RECONSTRUCTION 2023, Jan, 26 | 12:00 am

Extensively Coated Stems

 

Extensively Coated Stems

 

In 1983, the Food and Drug Administration approved the first porous-coated femoral implant for use without cement. This implant, the Anatomic Medullary Locking stem (AML, DePuy, Warsaw, IN), was characterized by a circumferentially porous-coated, straight, nontapered distal cylindrical rod coupled with a circumferentially porous-coated proximal metaphyseal triangular shape. Current studies of the AML have documented 98% femoral component survivorship at 20 years (1). Excellent results with this stem have also been reported in scenarios historically not deemed appropriate for porous-coated fixation, such as in patients with avascular necrosis, those with rheumatoid arthritis, and the elderly with osteoporosis (2,3,4). Today, extensively porouscoated femoral implants are available from many manufacturers.

 

INDICATIONS

The nonselective use of porous-coated femoral stems at our institution for all total hip replacements for 20 consecutive years has demonstrated no short-term difference in clinical outcome based on age, sex, diagnosis, or bone quality (5,6,7). Patients over 65 years of age or those with osteoporosis do not have poorer clinical results or less reliable osseointegration than do younger patients (8). The single most important factor predicting the clinical result of extensively porouscoated stems is the quality of the initial prosthetic fit within the femur, particularly within the diaphysis (9). This finding is more a characteristic of the surgical technique than of any patient parameter.

The requirement for using an extensively coated stem, therefore, is simple: the presence of bone capable of providing initial mechanical support for the implant and of mounting an osteogenic (i.e., typical fracture healing) response sufficient for ingrowth. Exceptionally few patients do not meet this criterion.

 

 

CONTRAINDICATIONS

The contraindications relate to the diameter or the femoral canal. Since stems are not available below 10.5 mm, patients with very small-diameter canals are not a candidate. The same is true for very large-diameter stems. The largest size is 22.5 mm. Patients with canals this large are perhaps treated best with a cemented stem or a revision-type stem that is available in the larger diameters.

 

 

PREOPERATIVE PREPARATION AND TEMPLATING

Surgical planning of a total hip replacement with an extensively porous-coated femoral component is similar to planning for other implants. Every patient who is scheduled for a total hip replacement must have an examination that allows the surgeon to plan if the hip needs to be kept the same length or lengthened. Not uncommonly, the standing leg length and the radiographic hip length are not

 

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the same, and the difference must be resolved so that a target surgical correction can be templated. The most common cause of a discrepancy is the presence of a contracture. Both a flexion contracture and an adduction contracture of the surgical hip will cause the hip to have false shortening. In contrast, an abduction contracture of

the operative hip will cause false lengthening. Other causes for a discrepancy between the radiographic hip length and the standing leg length include fixed pelvic obliquity and a difference in the leg length originating below the hip, such as arthritic knee deformities, leg fractures, or ankle and foot deformities.

There are six steps in the templating process for an extensively porous-coated femoral component. First, the leg length correction is determined as previously described. Second, the size and position of the acetabular component are determined. Third, the diameter of the distal, cylindrical portion of the stem is determined by placing the template in line with the center of the femur in the femoral diaphysis. The correct size is that which fills or is slightly larger than the isthmus so that porous coating will contact a distance of at least 3 to 5 cm of the medial and lateral endosteal cortices (Fig. 15-1). The fourth step involves determining the level of the femoral neck resection and the prosthetic neck length. With the center of the acetabulum previously marked and the femoral template aligned to the diaphysis, the template is raised or lowered until the head directly overlays the acetabular center or lies directly above the acetabular cup center a distance equal to the desired amount of increased limb length. The level of the neck cut is then marked and the neck length chosen. The fifth step involves selecting the appropriate proximal implant geometry and offset that adequately fills the proximal femur and recreates femoral offset based on the level of the neck resection. Extensively porous-coated systems usually contain two proximal geometries (Fig. 15-2). Femoral neck offset options vary by manufacturer. The final step involves templating with the lateral radiograph. With the lateral template at the desired level, the stem should not risk perforation of the anterior cortex in a femur with an exaggerated anterior bow.

 

 

 

 

FIGURE 15-1 The template is raised and lowered in line with the center of the femur in the femoral diaphysis. The correct size fills or is slightly larger than the isthmus so that porous coating will contact a distance of at least 3 to 5 cm of the medial and lateral endosteal cortices.

 

 

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FIGURE 15-2 The standard and high offset stems are shown to demonstrate how offset is increased. In this case, the high offset stem may not be needed because the template head center is medial well medial to the templated acetabular center.

 

SURGICAL TECHNIQUE

All surgical approaches to the hip have been used to place extensively porous-coated stems. However, the 6-inch straight stem design requires moderate retraction of the hip abductors, making these stems more difficult to insert with anterior abductor-sparing approaches. We will describe the posterior approach, which remains the most common surgical approach for total hip arthroplasty (9,10).

The patient is placed in the lateral decubitus position. We tape the lower (nonoperated) leg to the table, with the hip flexed 30 degrees and the knee at a 90-degree flexed position. If the upper leg is placed directly on top of the lower leg and both knees are flexed at 90 degrees, it is possible to compare leg lengths. The surgeon can use a right angle and a ruler to measure (at the knees) the apparent difference in femoral lengths (Fig. 15-3).

 

 

 

FIGURE 15-3 The lower leg is positioned with the knee flexed at 90 degrees and the difference in femur lengths is recorded. Typically, the operative femur appears short because it is in an adducted position. This measurement is repeated with trial components in place.

 

 

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After the superficial dissection and the arthrotomy, but prior to dislocation of the hip, a 5/32-inch threaded

Steinmann pin is placed through the gluteus medius muscle into the ileum. The pin is then bent twice at 90-degree angles so that the exposed tip touches the greater trochanter. This point is marked with a stay stitch to monitor offset and length after placement of the trial implants (Fig. 15-4).

After hip dislocation, the distance from the lesser trochanter to the center of the femoral head is measured and recorded (Fig. 15-5). One goal of femoral preparation will be to approximate this distance by repeating the measurement using the position of the femoral trial ball. The femoral head is resected with the provisional cut. Surgeons can elect femoral or acetabular first preparation. A femoral first technique has the surgeon place the trial at the templated level before acetabular exposure, thus avoiding a high initial neck resection, which can make acetabular exposure difficult. Additionally, the femoral anteversion that is difficult to change is accessed, and the acetabular position that can easily be adjusted can be matched to the femur.

 

 

 

FIGURE 15-4 The right hip is seen with the patient in the lateral position. The pin placed in the ileum prior to dislocation and bent to touch the greater trochanter can record leg length and offset. It is rotated out of the surgical field for the procedure and rotated back at the time of trial reduction to determine the change in length and offset.

 

 

 

 

 

FIGURE 15-5 A measurement from a point on the lesser trochanter to the center of the femoral head is recorded and repeated with trial components. Typically, this measurement does not change more than 5 mm comparing the native femoral head center to the trial head center.

 

 

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The first step in femoral preparation is to create a cylindrical tube with straight rigid reamers. Correct pilot hole positioning is critical for proper femoral component placement. The pilot hole should be at least 2 mm larger than the size planned for the intramedullary canal. Making the pilot hole larger than the anticipated distal implant size reduces the possibility of eccentric reaming of the intramedullary canal. As progressively larger reamers are

inserted into the medullary canal, these reamers must not contact the pilot hole. If this contact does occur, the hole must be enlarged to prevent the proximally located pilot hole from influencing the direction of the reamers distally (Fig. 15-6).

The distance reamed can be determined from marks on the reamer that reference either the greater trochanter or the medial neck cut depending on the system used. As the canal is reamed, the surgeon can feel and hear the reamer bite femoral cortex. The proper diameter reamer will bite cortical bone for 3 to 5 cm—that size should be within one size of the templated stem diameter. The absence of such correspondence should serve as a warning that the reaming is not being performed correctly. In this case, an intraoperative x-ray should be considered to check the reamer orientation.

Once the femoral diaphysis has been prepared, rasps are used to prepare the metaphyseal bone (Fig. 15-7). Care is taken not to remove excessive metaphyseal bone. Although the stem is distally

 

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fixed, proximal bone contact and ingrowth is important for long-term fixation and to prevent migration of wear debris distally. Most systems have two metaphyseal broach sizes. The smaller is used first and seated to the templated level based on the two intraoperative measurements. The more reproducible is the templated distance from the proximal tip of the greater trochanter to the lateral aspect of the femoral broach (Fig. 15-8). The second measurement is the distance from the lesser trochanter to the center of a femoral trial ball on the broach neck segment. If the broach is at the proper level and there is remaining metaphyseal cancellous bone, then the large triangular broach is used. At this point, the goal of recreating femoral anatomy is addressed. When the surgeon is happy with the femoral length, the acetabulum is prepared with a femoral first technique or a trial reduction is performed if the acetabulum has already been prepared.

 

 

 

 

FIGURE 15-6 The drill is being directed in part by the overhanging greater trochanter; therefore, the pilot hole needs to be enlarged to prevent varus positioning of the femoral component.

 

 

 

FIGURE 15-7 Broaches are used to prepare the femur for the triangular metaphyseal portion of the stem. This needs to be done carefully and precisely so that proximal gaps are minimized. Ingrowth cannot occur where gaps exist and gaps allow joint fluid access to the metaphyseal bone with the associated potential for osteolysis.

 

Limb length and offset can both be assessed during the trial reduction using the Steinmann pin and the stitch that were placed prior to femoral head removal (Fig. 15-4). Leg length should also be checked with measurements at the knees (Fig. 15-3). Adjustments can then be made in the femoral offset and limb length by changing the neck length of the prosthesis, the implant seating level (by recutting the femoral neck and driving the implant further distally), the configuration of the neck of the prosthesis (changing from the 135-degree to the 125-degree neck-shaft angle implant), or a combination of these. Options vary by system.

I assess stability of the hip with trial components. Posterior stability is assessed with the hip in flexion, adduction, and internal rotation. In most instances, the hip should be stable when flexed 90 degrees, when adducted 20 degrees, and when internally rotated at least 50 degrees. In addition to assessing posterior stability, the surgeon must confirm anterior stability by placing the hip in full extension and external rotation. Commonly, the anterior capsule has not been released and it prevents normal external rotation of the hip. When the anterior capsule is tight and external rotation is limited, it is possible to have excessive combined anteversion of the femoral and acetabular components. If this is the case, the hip will be stable posteriorly because of the excessive combined anteversion and stable anterior because of the intact capsule; however, as the patient regains range of motion and the anterior capsule stretches, anterior instability can develop. Measuring combined acetabular and femoral anteversion can prevent this scenario. Rarely is more than 40 degrees or combined anteversion required.

 

 

 

FIGURE 15-8 The distance from the top of the greater trochanter to the lateral aspect of the trial broach is measured on the template and reproduced at the time of surgery. This is one of the more accurate measurements that can help prevent leg length discrepancy.

 

 

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Once satisfied with the leg length, lateralization of the femur, and hip joint stability, the trials are removed and the components are inserted. At this point, the surgeon must make a decision regarding underreaming the femoral diaphysis by 0.5 mm or line-to-line reaming. The amount of interference fit or “scratch fit” is determined by placing the stem firmly into the canal until it stops. The stem should be approximately 5 cm proud (Fig. 15-9). On occasion, the stem sits higher than 5 cm or the stem seems tight and does not advance as expected with initial impaction. In this situation, the stem is removed and the femoral diaphysis, which was initially prepared 0.5 mm smaller than the stem, is enlarged to the same diameter as the final stem. The decision to utilize this so-called line-to-line fit should be made before the stem is hit forcefully and becomes stuck. This decision is based on the tightness of the last reamer, the length of tight reaming, and surgical judgment.

When impacting the stem, it may initially advance 5 mm with each hit, but as more porous coating engages the femur, the amount the stem advances decreases. Over the final 2 cm, the stem may only advance 0.5 mm with each hammer blow.

 

 

 

FIGURE 15-9 The final stem is inserted by hand until it stops. The measurement records the amount of scratch fit that can be expected. If this is more than 5 cm, the surgeon may consider reaming the canal to the same diameter as the stem rather than the usual 0.5 mm underreaming.

 

POSTOPERATIVE CARE

Historically, weight bearing has been limited for a 6-week period. Current protocols, however, allow immediate weight bearing as tolerated (11). While full weight bearing may be acceptable for the majority of patients, there are some in whom a 3- to 6-week period of 50% or less weight bearing is beneficial. The decision to limit weight bearing is typically based on the length of scratch fit, ease of stem insertion, and the appearance of the postoperative radiograph. Those patients with a good scratch fit intraoperatively and a normal x-ray postoperatively are routinely allowed to bear weight as tolerated. However, patients in whom stems are inserted easily or are undersized on the postoperative radiograph or in whom a diaphyseal hairline fracture exists should have limited weight bearing for 3 to 6 weeks postoperatively.

 

UNIQUE ADVANTAGES AND APPLICATIONS OF EXTENSIVELY POROUSCOATED STEMS

Extensively porous-coated femoral components can be used in patients who require a subtrochanteric femoral osteotomy at the time of arthroplasty. Cases of high-riding developmental dysplasia that need shortening, angular, or rotational osteotomy are well suited to this implant design

 

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(Fig. 15-10). Distal rotational control is excellent with the rough cylindrical scratch fit, and the proximal fragment of the osteotomy is well controlled by the triangular metaphyseal segment of the implant. This technique can also be used in patients who have a deformity secondary to a previous varus or valgus reconstructive osteotomy. The distal fixation with an extensively porous-coated femoral component is also helpful in cases that require a greater trochanteric osteotomy. A trochanteric osteotomy weakens the proximal femur and might prohibit the use of a

proximally coated or wedge-shaped stem. An example would be a conversion of a hip fusion to a total hip in which a greater trochanteric osteotomy facilitates exposure of the hip joint.

 

 

 

 

FIGURE 15-10 The angular correction and shortening were accomplished with resection of the marked portion of the femur. The triangular proximal portion of the stem controls the proximal fragment, and the scratch fit of the cylindrical portion of the stem controls the distal portion of the femur. Typically, there is no need for a step cut or additional fixation at the osteotomy site.

 

 

COMPLICATIONS

Insertional Femoral Fracture

Insertion of extensively porous-coated stems, like other press-fit cementless stems, can be associated with femoral fractures that must be addressed. In choosing a stem size, surgeons must balance between undersizing the stem, which can result in a loose prosthesis, and oversizing the stem to obtain more rigid fixation, which can result in an insertional femur fracture.

A study in 1989 reviewed the type and treatment of femoral fractures that occurred with insertion of an extensively porous-coated stem (12). In 1,318 hip arthroplasties performed between 1977 and 1986, 39 insertional fractures (3%) were reported (12). Only half of the fractures were diagnosed intraoperatively. Proximal fractures were less common than distal fractures and were more likely to be diagnosed intraoperatively. Incomplete proximal fractures were treated simply with protective weight bearing, while displaced proximal fractures were treated with cerclage wires. The most

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common type of insertional fracture was a nondisplaced diaphyseal fissure. These were typically

discovered on postoperative radiographs, only visible on a single view, and did not involve the posterior femoral cortex. Surgeons should suspect this type of fracture intraoperatively if the stem that advanced 0.5 to 1 mm with each impaction suddenly advanced 2 mm or more. It can be treated with protective weight

bearing without compromising long-term fixation. Displaced distal fractures are treated with open reduction and internal fixation using a cable screw plate, retaining the femoral component if the stem is stable.

Stem Removal: Bone Ingrown Stem

Occasionally, extensively porous-coated stems will need to be removed for pain, stem breakage, or infection. To date, we have only removed one ingrown stem for pain. Infection remains the most common reason to remove an ingrown femoral stem. Our institutional database follows 7,577 primary total hips, and there have been 11 stem fractures. All but one of these hips had stems 13.5 mm in diameter or smaller. The technique for removal was described in 1992 (13). Emphasis was placed on obtaining serial radiographs to determine whether the stem to be removed was stable or loose. Loose stems that demonstrate migration are easily removed. Stable stems are removed by sectioning the stem between the proximal triangular portion and distal cylindrical portion with a metal cutting burr through a cortical window or an extended trochanteric slide osteotomy (Fig. 15-11). An osteotomy may not usually be needed in cases with extensive proximal osteolysis or if the stem is broken because the proximal portion of the stem is easily removed.

Otherwise, it is necessary to perform an extended trochanteric osteotomy and use a giggly saw to separate the medial bone/implant interface and remove the metaphyseal portion of the stem. The cylindrical distal portion of the stem is removed using trephines designed to match the implant diameter. The trephines must be irrigated frequently to prevent heat necrosis of the bone. In addition, the trephines become dull with use and several of the same diameter trephines are typically necessary for a single case. In cases where a new stem will be inserted, surgeons should consider bypassing the trephined area to avoid failure of fixation that can occur because of proximal heat necrosis, which may have occurred while removing the stem.

 

 

 

FIGURE 15-11 Extensively porous-coated stems can be removed by sectioning the stem and subsequent removal of the proximal portion and trephining the distal fragment. This takes time and multiple trephines but can be done with minimal bone loss.

 

 

 

Periprosthetic Bone Loss Secondary to Stress Shielding

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To address concerns about bone loss secondary to stress shielding, the clinical and radiographic results of 48 THAs in patients with easily visible proximal bone loss were compared to results from 160 THAs without proximal bone loss (14). At a mean 14-year follow-up, the study showed that the patients with

radiographically evident proximal bone loss secondary to stress shielding were no more likely to be revised,

have particle-induced osteolysis, or have thigh pain than were patients without proximal bone loss.

Thigh Pain

Although thigh pain has been reported in 8% to 14% of patients implanted with extensively porouscoated stems (15,16,17,18), it rarely limits a patient's activity or satisfaction with the hip replacement (18,19,20). McAuley et al. (18) reported a 12% incidence of thigh pain but noted that the pain limited activity in only 3% of patients. Likewise, a study of 1,415 extensively porous-coated femoral components assessed outcome as a function of stem diameter (19). Activity-limiting thigh pain was present in 3.6% of patients with the smallest diameter stems and 2.5% of the patients with the largest diameter stems (18 to 21 mm). The study concluded that patients with large-diameter extensively porous-coated femoral components were no more likely to be revised, loosen, or have thigh or activity-limiting pain than were patients with smaller diameter stems.

 

 

 

RESULTS

While the 20-year 98% femoral survivorship data alone are a compelling statistic, many other retrieval and clinical studies are worth reviewing. Early studies were performed on radiographs and postmortem-retrieved specimens. These studies were followed by publications on the clinical results of these stems.

The first autopsy study on extensively coated stems confirmed the occurrence of bone ingrowth into the stem's surface (21). It revealed that, on average, bone grew into 35% of the available porous implant surface (range, 25% to 43%). Moreover, it showed that the pattern of ingrowth was predictable, with the most consistent growth occurring at the termination of the porous coating (21). Following confirmation of osseointegration, the stability of ingrown stems was then tested. Mechanical testing of autopsy-retrieved specimens demonstrated that the relative motion between bone and the implant was less than 20 μm and was completely elastic (22). Subsequent research documented the effect of this ingrowth on how the femur remodeled (23,24,25,26). This bone loss occurred on a gradient and was typically greatest adjacent to the proximal one-third of the implant. Moreover, the studies showed that the magnitude of the loss was highly correlated with the patient's initial quality of bone (26,27). Patients with low bone mineral content preoperatively (osteopenia or osteoporosis) had more pronounced proximal bone loss after arthroplasty than did patients with high initial bone mineral content. Bone loss secondary to implantation of an extensively porous-coated stem was not correlated with stem size, patient weight, duration of implantation, or patient age (26,27).

Postmortem-retrieved specimens were also used to examine the effect of circumferential porous coating on the migration of wear debris adjacent to uncemented femoral components (28). One study analyzed five femoral specimens with bone-ingrown and fibrous-encapsulated femoral implants retrieved at autopsy after a mean 95 months in situ (range 53 to 132 months) (28). Histologic examination of the bone confirmed that circumferential porous coating could prevent distal migration of polyethylene wear debris along the bone-implant interface in both bone-ingrown and fibrousencapsulated femoral implants.

Early clinical studies of extensively porous-coated stems focused on defining the radiographic appearance of bone ingrown, fibrous stable, and loose cementless femoral components. In a study of 97 hips implanted with AML stems and 51 hips implanted with other cementless stems, the authors devised a means of grading fixation that could be applied to any cementless femoral implant (29). Signs that a stem had bone ingrowth included the presence of spot welds to endosteal bone at the termination of the porous coating, calcar atrophy, and the absence of migration. In contrast, signs of failed ingrowth included the presence of extensive radiolucent lines adjacent to the porous coating, stem migration, and the presence of a distal

pedestal.

With direct confirmation of osteointegration through autopsy analysis and a clear radiographic definition of an ingrown stem, it became easier for clinical researchers to document the survivorship of extensively porous-coated stems (1,16,17,30,31). Although the majority of publications referenced originate from the institute that developed extensively porous-coated stems, other centers have duplicated these clinical results and some have similarly confirmed better than 95% survivorship of porous-coated stems (32,33,34). Studies have also documented high porous-coated stem survivorship in patients with altered bone quality. One study of 203 patients who were 65 years or older at the time of hip replacement surgery reported 97% femoral survivorship at 12-year postarthroplasty

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(4). Another study of 64 patients with rheumatoid arthritis implanted with extensively porous-coated stems demonstrated 98% femoral stem survivorship at 10 years (3). A study of 45 young patients (mean age at surgery of 31 years) with avascular necrosis thought to have altered femoral physiology reported no failures at a mean of 9-year follow-up (2).

 

 

REFERENCES

  1. Belmont PJ Jr, Powers CC, Beykirch SE, et al.: Results of the anatomic medullary locking total hip arthroplasty at a minimum of twenty years: a concise follow-up of previous reports. J Bone Joint Surg Am 90: 1524-1530, 2008.

     

     

  2. Hartley WT, McAuley JP, Culpepper WJ, et al.: Osteonecrosis of the femoral head treated with cementless total hip arthroplasty. J Bone Joint Surg Am 82: 1408-1413, 2000.

     

     

  3. Jana AK, Engh CA Jr, Lewandowski PJ, et al.: Total hip arthroplasty using porous-coated femoral components in patients with rheumatoid arthritis. J Bone Joint Surg Br 83: 686-690, 2001.

     

     

  4. McAuley JP, Moore KD, Culpepper WJ II, et al.: Total hip arthroplasty with porous-coated prostheses fixed without cement in patients who are sixty-five years of age or older. J Bone Joint Surg Am 80: 1648-1655, 1998.

     

     

  5. Sotereanos N, Engh CA, Glassman AH, et al.: Cementless femoral components should be made from cobalt chrome. Clin Orthop 313: 146-153, 1995.

     

     

  6. Jana AK, Engh CA Jr, Lewandowski PJ, et al.: Total hip arthroplasty using porous-coated femoral components in patients with rheumatoid arthritis. J Bone Joint Surg Am 83: 686-690, 2001.

     

     

  7. Hartley WT, McAuley JP, Culpepper WJ, et al.: Osteonecrosis of the femoral head treated with cementless total hip arthroplasty. J Bone Joint Surg Am 82: 1408-1413, 2000.

     

     

  8. McAuley JP, Moore KD, Culpepper WJ II, et al.: Total hip arthroplasty with porous-coated prostheses fixed without cement in patients who are sixty-five years of age or older. J Bone Joint Surg Am 80: 1648-1655, 1998.

     

     

  9. Engh CA, Glassman AH, Bobyn JD: Surgical principles in cementless total hip arthroplasty. Tech Orthop 1:

    35-53, 1986.

     

     

  10. Engh CA, Bobyn JD: Biological fixation in total hip arthroplasty. Thorofare, NJ: Slack, 1985.

     

     

  11. Woolson ST, Adler NS: The effect of partial or full weight bearing ambulation after cementless total hip arthroplasty. J Arthroplasty 17: 820-825, 2002.

     

     

  12. Schwartz JT Jr, Mayer JG, Engh CA: Femoral fracture during non-cemented total hip arthroplasty. J Bone Joint Surg Am 71: 1135-1142, 1989.

     

     

  13. Glassman AH, Engh CA: The removal of porous-coated femoral hip stems. Clin Orthop Relat Res 285: 164-180, 1992.

     

     

  14. Engh CA Jr, Young AM, Engh CA Sr, et al.: Clinical consequences of stress shielding after porous-coated total hip arthroplasty. Clin Orthop Relat Res 417: 157-163, 2003.

     

     

  15. Engh CA, Bobyn JD, Glassman AH: Porous-coated hip replacement: the factors governing bone ingrowth, stress shielding, and clinical results. J Bone Joint Surg Br 69: 45-55, 1987.

     

     

  16. Engh CA, Massin P: Cementless total hip arthroplasty using the anatomic medullary locking stem: results using a survivorship analysis. Clin Orthop Relat Res 249: 141-158, 1989.

     

     

  17. Engh CA Jr, Culpepper WJ II, Engh CA: Long-term results of use of the anatomic medullary locking prosthesis in total hip arthroplasty. J Bone Joint Surg Am 79: 177-184, 1997.

     

     

  18. McAuley JP, Culpepper WJ, Engh CA: Total hip arthroplasty: concerns with extensively porous coated femoral components. Clin Orthop Relat Res 355: 182-188, 1998.

     

     

  19. Engh CA Jr, Mohan V, Nagowski JP, et al.: Influence of stem size on clinical outcome of primary total hip arthroplasty with cementless extensively porous-coated femoral components. J Arthroplasty 24: 554-559, 2009.

     

     

  20. Brown TE, Larson B, Shen F, Moskal JT: Thigh pain after cementless total hip arthroplasty: evaluation and management. J Am Acad Orthop Surg 10: 385-392, 2002.

     

     

  21. Engh CA, Hooten JP Jr, Zettl-Schaffer KF, et al.: Evaluation of bone ingrowth in proximally and extensively porouscoated anatomic medullary locking prostheses retrieved at autopsy. J Bone Joint Surg Am 77: 903-910, 1995.

     

     

  22. Engh CA, O'Connor D, Jasty M, et al.: Quantification of implant micromotion, strain shielding, and bone resorption with porous-coated anatomic medullary locking femoral prostheses. Clin Orthop Relat Res 285: 13-29, 1992.

     

     

  23. Engh CA, McGovern TF, Bobyn JD, et al.: A quantitative evaluation of periprosthetic bone-remodeling after cementless total hip arthroplasty. J Bone Joint Surg Am 74: 1009-1020, 1992.

     

     

  24. Maloney WJ, Sychterz C, Bragdon C, et al.: Femoral bone remodeling after total hip arthroplasty: the skeletal response to well fixed femoral components inserted with and without cement. Clin Orthop Relat Res 333: 15-26, 1996.

     

     

  25. McAuley JP, Sychterz CJ, Engh CA Sr: Influence of porous coating level on proximal femoral remodeling: a postmortem analysis. Clin Orthop Relat Res 371: 146-153, 2000.

     

     

  26. Sychterz CJ, Engh CA: The influence of clinical factors on periprosthetic bone remodeling. Clin Orthop Relat Res 322: 285-292, 1996.

     

     

  27. Sychterz CJ, Topoleski LD, Sacco M, et al.: Effect of femoral stiffness on bone remodeling after uncemented arthroplasty. Clin Orthop Relat Res 389: 218-227, 2001.

     

     

  28. Von Knoch M, Engh CA Sr, Sychterz CJ, et al.: Migration of polyethylene wear debris in one type of uncemented femoral component with circumferential porous coating: an autopsy study of 5 femurs. J Arthroplasty 15: 72-78, 2000.

     

     

  29. Engh CA, Massin P, Suthers KE: Roentgenographic assessment of the biologic fixation of porous-surfaced femoral components. Clin Orthop Relat Res 257: 107-128, 1990. Erratum in Clin Orthop Relat Res

    284: 310-312, 1992.

     

     

  30. Engh CA Jr, Claus AM, Hopper RH Jr, et al.: Long-term results using the anatomic medullary locking hip prosthesis. Clin Orthop Relat Res 393: 137-146, 2001.

     

     

  31. Chen CJ, Xenos JS, McAuley JP, et al.: Second-generation porous-coated cementless total hip arthroplasties have high survival. Clin Orthop Relat Res 451: 121-127, 2006.

     

     

  32. Nercessian OA, Wu WH, Sarkissian H: Clinical and radiographic results of cementless AML total hip arthroplasty in young patients. J Arthroplasty 16: 312-316, 2001.

     

     

  33. Chiu KY, Tang WM, Ng TP, et al.: Cementless total hip arthroplasty in young Chinese patients: a comparison of 2 different prostheses. J Arthroplasty 16: 863-870, 2001.

     

     

  34. Kronick JL, Barba ML, Paprosky WG: Extensively coated femoral components in young patients. Clin Orthop Relat Res 344: 263-274, 1997.