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Navigation of Short-stem Implants

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Abstract

Short-stem prostheses with modular necks were implanted using a modified Watson-Jones approach in 55 cases from November 2004 to May 2006. Twenty-eight cases were navigated using the OrthoPilot navigation system (B. Braun Aesculap, Tuttlingen, Germany). Primary stability, ease of minimal invasive implantation, and restoration of the biomechanics were evaluated. Short-term results showed a good functional outcome and a low complication rate without any dislocations. Apart from cup navigation, the navigation system helps to restore biomechanics in terms of center of rotation, leg length, and offset by advising surgeons on the modular neck offering best fit and predicting the safe range of motion (ROM) reliably.

An increasing demand for less traumatic total hip arthroplasty (THA), combined with a faster recovery period, has led to both minimally invasive surgical approaches and bone- and soft tissue preserving short-stem prostheses. The need for enhanced joint stability and high mobility after THA has led to navigated hip surgery with the use of modular necks to restore biomechanics.

Morrey demonstrated good results with short-stem prostheses.¹ The preservation of bone stock, reduced soft tissue trauma, facilitation of minimally invasive surgery with reduced blood loss, reduction of surgery time, shorter hospital stay, faster recovery time, and immediate stability under full weight are the goals of implanting short-stem prostheses.^2,3 To achieve primary stability, the short-stem prosthesis has multipoint contact with cortical areas in the metaphyseal femur. Multipoint contact forces the prosthesis stem into a virtually preset implant position with only few possibilities for variation that can affect the biomechanics of the hip in terms of leg length and offset. The modular construction of the prosthesis decouples the stem position from the joint reconstruction, offering the possibility influencing joint geometry with greater variability and without compromising secure fixation in the femur.⁴

Navigation has proven to be a useful tool to achieve more precise positioning of THA implants.⁵ Another advantage of a modular implant is that the changes in the biomechanics of a hip are displayed and can be easily influenced intraoperatively. The suitability of a modular short stem to meet the expectations was evaluated in this study.

Materials and Methods

From November 2004, 55 cementless modular short-stem prostheses (Metha, B. Braun Aesculap, Tuttlingen, Germany) were implanted (Figure 1). In 27 cases (group 1), the stems were implanted without navigation, and in 28 cases (group 2), the authors used the OrthoPilot THA navigation system (B. Braun Aesculap).Group 1 included 15 female patients and a mean age of 46 years (range: 30-61 years). Group 2 included 11 female patients and a mean age of 49 years (range: 32-61 years).

The indications for the short-stem prosthesis were primary coxarthritis (22%), or dysplastic coxarthritis (54%), or femoral head necrosis (24%); having no affect on the femoral neck; patient age younger than 60 years; and good bone density. Patients with polyarthritis or osteoporosis or patients who underwent previous surgery changing the proximal femoral anatomy were excluded from the study. In all cases, the authors used a cementless plasma-sprayed hemispherical press-fit cup (Plasmacup, B. Braun Aesculap) with a ceramic liner and a cementless plasma-sprayed short stem with modular neck adapters with a ceramic head.

The stem is designed to achieve a multipoint fixation in the femoral metaphysis. The modular necks offer the choice of antetorsion-retrotorsion of ±7.5° or neutal (0°) and different caput-collum-diaphysis (CCD) angles of 130°, 135°, and 140°, so that nine different options are possible. The standard neck lengths of the ceramic heads are small (s=–3.5 mm), medium, (m=0 mm), and large (l=+3.5 mm).

The image-free infrared kinematic system with passive trackers (OrthoPilot) was used in our series of THA navigation.⁵ The rigid bodies were first fixed on the pelvis with a Steinmann nail and later with a screw through an additional incision into the iliac crest and at the femur with a clamp on the greater trochanter. The software was adapted to the special requirements for the short-stem prosthesis. The cup navigation takes into account inclination, anteversion, reamer depth, and center of rotation. Metha THA navigation also takes into account femoral leg length and offset change data, anatomic antetorsion and implant range of motion (ROM) in dependence on the achieved stem position, and selection of modular neck adapter and prosthesis head. The implant sizes and the postoperative overall length and offset change were registered.

Surgical Technique

Surgery was performed with the patient in the supine position using a modified Watson-Jones approach with a less invasive technique. After exposition and flap incision of the capsule, the rigid bodies for the pelvic passive tracker were fixed on the iliac crest. The femoral rigid body was fixed at the major trochanter using a specific c-clamp. Before head dislocation, the data of the initial body attitude and the femoral plane were collected (Figure 1). The head resection was performed with a minimum of 5-mm proximal neck length. The preparation of the acetabulum and the implantation of the press-fit cup were performed under the control of the navigation system. With the ceramic liner set, the new center of rotation was determined for subsequent calculations.

The implant bed for the femoral stem was prepared manually without navigation. The objective of Metha short-stem navigation is to achieve the best fitting position with the highest stability by multipoint contact with the cortical bone first, which should not be compromised by forcing the stem into another rotation or valgus position. Navigation was used for the subsequent joint reconstruction. In the authors’ series, implant position was verified using C-arm radiography during rasping and trial reduction.

After positioning the prosthesis stem, the biomechanics of the hip were restored through the modular neck adapters and different head sizes with the help of the navigation system. The final stem position was registered, and the navigation system allowed the simulation of biomechanics with regard to a safe ROM without risk of dislocation or impingement. With the best fitting modular neck and head combination inserted, the hip was reducted, and a final joint motion control was performed. Wound closure involved only the closure of the capsule and the iliotibial tract and the use of subcutaneous and cutaneous sutures, because no muscles were detached. Immediate full weight bearing was allowed in the postoperative protocol.

Results

One case of intraoperative perforation of the lateral cortical bone during the insertion of the first entrance rasp was observed, but it did not influence the remaining procedure or outcome. No other complications such as fractures or fissures occurred. Average operating time was 67 minutes. Intraoperative blood loss was 350 mL.

The mean cup size was 52 mm in both groups. The cup position was 45.3° inclination and 16.8° anteversion in group 1 compared with 45° inclination and 19.7° anteversion in group 2. The center of rotation changed 2.0 mm laterally, 1.1 mm distally, and 0.6 mm posteriorly in group 1 compared with 2.3 mm laterally, 2.2 mm distally, and 1.4 mm anteriorly in group 2.

Apart from the largest stem size, number 6, which was not available at the beginning of the series, the size distribution of the Metha stems is nearly even (Figure 2). In group 1 (non-navigated), the most used modular neck adapter was the standard 135° with neutral or 7.5° retrotorsion-antetorsion (12=7.5° retro, 15=0°). In group 2 (navigated), different variants of neck adapters were used more often. The CCD angle used most often was 130°. The antetorsion was primarily 7.5° retro and, in a few cases, neutral or anteverted (20=7.5° retro, 5=0°, 3=7.5° anteriorly) (Figures 3 and 4).

The neck sizes of the ceramic heads were 10 short, 10 medium, and 7 long in group 1, and 8 short, 16 medium, and 4 long in group 2 (Figure 5). Reliable length and offset measurements could be obtained through the navigation system for group 2. The mean lengthening in group 2 was 11.2 mm for the stem and 7 mm overall, with 4.5 mm lateralization offset for the stem and 3.5 mm of medialization overall. The average antetorsion of the stem was 20.1°, with a range of 8.2° to 56.6° (Figure 6).

The average preoperative Harris hip score was 43. The postoperative Harris hip score was evaluated after 6 months. The number is small (11), and the value of 92 is still preliminary. Generally, patients were satisfied with the outcome, but 2 patients complained of thigh pain. Nonetheless, both patients had good functional and radiologic results. In 3 other patients, the leg length could not be adjusted; the elongation was 10 mm to 15 mm.

In the short follow-up period, no radiolucent signs could be detected. Two cases of subsidence averaging 3 mm and 4 mm occurred, both without clinical complaint. The authors assume that the stem, chosen intraoperatively, was too small. Some residual spongious bone between the prosthesis and the cortical bone of the neck in the axial view was noted in radiography controls. In 3 patients, the distal tip of the prosthesis did not reach the lateral cortex, and the prosthesis was in valgus position. The position did not change in follow-up radiography.

Discussion

The primary reason to use short-stem prostheses is to preserve bone stock and muscular structures (especially in combination with a minimally invasive surgical technique) without compromising primary stability and long-term outcomes.¹ The long-term outcome cannot be estimated from the immediate results of this study.

Bone loss appears to be minimal with short-stem and metaphyseal anchorage. The neck is cut higher than with a straight-stem prosthesis, and the greater trochanter region can remain untouched. Also, the metaphysis is not filled, but leaves spongious bone, and the prosthesis has contact with the cortical bone on different points. The amount of bone preserved in case of loosening cannot be predicted, however.

Another advantage of the short-stem prosthesis is its suitability for minimally invasive procedures. In this series, the authors used a modified anterolateral approach according to the Watson-Jones method. Muscle detachment was avoided by the combination of using the prosthesis and approaching through a skin incision of reasonable length. The approach was not affected by the navigation system, which requires fixation of the rigid bodies on the pelvis and femur. The femoral C-clamp, as well as the Steinmann nail, could be positioned through the approach. For better stability, a screw inserted into the iliac crest, which required another incision, replaced the nail. The procedure can be carried out with incisions 8 cm to 10 cm long. Significant primary stability was achieved. In 2 of 55 stems, slight subsidence occurred in the first 6 weeks after implantation. In both patients, the stem size was chosen incorrectly. In the first patient, subsidence occurred while the authors were still in a learning curve. There had been previous surgery involving a ceramic bone plug in the femoral neck. Intraoperatively, this plug affected the rasping, because it was mistaken for the lateral cortical bone. This led to the selection of a stem size too small to fill the femoral neck. With the second patient, no reason could be found for the choice of an insufficiently sized stem. As seen in control cases, subsidence in these patients did not have any clinical consequences. Choosing the right stem size, which cannot be determined exactly by preoperative radiography, remains a major issue in this type of surgery, however. Three of 55 stems were in a valgus position, and the stems had a more proximal contact with the lateral cortical bone. Change of position was not evident in the control cases, indicating that the fixation of the tapered, short stem relies on multiple points that are not always evident in radiographic control cases.

Modularity leads to better restoration of hip biomechanics but also involves risks of fatigue and disconnections.^4,6 Biomechanical investigations have shown a high resistance against fretting corrosion and mechanical failures. In addition, disconnection tests demonstrate that the Metha modular cone design ensures a high-quality fixation of the components.⁷ Also, modularity did not result in any clinical problems. The authors evaluated the modularity with respect to the geometry of the hip. In the non-navigated group 1, surgeons used a neutral antetorsion and a midsize CCD angle. In the navigated group 2, the retroverted neck was more often recommended by the system and even more retroversion was desirable. The CCD angle was given mainly by a larger varus angle resulting from patient selection. The patients in the navigated group were younger than patients typically undergoing THA. Incidence of dysplastic coxarthritis in the navigated group was higher than in the non-navigated group.

Consequently, the stem followed the more valgus and antetorted position of the femoral neck. Thus, the simulation of the navigation system led to corrected values for CCD and antetorsion. Also, the implanted prosthesis heads show an improved distribution: in more than 57% of cases, prosthesis heads of medium neck length were used. The overall length could not be adjusted in all cases, however, and a reduced offset had to be registered. More modular necks sizes would be beneficial to correct the deformities to average biomechanics of the hip. Navigation is considered to increase the accuracy of the positioning in total hip and knee arthroplasty.^8,9 Its reliability is proven for the acetabulum and for the femoral stem.^5,10 Although the aim for the placement of the acetabulum is clear, the parameters for the stem differ. For the cup, the center of rotation should be undisplaced, and the values for inclination and anteversion should be in the range cited by Lewinnek et al.¹¹

The navigation of the short-stem prosthesis differs from navigation of standard-stem prostheses. The positioning of the stem only aims for maximum stability in the metaphyseal femur. The anchorage of the stem is separated from the restoration of the biomechanics of the hip. Therefore, the stem itself is not navigated, but the choice of the optimal modular neck and the joint reconstruction is navigated. The criteria for restoration are offset, leg length antetorsion, center of rotation of the head, and implant ROM. The parameters can be controlled using navigation. A general concern is the actual anatomy, which might be pathologic, as in dysplastic hips or secondary coxarthritis after epiphysiolysis or Perthes disease. In this series, the authors changed biomechanics in cases with a high center of rotation of the acetabulum or extreme antetorsion of the femoral neck. By calculating the safe range of motion and the maximum flexion without impingement, the navigation system could help find the right biomechanics independent from changes in the hip geometry. Dislocation was not observed in the intraoperative test or postoperative follow-up. In this experience, the navigated short-stem prosthesis offered good intraoperative handling and good preliminary results.

References

1. Morrey BF, Adams RA, Kessler M. A conservative femoral replacement for total hip arthroplasty. A prospective study. J Bone Joint Surg Br. 2000; 82:952-958.
2. Sculco TP. Minimally invasive total hip arthroplasty: in the affirmative. J Arthroplasty. 2004; 19:78-80.
3. Hube R, Zage M, Hein W, Reichel H. Early functional results with the Mayo-hip, a short stem system with metaphyseal–intertrochanteric fixation [in German]. Orthopädie. 2004; 3:1249-1258.
4. Bobyn JD, Tanzer M, Krygier JJ, Dujovne AR, Brooks CE. Concerns with modularity in total hip arthroplasty. Clin Orthop Relat Res. 1994; 298:27-36.
5. Lazovic D, Kaib N. Results with navigated bicontact total hip arthroplasty. Orthopedics. 2005; 28:S1227-S1233.
6. Chmell MJ, Rispler D, Poss R. The impact of modularity in total hip arthroplasty. Clin Orthop Relat Res. 1995; 319:77-84.
7. Grupp T, Blömer W. Investigation of the loading stability of the modular conical connection [in German]. Implant. Aesculap AG & Co. KG. January 2005:6-8.
8. DiGioia AM, Jaramaz B, Blackwell M, et al. Image guided navigation system to measure intraoperatively acetabular implant alignment. Clin Orthop Relat Res. 1998; 355:8-22.
9. Stulberg SD, Loan P, Sarin V. Computer-assisted navigation in total knee replacement: results of an initial experience in thirty-five patients. J Bone Joint Surg Am. 2002; 84:90-98.
10. Kiefer H, Othman A. OrthoPilot total hip arthroplasty workflow and surgery. Orthopedics. 2005; 28:S1221-S1226.
11. Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am. 1978; 60:217-220.

Authors

Dr Lazovic and Mr Zigan are from the Clinic for Orthopedics, Pius-Hospital, Oldenburg, Germany.