Introduction                       

Total hip replacement has been termed as “Operation of Century” as it has revolutionized the treatment of patients with advanced hip disorders.1 An insight and awareness of evolution and design principles of hip arthroplasty is not only of historical significance, but also carries a clinical importance because it will improve the understanding of how to treat the patients in need of hip replacement.

The history of hip arthroplasty can be traced back to early 1800s,2,3 when resection arthroplasty was described. Since then, it has continued to evolve and is still evolving. Advances in bioengineering technology have driven the development of hip prostheses. Both cemented and uncemented hips can provide durable fixation. Better materials and design have allowed use of large bearings, which provide an increased range of motion with enhanced stability and very low wear. Minimally invasive surgery limits soft-tissue damage and facilitates accelerated discharge and rehabilitation.

 

Resection   (Excision)   Arthroplasty   (Fig.   1.1)          

Resection arthroplasty was first described for the treatment of infected hips and then used for all kinds of painful conditions around the hip. Schmalz first described this procedure in 1817 followed by White in 1821, both for tubercular arthritis of the hip.4 Between 1921 and 1945,5-8 GR Girdlestone refined the indications and technique for resection arthroplasty. He advocated the resection of the femoral head and neck along with a portion of the lateral aspect of acetabulum for the treatment of advanced tubercular and pyogenic arthritis of hip. Later he also recommended this procedure for unilateral hip osteoarthritis.

This procedure became famous as “Girdlestone Arthroplasty”. The encouraging results of Girdlestone arthroplasty led to its use as a primary treatment for the degenerative disorders of the hip in Europe before the development of total hip arthroplasty.

The surgical technique has been refined over the years. The current technique usually leaves greater trochanter and abductor muscles intact. The capsule is either excised or interposed. The femoral neck is usually osteotomized proximal to the intertrochanteric line. On the acetabular aspect, only osteophytes are removed. The cartilage of acetabulum is removed with reamers till bleeding cancellous bone is exposed. A complete debridement, irrigation and closure with suction drain complete the surgery.

Postoperative instability and shortening remain the major postoperative concerns. Several authors like Lorenz (1919), Schanz (1922), Batchelor (1948), Milch (1955), etc. described several osteotomies for providing stability to this procedure.9-13 The controversy over

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Total Hip Arthroplasty

 

Figure 1.1: Girdlestone arthroplasty on right side

 

whether to perform a concurrent or a staged osteotomy after resection arthroplasty has now been clearly overshadowed by the development of current technology of hip arthroplasty.9-13

Inspite of all modern hip arthroplasty technology, the hip surgeons might have to go back to this procedure occasionally. This is usually performed as a salvage procedure for infected hip arthroplasties. Resection arthroplasty when used as a salvage procedure for infected hip is marginally different, as the goal is to remove the implant, cement and all infected tissue.13 This resection arthroplasty is also used frequently as a first stage procedure for two-stage revision of infected hip arthroplasty.

 

Interposition Arthroplasty14,15

 

Thompson14 first reported the use of wood as an interposition material in temporomandibular joint, which was resected because of ankylosis. European surgeons employed the same technique for hip disorders and various materials, biologic and synthetic, have been described for this purpose. Corrective osteotomy of the proximal femur was also done simultaneously for fixed deformities.

Sir Robert Jones (1912) used gold foil as an interposition material. Murphy (1902) started the use of muscle and fascia as an interposition material. Campbell and MacAusland preferred to use fascia lata as an interposition material. However, all had uniformly unsuccessful results.

 

Cup (Mould) Arthroplasty (Fig. 1.2)15,16

 

Smith-Peterson gave the concept of mould arthroplasty in 1923. This concept was based on their experience in a patient, in whom a synovial tissue formed around an impregnated glass piece in the thigh. They tried putting a mould of inert material (glass) between the articular surfaces of hip joint as an alternative to interpositional membrane, which would guide nature’s repair to form smooth articulating surfaces. The implants could be removed once such surfaces were formed. As glass was too fragile, alternative materials like Bakelite and Pyrex were attempted, but they were not durable either.

Smith-Peterson began using Vitallium (an alloy of cobalt, chromium and molybdenum) cups and performed 500 cup arthroplasties between 1938 and 1948. It was like hemi-surface replacement of hip. Cup arthroplasty remained a procedure of choice for many years for patients with advanced arthritis of hip; however, it was not possible to correct bony abnormalities, shortening or manage the bone defects.

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Evolution and Design Principles of Hip Arthroplasty

 

Figure 1.2: Smith-Peterson cup arthroplasty

 

Double Cup Arthroplasty15,17-19 (Fig. 1.3)

 

This was the beginning of current surface replacement prosthesis. During 1950s, various modifications of cup arthroplasty were introduced. In 1953, Haboush reported two cases of double cup arthroplasty, in which two metallic cups were fixed with acrylic cement, one onto the femoral head and the other into the acetabulum. This was the perhaps the first described use of PMMA for hip arthroplasty. Townley introduced hemi-surface replacement (1952) and then total surface arthroplasty (1960) with polyurethane acetabular component. He started using polyethylene acetabular component in 1977 and termed it as Total Articular Replacement Arthroplasty (TARA). Numerous modifications have been attempted since then, particularly as an alternative to total hip replacement in young adults.

 

 

 

 

Figure 1.3: Smith-Peterson double cup arthroplasty, metallic femoral component and polyethylene acetabular component

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Total Hip Arthroplasty

 

Figure 1.4: Austin-Moore and Thompson unipolar femoral prostheses

 

Femoral Endoprostheses15, 20-22

 

Delbet used reinforced rubber as a replacement for the femoral head in 1919. The first endoprostheses of hip is credited to Hey Groves in 1923. After this, various types of endoprostheses were developed all over the world. AT Moore credited Bohlman for the use of a cobalt chrome ball fitted to a Smith-Peterson nail in 1939. In early 1940s, Moore and Bohlman replaced the upper end of femur in a patient with a malignant giant cell tumor using 12-inch stainless steel prosthesis. In 1946, Judet brothers used an endoprostheses with a femoral head of acrylic attached through an acrylic stem. Because of severe wear, the acrylic was changed to a metal (cobalt-chrome) alloy. In 1950, Thompson developed a short stemmed metal device that was termed as “light-bulb prosthesis”. It soon became apparent that the long stemmed devices generally were superior to shorter devices, which they soon replaced.

Moore and Thompson individually developed long stemmed metal prosthesis (Fig. 1.4), which began the era of hemiarthroplasty. Both were unipolar prostheses with long stems that allowed transmission of weight bearing forces along the axis of femoral stem, thus avoiding the high shear forces seen with short stems. The Moore’s prosthesis is fenestrated to allow bony ingrowth. Thompson prosthesis is smooth and is used with cement. Both these prostheses functioned so well that they are used with only minor modifications even

 

 

 

 

Figure 1.5: Bipolar prosthesis and its modular parts

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to this date. These prostheses initially designed for patients with hip fractures; however they were frequently used as an alternative to cup arthroplasty in degenerative hip arthritis before the advent of current total hip arthroplasty. The acetabulum was reamed to a concentric fit. These prostheses were later combined with various types of acetabular components and used as the femoral components of early total hip arthroplasties.

Evolution and Design Principles of Hip Arthroplasty

 

In an attempt to improve the prosthetic design, Mckeever and Collison employed Teflon lined metal cups placed over metallic femoral endoprostheses. The concept of current metallic bipolar prosthetic device was introduced by Bateman23 and Gilberty24 in 1974, with the belief that it would function without damaging the acetabular bone. The device consisted of a metallic cup with high density polyethylene that was locked onto the head of the femoral component (Fig. 1.5). This prosthesis conceptually enjoyed motion between metal head and polyethylene socket as well as between metallic cup and the acetabulum. Despite the initial enthusiasm for their use, many surgeons think that the bipolar endoprostheses have only little advantage over a simple unipolar endoprostheses.

 

Total          Hip          Arthroplasty                   

The origin of total hip arthroplasty dates back to 1890, when Gluck15 in Germany reported first total hip replacement with ivory femoral and acetabular components cemented to bone. Kenneth McKee modified metal on metal (MOM) arthroplasty in 1950s. He introduced cobalt-chrome alloy articulations. A metal acetabular component mated to the Thompson femoral endoprostheses (Figs 1.6 and 1.7) was used. Watson-Farrar modified the neck to reduce impingement.2 Their first series (1956-1960) had a high incidence of complications. The failures were due to poor designing and implant placement besides poor aseptic techniques. Ring from England developed first uncemented metal prosthesis, consisting of a metallic shell with screws into acetabulum that articulated with cementless Moore prosthesis.2 Tronzo2 modified the acetabular design by replacing screws with one large and three small prongs, which when driven into acetabulum, prevented rotation. After this several modification of this design were introduced. In 1969, the prosthesis was modified and a sintered stainless steel surface was employed, and thus the first true cementless hip replacement with potential of biologic ingrowth was designed.

Sir John Charnley (Fig. 1.8)25-27 is credited as the “Father of total hip arthroplasty” for ushering the era of modern hip arthroplasty. He introduced the concept of ‘low friction torque arthroplasty’ and ‘self curing acrylic cement’. He also gave the concepts of lubrication, materials, design, trochanteric osteotomy, asepsis and operating room hygiene. He also determined that the coefficient of friction of a steel ball against polytetrafluoroethylene (PTFE/

 

 

 

Figures 1.6A and B: McKee-Farrar (A) and Ring prosthesis (B)

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Total Hip Arthroplasty

 

Figure 1.7: Postoperative X-rays of early metal on metal hip prostheses

 

 

 

Figure 1.8: Sir John Charnley: Father of modern hip arthroplasty

 

 

Teflon) was close to that of a normal joint. In 1961, he realized that the best engineering practice would be to use a 22.225 mm diameter head to reduce the frictional torque. The procedure of cementing, use of 22.225 mm head, metal on polyethylene articulation and trochanteric osteotomy together form the central concept of ‘low friction torque arthroplasty’ (Figs 1.9 and 1.10).

The size of head is important as increasing the size leads to an increase in range of motion (Fig. 1.11). A 22 mm head provides 90° range of motion, while a 32 mm head provides 106° range of motion. The range of motion can also be increased by chamfering the acetabular cup or by decreasing the depth of the acetabular cup (Fig. 1.12).

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Evolution and Design Principles of Hip Arthroplasty

 

Figure 1.9: Evolution of Charnley’s prosthesis

 

 

 

Figure 1.10: The procedure of cementing, use of 22.225 mm head, metal on polyethylene articulation and trochanteric osteotomy together form the central concept of ‘low friction torque arthroplasty’

 

 

 

 

Figures 1.11A and B: Increasing the head size increases range of motion

Figures 1.12A to D: (A) Shallowing the acetabulum (B and C) and chamfering of acetabulum (D) also lead to increased range of motion 7

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Total Hip Arthroplasty

 

Figure 1.13: Cemented acetabular cups

 

Total hip arthroplasty has witnessed a great evolution in the field of acetabular and femoral components, both cemented as well as cementless designs. They can be discussed as follows:

CEMENTED ACETABULAR COMPONENTS 15, 28-30

There is now a great deal of experience going back more than four decades with cemented acetabular components mainly polyethylene. The cemented cups are thick walled polyethylene cups (Fig. 1.13). Metal backed cups are not preferred as they produce back side wear. The original Charnley cup had a peripheral flange that helped trap and pressurize the cement into acetabulum. Charnley further modified it by designing a thin extended flange that could be trimmed to meet the individual needs. Oh et al29 demonstrated that it is desirable to have a uniform thickness cement mantle and this can be achieved by adding 2.5 mm pods to the socket, which should be blunt tipped to prevent “hang-up” by catching on the acetabular wall at the time of insertion.

Vertical and horizontal grooves on the external surface increase stability. Newer designs with PMMA spacers typically 3 mm in height ensure a uniform cement mantle and avoid the phenomenon of bottoming out, which results in a thin or discontinuous cement mantle at the summit of the cup.

Some ceramic cemented cups are also available. The proposed advantage is that ceramic is a much better heat sink than plastic and therefore reduces the thermal trauma to bone and thus would liberate less wear debris. However, these cups have been shown to develop radiolucencies suggesting that the much stiffer ceramic creeps into the cement, especially superiorly.

Cemented acetabular fixation is satisfactory in elderly, low-demand patients because of simplicity and low cost. A cemented acetabular component is often used with an acetabular reinforcement ring in revision settings.

CEMENTLESS ACETABULAR COMPONENTS15,30,31

In an attempt to increase longevity and lessen the incidence of aseptic loosening, cementless cups were introduced. The following types have been described:

Press Fit cups

There are no pure press fit cups. Some additional fixation form is always present. Mathys hemispherical cups (Fig. 1.14) are such examples which have two plastic pegs superiorly

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Evolution and Design Principles of Hip Arthroplasty

 

Figure 1.14: Mathys hemispherical press-fit cup Figure 1.15: Spitorno hip system

 

 

that are driven into predrilled holes. Two metal screws through the rim of the cup are used for supplementary fixation. Ring’s oval cup is another example, which has a ridged peg over its dome. Both have shown good early results; however long term results (> 10 years) are not encouraging.

Spitorno cup (Fig. 1.15) is a metal backed press fit cup that has flexible metal spiked ribs that are driven into host bone. Spiked cups have also been described in which spikes vary from small rounded spikes to huge pagoda like structures. Some cups also have pegs, which are driven into predrilled holes.

Most of the current acetabular sockets (Figs 1.16 and 1.17) are ingrowth cups, which have some sort of surface or coating to facilitate bony ingrowth. Most of them are porous coated. They may be hydroxyapatite coated for bony ingrowth. They are available in various sizes. The liner may be of metal, polyethylene or ceramic. The outer diameter of liner matches that of shell and inner diameter varies according to the size of the femoral head. Most ingrowth cups are held in place with screws, usually 6.5 mm cancellous screws. They are useful, but fretting is a potential drawback. Because of this, some companies have come-up with pure press fit cups. However, press fit cups need precise and meticulous surgical technique. Some cups are also available with double radius, providing more stability; however, exact reaming required is difficult.

 

 

 

Figure 1.16: Modern porous coated press fit acetabulum Figure 1.17: Hydroxyapatite coated Omnifit cup

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Total Hip Arthroplasty

 

Figure 1.18: Mittlemeir threaded cup

 

Threaded Cups

The threaded cups of Mittlemeir (Fig. 1.18) and Lord have been used in Europe with quite acceptable results. Threaded cups can be divided into three broad groups:

1. Truncated cones: Conical implant with a flat base, e.g. Lord prosthesis

2. Threaded ring: Threaded hemispherical ring with a large apical hole, e.g. Mecron ring

3. Hemispherical shells: Majority have conical threads.

A search for additional fixation led to the development of hybrid, threaded porous cups, where threads provide initial fixation and later, bony ingrowth on porous surface provides long term stability.

 

FEMORAL STEMS

 

 

Femoral stems can be cemented or cementless. However, common to both are some design features (Fig. 1.19), which are very important to consider for a successful outcome:

 

Figure 1.19: Design features common to both cemented and cementless femoral stems

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Vertical height: It determines leg length. Vertical height increases with the length of neck, so proper reconstruction of the neck length is essential.

Medial offset: It is also known as Horizontal offset or simply “offset”. It determines the abductor function. Inadequate restoration leads to increased joint reaction force, bony impingement and dislocation, while excessive offset-stem can lead to fracture or loosening.

 

Evolution and Design Principles of Hip Arthroplasty

 

Cemented Femoral Stems15,25-27,31

McKee-Farrar and Charnley stems (Figs 1.6, 1.10 and 1.20) were initial cemented femoral stems. Although a lot of newer designs have come up, no long term comparative trials are available and Charnley’s cemented femoral stem still remains the standard. The Charnley stem had a 22.225 mm head articulating with polyethylene. It had a straight stem with a broad medial face. Muller disliked the idea of trochanteric osteotomy principle of Charnley and introduced a curved stem that was easier to insert. He also used a 32 mm head with the idea of achieving increased range of motion. He added a collar to his stem and also used three different neck lengths. It was triangular in shape and wedged in cross-section. This prosthesis did not do well in the long-term.

The Aufranc-Turner stem was similar to Muller’s stem, except the head was more undercut and the neck was oval in cross-section. The anteroposterior diameter of neck was 3 mm less than the mediolateral direction to allow more movement without impingement. Amstutz modified the Charnley stem by using 28 mm head, making stem thicker and undercutting the head considerably. Harris used 32 mm head and made cross-section of neck oval with a decreased anteroposterior diameter. He also designed a modular system with a coating of PMMA on proximal 1/3rd of stem for better bonding with cement (Figs 1.21A to E).

Certain design features of cemented stems have been found to be more successful. If we look at engineering principles, a cemented stem must have a broad medial border proximally and a broad lateral border distally. It must be wedge shaped in all planes. Designs with sharp edges are avoided as these may initiate fracture in the cement mantle. All stems are now straight distally but the degree of proximal curve varies. Excessive proximal curve should be avoided to prevent cement overhanging proximally and laterally, which may be problematic, if revision is required. There is no definite benefit of collared stem over collarless stem regarding subsidence or load transmission; however, a collar may be useful as an aiming device for determining version and as a stop point while inserting the stem. Noncircular cross-sections such as a rounded rectangle or an ellipse, with or without surface grooves or longitudinal slots improve rotational stability of the stem into cement mantle. Stems may be textured or polished smooth. Polished stems are preferred as they have been shown to have a longer life. As loosening usually occurs at cement-bone interface rather than stem-cement interface; porous coating or pre-coating are of limited use in cemented stems. They are also

 

 

 

Figure 1.20: From left to right: Original first-generation Charnley flatback, second-generation roundback stem, third-generation, flanged Cobra stem, triple taper C-stem

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Total Hip Arthroplasty

 

Figures 1.21A to E: Earlier designs of cemented femoral stems.

(A) Charnley (B) Muller (C) Aufranc-Turner (D) Amstutz

(E) Harris

 

 

difficult to revise. Also, polished stems produce mainly compressive forces as compared to shear forces towards bone cement interface in matted finish stem (Fig. 1.22). A variety of collarless polished stems like CPT and C-stem have been shown to have very good long term functional results. A primary cemented stem does not need to enter the bowed area of femur, so a bowed stem is not required and a straight stem is preferred (Fig. 1.23).

For cemented stems, almost any metal can be used. However modern designs are composed of high strength super alloys like cobalt chrome alloy, which has a higher modulus of elasticity and thus reduces the stresses within the proximal cement mantle and also the resulting incidence of fracture of bone cement. The cemented stems may be monoblock or modular. Morse-Taper lock concept allows for modular heads. It is usually a 3 degree taper. Various taper sizes are available, 12/14 is standard.

 

Cementless Femoral Stems15,20,21,30

As previously discussed, Moore’s prosthesis was the original cementless femoral stem. Later Charnley gave the concept of cemented stem and these became popular. As loosening and

 

 

 

Figure 1.22: Matt finished stems produce shear forces at bone cement interface rather than compressive forces as seen with polished stems

Figure 1.23: X-ray showing C-stem with good cementing

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Evolution and Design Principles of Hip Arthroplasty

 

Figures 1.24A to C: Earlier cementless femoral stems. (A) Judet (B) Lord (C) Sivash

 

osteolysis on the bone cement interface were frequently seen complications, cementless fixation again came into vogue. The basic principle of cementless fixation is osteointegration, which refers to the attachment of lamellar bone to implant surface without any intervening soft tissues.32 This process takes 4-12 weeks after implantation and may continue for as long as 3 years.33,34 A good and adequate bony contact along with firm fixation of implant to avoid micromotion is essential to achieve osteointegration.33 A micromotion of <20 μm would result in bone formation,40-150 μm leads to bony and fibrous tissue formation whereas micromotion >150 μm would result in fibrous tissue formation.35-37 Three types of cementless stems are available:

Press fit stems: R Judet is credited for the development of first press fit total hip replacement. Lord and Sivash also developed press fit stems (Figs 1.24A to C). These stems can be calcar support or wedge fit. Moore implant is an example of calcar support press fit stem. Usually all other stems are wedge fit stems. If a collared stem is used and the collar makes contact with calcar, before the stem becomes wedge fit, implant would not be stable. On the other hand, if stem becomes wedge fit before the collar touches calcar, collar is of no use.

The problem with conventional press fit stems is one of sizing. As the medullary canals are of not same size in different individuals, a large number of implant sizes should be available to achieve a press fit implant. This difference is more marked between elderly people and young adults.

These implants can be either metaphyseal fit or diaphyseal fit. It is better to have a metaphyseal fit as proximal end of femur must support the vertical load and provide torsional resistance while the distal end serves to resist toggle only. For achieving these goals, either proximal or distal modularity is employed in current cementless press fit stems.

The distal part of stem can be made to fit by sleeves (Whiteside), highly polished bullet (Omniflex system) or metal tubes (Precision Osteolock, APR II systems). A possible complication is difficulty in removing the distal sleeve if bone grows around its proximal end. To achieve proximal fill, a conical sleeve may be added to the stem. These sleeves are available in different sizes (S-ROM) (Fig. 1.25). Another method of proximal modularity is to attach a wedge to the conventional stem.

Macrointerlock fixation In these stems, press fit is supplemented by some mechanical interlocking. Various designs to achieve this interlocking have been described, e.g. steps, ribs, threads, dimples, flutes, wings.

Metal coatings (bioinert and bioactive) (Fig. 1.26) Ingrowth is the formation of bone inside a porous surface whereas ongrowth refers to bone growth over a roughened surface. The surface characteristics and coatings over an implant decide whether ingrowth or ongrowth would occur. For bone ingrowth to occur, the pore size needs to be between 50-100 μm.32,38 The surfaces which result in bone ingrowth are porous metals, sintered beads and fibre

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Total Hip Arthroplasty

 

Figure 1.25: S-ROM hip system

 

 

 

 

Figure 1.26: Modern cementless porous coated stem. Left to right: Versys (Zimmer), AML (Depuy), Synergy (Smith and Nephew), Solution (Depuy), Accolade (Stryker)

 

mesh. Porous metals are highly porous and have a uniform 3 dimensional network with high interconnectivity of the voids.39 Fibre mesh have metal pads attached with diffusion bonding.40 Sintered beads are microsphere of alloys attached by high temprature.40,41

Ongrowth surface are made by grit blasting or plasma spraying. These porous coated stems are usually bioinert and depend upon a press fit initially and bone ingrowth over a period of time. It is very essential to achieve implant stability initially. If implant is unstable, it will lead to fibrous ingrowth (Fig. 1.27) AML stem is an example of porous coated stem. Porous coating should be circumferential. Non-circumferential coating allows wear debris access to femoral canal, enlarging the “effective joint space”, which is very detrimental and causes excessive wear. Also less surface area for bone contact is available.

Although the extent of porous coating necessary is controversial, most agree that the porous coating should be circumferential at its proximal extent. Extensively porous coated stems are not preferred by some authors as incidence of proximal stress shielding and thigh pain is high. Also the revision is difficult if required. Proximal coated stems have lesser incidence of thigh pain as load transfer is through metaphysis. These stems can be parallel sided cylindrical or tapered stems. Tapered stems establish bony contact in metaphyseal region, permitting proximal loading. Tapered stems produce constant stress along their length, while cylindrical stems have a stress gradient, high stresses in distal portion and low stresses in proximal aspect, causing stress shielding (Fig. 1.28).

Bioactive coatings actively stimulate bone growth. Hydroxyapatite coated stems (Fig. 1.29) are such an example. The overall strength of fixation is improved if hydroxyapatite is laid down on a rough surface. The optimal thickness of hydroxyapatite coating is 50 μm.42,43-46

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Figure 1.27: Stable fixation is necessary for bony ingrowth

Figure 1.28: Tapered stems produce constant stress along their length, while cylindrical stems have a stress gradient

 

 

 

 

Evolution and Design Principles of Hip Arthroplasty

 

Figure 1.29: HA coated Corail® stem

 

The advantage of hydroxyapatite coating is that it is osteoconductive and enhances bone growth onto the implant.42,47-48

As with porous coatings, the better the initial press fit, the more rapid and stronger is the bony ingrowth. Other bioactive coatings such as bioglass have not shown promising results. Other materials like fluoroapatite are in experimental stages.

Cementless stems may be straight or anatomical. The anatomical stems with in built neck versions are side specific and they have a posterior bow proximally and a variable anterior bow in diaphyseal region. The straight stems are not side specific.

The cementless stems have been classified into 4 categories based on the geometry and fixation into by Berry et al.49 Khanuja et al50 further modified these categories into 6 types based on shape, amount of osseous contact and the progression of stem fixation from proximal to distal. Type 1-3 are tapered stems designed to have proximal fixation. Type 4 is fully coated to obtain distal fixation. Type 5 is a modular prosthesis whereas type 6 stems are curved anatomic designs.50

Type 1 stems are single wedge stems e.g. Trilock stem® (Depuy). They are narrow mediolaterally and have flat and thin profile in the anteroposterior plane. They are straight stems with tapered proximal fixation in the metaphysis. These stems are usually proximally coated. They obtain fixation in the femoral canal by wedge fixation and 3 point fixation along the stem length as the implant contacts the canal posteriorly, proximally and distally as well

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Total Hip Arthroplasty

 

Figure 1.30: Classification of cementless stems50

 

 

 

as anteriorly in the midportion. The canal preparation requires only proximal broaching and no distal reaming.50

Type 2 stems are double wedged or metaphysis filling designs. These stems narrows distally in both mediolateral and anteroposterior planes. These stems, e.g. Omnifit HA® (Stryker) stem are wider than the type 1 stems and are metaphyseal filling. They obtain fixation in the metaphysis by proximal contact in both anteroposterior and medial lateral planes.The distal position of these stems is either tapered or rounded for canal fill. The preparation for these stems involves proximal broaching and distal femoral reaming.50

Type 3 stems achieve fixation in the metaphyseal–diaphyseal junction and proximal diaphysis. They are tapered mediolaterally and anteroposteriorly along the length of stem and don’t have abrupt change in geometry or coating.

Type 3a stems are tapered and rounded in geometry and may have proximal fins for rotational stability, e.g. Mallory Head® (Biomet) stems. They are proximally coated and require preparation with reamers distally and broaches proximally.18

Type 3b stems have a conical taper with longitudinal raised splines to provide rotational stability and fixation, e.g. Wagner cone prosthesis stem® (Zimmer). The profile of these stems is very narrow providing the ability to change version in difficult cases. The preparation for these stems is done by conical reamers.50

Type 3c stems are tapered with a rectangular cross-section providing a four point rotational support in the metaphyseal diaphyseal region, e.g. Alloclassic Zweymuller system® (Zimmer). Rectagular femoral broaches are required for the femoral canal preparation for these stems.50

Type 4 stems are distally fixed cylindrical fully coated stems. They achieve fixation primarily in the diaphysis by diaphyseal scratch fit, e.g. Anatomic medullary locking hip prosthesis Stems® (Depuy). They have extensive porous coating with a proximal collar to

 

 

 

 

 

Evolution and Design Principles of Hip Arthroplasty

 

Figure 1.31: Kinectiv stem with additional modular neck

 

 

enhance bone loading and axial stability. The preparation for these stems involves proximal broaching and distal reaming.50

Type 5 stems are modular stems with metaphyseal and diaphyseal components being prepared separately. They achieve fixation in both metaphysis and diaphysis, e.g. S-ROM stem® (Depuy). These stems have a separate metaphyseal sleeve and diaphyseal stem components. These stems are of use in patients with rotational malalignments and anatomic abnormality, e.g. hip dysplasia.50

Type 6 stems are curved anatomic stems, e.g. Porous Coated Anatomic Stem® (Howmedica). Their proximal portion is wide in both lateral and posterior planes. They have a posterior bow in metaphysis and an anterior bow in diaphysis. These stems achieve fixation in metaphysis. They are side specific as they have anterversion of the neck. The preparation the stem is done by metaphyseal broaching and distal reaming.

 

Newer   Concept   Hip   Arthroplasty   Designs          

Hip arthroplasty has witnessed a great evolution and is still evolving. Zimmer has come up with an implant that gives surgeons the control to make independent adjustments to each part of the implant. Each component is chosen—right during surgery—that fits the patient best. The Zimmer Kinectiv® Hip (Fig. 1.31) has four parts. Traditional hip implants only have three parts because the stem and neck are one component. With the Zimmer Kinectiv Hip, a system of modular stem and neck components help the surgeon restore the natural hip joint center intraoperatively by addressing leg length, offset, and version independently. The other useful concepts are:

HIP RESURFACING (SURFACE REPLACEMENT) (FIG. 1.32)

This is basically an improved concept of cup arthroplasty as we discussed above. The conventional total hip replacement surgery entails amputation of the femoral head and invasion of the femoral bone marrow cavity. Obviously, large quantity of healthy tissue is sacrificed in this operation. Because in many patients who are subjected to the total hip replacement only the hip joint surfaces are damaged by the disease, it seems reasonable to remove only these damaged surfaces and replace them with artificial material. This is the idea of the hip surface replacement operation. Such surface replacement is the procedure carried out on the knee joint although there it is called “total knee replacement”.

 

 

 

 

Figure 1.32: Birmingham hip surface replacement

 

Total Hip Arthroplasty

 

The surgeons endorsing the modern hip surface replacements like Birmingham hip replacement (BHR) believe that the cause of failure of earlier surface replacement prostheses was the material used for fabrication of the surface replacement shells and not the concept of the operation itself. In vitro simulations have shown wear rates with modern metal on metal resurfacing prosthesis to be consistently low, 40-100 times less than those with metal on polyethylene. There is concern of the toxicity of metal, but there is currently no definitive evidence that metal ions cause cancer.

In the 1960’s when the first metal-on-metal (McKee-Farrar) total hips competed with Charnley’s metal-on- polyethylene total hips, John Charnley’s argument against the competing metal-on-metal total hips was just that the metal-on-metal hips have very high friction, because the space (clearance) between the bearing surfaces of these old metal-on-metal total hips was uneven and too large.

Modern surface hips have very small clearance (Fig. 1.33) between bearing surfaces (nominally about 0.1 mm in Birmingham hip) which produces a good lubrication when the surfaces are moving and good clinical results. In the quest to achieve even better lubrication of bearing surfaces, the engineers at DePuy produced their ASR surface hip with clearance of only 0.05 mm. This “improvement” had unexpected consequences. The ASR cup was shallower, providing less coverage. This lead to unexpected overloading at the edges, leading to excessive wear. The wear of the ASR is substantially greater than the wear of Birmingham Hip, so it has been withdrawn from the market.

MONO-BLOCK CUPS (DELTA-MOTION)51

Dislocation is a major concern after THA. Larger bearings allow a greater range of motion and higher stability than conventional 28 mm bearing couples, leading to a better postoperative mobility as well as stabilty. On the other hand, size limitations on the acetabular side are provided by the anatomy of the human pelvic bone as well as the deformation and fracture behavior of the used artificial materials. Investigating the wall thickness of the metal

 

 

 

 

Figure 1.33: Large clearance leads to increased wear while small clearance provides even loading and less wear

 

 

 

 

Evolution and Design Principles of Hip Arthroplasty

 

Figure 1.34: Delta motion monoblock ceramic on ceramic hip system

 

shell which is press-fitted in the human pelvic bone, the general trend towards a smaller wall thickness yielding an increased compliance can be observed with larger bearing diameters. This may lead todeformations of the metal shell making it difficult for the surgeon to properly introduce the insert. With decreasing overall wall thickness of the acetabular components, the volumetric stresses increase by definition. Therefore, an optimal component coupling between insert and metal shell is necessary in order to avoid point loads and resulting stress concentrations. With pre-assembled systems, this optimal coupling is reached by the force-controlled insertion of the insert in the metal shell without any prior deformation of the shell. This procedure enables to design acetabular components with a much lower overall wall thickness than conventional systems. As an example, in the case of the DELTA motion® system (Fig. 1.34), this overall wall thickness has been decreased to 5 mm allowing. For example, a usage of a 36 mm bearing couple together with a 46 mm outer diameter of the metal shell.

 

SHORT STEMS (FIG. 1.35)

The concept of a short femoral stem originated in the mid 80’s with the Mayo® stem (Zimmer) designed by Morrey. Advantage of using a short femoral stem is that it conserves the greater trochanter and the ring of the femoral neck, thus offering a promising bone conserving solution for younger patients. Preservation of bone stock is especially attractive for young, active patients who are likely to outlive their first hip arthroplasty operation and will require one or more revision surgeries during their lifetime. Today, femoral components such as the METHA (Aesculap), NANOS (Smith and Nephew), CFP stem (Link), PROXIMA (DePuy) femoral components successfully use the same short-stem principles. Other “clinically proven” straight stems, including the Trilock (DePuy) and Taperloc (Biomet), recently became available in shorter versions.

A correct indication is a prerequisite for successful implantation of short-stem prosthesis.

In particular, the quality and shape of the bone must be considered.

 

 

 

Figure 1.35: Short stems. Left to right: METHA (Aesculap), CFP stem (Link), PROXIMA (DePuy)

 

 

 

 

 

Figure 1.36: Birmingham mid-head resection prosthesis

 

Total Hip Arthroplasty

 

MID-HEAD RESECTION PROSTHESIS (FIG. 1.36)

The Birmingham Mid Head Resection (BMHR) prosthesis is a recently developed hip prosthesis for patients who are keen on having metal on metal hip resurfacing but do not have enough bone stock in their femoral head to accept a resurfacing implant. The acetabular component of the Birmingham Mid Head Resection prosthesis is the same as that of the BHR prosthesis. The femoral side is constituted by two modular components—a stout stem with a fusicone contour in the proximal portion and anti-rotation splines at the distal end. The special BMHR head goes over the taper adapter present at the top of the stem.

 

Summary                        

Total hip arthroplasty continues to enjoy progressively evolving concepts and designs. The history of hip arthroplasty also explains how our understanding regarding hip arthroplasty has evolved. The aim is towards more and more preservation of bone stock. Preoperative work-up including templating still remains the most important determinant of a successful outcome of hip arthroplasty. This also minimizes any intraoperative surprises or complications. A variety of options are available today, however choice needs to be individualized.

 

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