft angle on the normal side was 130 degree

     

    Figure 20.12B: AP and lateral radiographs of the right hip showing old healed tuberculosis

     

     

     

     

    Figure 20.12C: Intraoperative picture after acetabular cup implantation with delta ceramic liner 32 mm and femoral stem implantation with 130 degree, + 7.5 anteversion modular taper to optimize hip biomechanics

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    Figure 20.12D: Immediate postoperative radiograph showing good alignment, note that neck cut is too steep, however there is good lateral support

     

     

     

     

    Figure 20.12E: Follow-up X-ray after 6 months, Note ability to recreate offset and length identical to the normal hip

     

     

  • On examination the patient had fixed adduction and external rotation deformity of 10 degree and true shortening of 1 cm. Radiographs revealed advanced arthritis (Figs 20.12A and B)

  • She had fibrous ankylosis of the right hip with only painful jog of movements possible

  • She was taken up for right total hip arthroplasty using METHA short stem, Plasma cup and ceramic on ceramic articulation (Figs 20.12C to E)

  • Postoperatively patient was put on ATT for 6 months.

     

    References                       

    1. Whiteside LA, White SE, McCarthy DS. Effect of neck resection on torsional stability of cementless total hip replacement. Am J Orthop 1995;10:766-70.

    2. Jasty M, Krushell R, Zalenski E, O’Connor D, Sedlacek R, Harris W. The contribution of the nonporous distal stem to the stability of proximally porous-coated canine femoral components. J Arthroplasty 1993;8(1):33.

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    3. Morrey BF, Adams RA, Kessler M. A conservative femoral replacement for total hip arthroplasty. A prospective study JBone Joint Surg Br 2000;82(7):952-8.

    4. Morrey BF. Short-stemmed uncemented femoral component for primary hip arthroplasty. Clin Orthop Relat Res 1989;249:169-75.

    5. Swanson TV. The tapered press fit total hip arthroplasty: a European alternative. J Arthroplasty 2005;20[4 Suppl 2]:63-6.

       METHA: Short Stem THA

        

    6. Zmukler-Moncler S, Perrin D, Piattelli A, Scarano A. Evaluation of a soluble calcium phosphate coating obtained by electrochemical deposition: A pilot study in the pig maxillae. In: Davidovitch Z, Mah J, (Eds). Biological mechanisms of tooth eruption, resoption and replacement by implants. Boston: Harvard society for the advancements of orthodontics 1998.pp.481-5.

    7. Grupp TM, Blomer W. Modulare kurzschaft- Huftendoprothese-Untersuchungen der Belastungs stabilitat der konusverbindung. Implant 2005;1/05:9-12.

    8. Kaddick C, Wimmer A. Hip simulator wear testing according to the newly introduced standard ISO 14242. Pro Instn Mech Engrs 25, Part H 2001.pp. 429-42