Silent Hip
Introduction
Short stem femoral implants are rapidly gaining importance because an increasing numbers of young patients require joint replacement and hip resurfacing is not always a suitable option. Conventional cementless total hip arthroplasty has serious attendant issues of stress shielding of the proximal femur1-3 and thigh pain4-6 even when the joint is otherwise performing well. Short uncemented proximally fixed anatomic implants were introduced to conserve bone and reduce stress shielding in the femur through a more physiological stress distribution to the proximal femur.7-9 Additionally, use of a metaphyseal loading implant has been reported to eliminate thigh pain completely.10 Short stems aim to bridge the gap between conventional straight design and hip resurfacing. Early experiences reporting good short term results are available.11,12 Preservation of neck has been shown to reduce micro motion and increase torsional stability, which are key factors in the prevention of loosening of the femoral components of the THA.13,14 The likelihood of a revision surgery in young patients undergoing a hip replacement has led to the designing of newer implants which offer preservation of bone and soft tissues. Resurfacing as an option is restricted to large metal on metal bearings and has limitations in addressing deformities of the femoral head and restoring leg length and offset.
SilentTM Hip
The Silent hip is a bone conserving femoral implant of novel design. It is a modular, cementless prosthesis with femoral neck fixation for use in primary total hip arthroplasty.
The Silent hip is cigar shaped cylindrical short hydroxyapatite and porous coated collarless tapered femoral stem which achieves fixation in the femoral neck only and does not violate the femoral canal (Fig. 17.1). The Silent hip utilizes the 12/14 taper thereby allowing a choice of bearing type and size of femoral head.
The preservation of femoral neck as a result of the very high neck cut done to implant Silent hip enables bone conservation as well as bone preservation in the greater trochanteric, metaphyseal as well as diaphyseal regions of femur. The Silent hip can be implanted by minimally invasive techniques resulting in an earlier recovery and rehabilitation. The tapered implant profile of the Silent hip engages in the femoral neck and ensures loading of femur in a more physiological way so as to eliminate any bone loss as a result of stress shielding. The Silent hip achieves primary stability through its taper design and secondary stability by the hydroxyapatite coating which promotes bone ingrowth thereby sealing the femur and acetabulum from particulate debris and thus reducing the risk of osteolysis and late implant loosening.
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Figure 17.1: The SilentTM hip implant
Indications
The Silent hip is indicated for young patients requiring total hip replacement for primary or secondary arthritis who may not be good candidates for surface replacement.
Contraindications
The following are contraindications that could compromise the performance of a Silent hip.
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Poor bone quality in the femoral neck
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Gross deformity or bone loss in the region of the neck of the femur, including osteoporosis
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Severe anteversion of the femoral neck (greater than 20 degrees)
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Anatomic CCD angle equal to or less than 120 degrees
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Males aged above 65 years
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Females aged above 60 years
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Morbid obesity
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Active local or systemic infection
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Previous exposure to radiation of the hip
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Paget’s disease
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Loss of musculature, neuromuscular compromise or vascular deficiency in the affected limb.
Preoperative Templating
One of the main objectives of preoperative templating is to determine the appropriate size of the Silent hip system as well as to ensure precise femoral and acetabular preparation.
The sizing of the SilentTM Hip system refers to the diameter and length of the implant.
The diameter is measured at neck resection level.
The optimal SilentTM Hip implant for any given patient will typically be the largest compatible with the femoral neck structure.
Acetabular templating should be performed to establish the centre of rotation for the femoral head. The femoral implant is templated by aligning its axis with the anatomic CCD angle. The Silent hip is contraindicated in patients who have a CCD <120°. In patients with a CCD between121°-130°, the implant should be placed in as much valgus as possible. In patients with CCD>130°, the anatomic CCD is used to place the implant.
The length of implant is determined using an A/P X-ray (Fig. 17.2A). The ideal implant length positions the shoulder of the implant 3 mm proximally of the neck isthmus and leaves
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Silent Hip
Figure 17.2A: Templating on an anteroposterior X-ray Figure 17.2B: Templating on a lateral X-ray
a 3 mm gap distally between the tip and the inner wall of the lateral cortex. This will provide optimal proximal implant support and will eliminate the potential for end loading.
The implant diameter is selected to give the best fit to cortical bone in the medial region of the neck. Typical neck diameters are smaller in the anterior-posterior plane. Use of a true lateral X-ray is recommended for this step (Fig. 17.2B).
The initial resection is positioned at least 5 mm above the final osteotomy plane, as indicated by the cut-out window in the template.
Surgical Steps
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The Silent hip can be implanted by any of the conventional surgical approach to the hip (Figs 17.3 and 17.4).The femoral head is resected before or after dislocation (Figs 17.5 and 17.6). The orientation of resection is perpendicular to intended Silent hip axis. A conservative neck cut be done (10 mm from the head-neck junction) so as to have greater intraoperative
Figure 17.3: Incision given for posterior approach to the hip
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Figure 17.4: The hip joint has been exposed and hip dislocated
Figure 17.5: The head neck junction is marked
flexibility for selection of implant. A calcar mill can be used later on to achieve the optimum neck resection level.
The acetabulum preparation is now done and appropriate acetabular cup implanted (Figs 17.7 to 17.9).
After making the neck cut, the cut surface is marked with cross-hairs along the anteroposterior and superior-inferior plane (Fig. 17.10). The target device of the Drill Guide is located onto the resection plane in line with the axis of the femoral neck and the Goniometer is then advanced over the main shaft of the Drill Guide.
The telescopic arm of the Goniometer is extended and aligned with the long axis of the femur. The fulcrum is then positioned over the intertrochanteric crest by translating the Goniometer along the shaft of the Drill Guide. The Goniometer is used to recreate the pre-
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Silent Hip
Figure 17.6: A conservative neck cut is done at 5-10 mm from head-neck junction
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Figure 17.7: Acetabular reamers are used to prepare the acetabulum
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Figure 17.8: The acetabular cup is implanted
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Figure 17.9: The Ceramic liner is positioned in the cup
Figure 17.10: The neck cut surface is marked with cross-hairs along the anteroposterior and superior-inferior plane
planned implant angle. The appropriate neck anteversion alignment is achieved by rotating the Drill Guide until it is in line with the natural version of the neck (Fig. 17.11).
Once the desired axis has been determined, the 8 mm Reamer is inserted into the Drill Guide and used to ream the femoral neck through to contact with the lateral cortex (Fig. 17.12). The entry point is always superior to the A/P line to ensure a valgus position of the implant (Fig. 17.13). Extreme care should be taken to ensure that the lateral cortex is not penetrated by the drill.
Depth markings on the 8 mm Reamer and the Depth Gauge instrument should be used to compare actual and intended cavity depth. These markings correspond to the implant size and not the actual depth of the cavity.
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Silent Hip
Figure 17.11: A goniometer is used to recreate the implant angle
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Figure 17.12: A 8 mm reamer is inserted into the drill guide to ream the femoral neck
Figure 17.13: The entry point is always superior to the A/P line to ensure a valgus position of the implant
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Figure 17.14: The depth gauge is used to check that the cavity depth is greater than the length of the implant planned
The 8 mm Reamer, Drill Guide and Goniometer are removed and an intraoperative check is done using the depth gauge to ensure that the cavity depth is greater than the length of the implant planned (Fig. 17.14).
The conical cavity within the femoral neck is initiated by sequential reaming. It is critical to use the reamers sequentially as they are designed to maintain the alignment of the conical cavity.
Progressive sizes of Conical Reamers are then used to increase the diameter of the cavity until contact is made with the inner cortex of the femoral neck (Figs 17.15A to C).
Exposing too much cortical bone will reduce the potential for the femur to accommodate the intended press-fit. Initial exposure of cortical bone may take place at any point along the length of the cavity, so care should be taken throughout conical reaming not to remove too much cancellous bone.
The diameter of the final reamer used will determine the diameter of the final implant and should be recorded. The horizontal grooves located in the cutting region of the reamers indicate the depth of the implant. These should be used together with the preoperative planning data to assist initial trial implant selection.
Trial Implant is selected based on the Final Reamer diameter and planned implant length. The trial implant is inserted into the prepared femoral cavity using the T-Handle (Fig. 17.16A). At this stage it is important to check that the trial implant sits recessed into the initial resection (Figs 17.16B and C). This is the correct position, due to the conservative neck cut. If the Trial is proud, a smaller final implant should be considered.
A trial Reduction is then done and appropriate adjustments made (Fig. 17.16D).
Once trialing has been performed satisfactorily and the implant size selected, the Calcar Mill should be used (Fig. 17.17). The Calcar Mill is located over the Trial Implant and used under power to mill the excess bone until the depth stop position is achieved.
The bone level will now be flush with the face of the trial (Fig. 17.18).
The trial is extracted (Fig. 17.19) and the final implant placed into the initial cavity by hand. If initial hand placement of the implant leaves the shoulder of the implant more than 8 mm proud of the milled plane, it will be difficult to achieve a final seating height. In this case, the implant should be removed from the cavity and re-reaming done with the last reamer used. The implant is then reinserted and checked.
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Figures 17.15A to C: Conical reamers are used to create a conical cavity within the femoral neck until contact is made with the inner cortex of the femoral neck
The Implant Impactor instrument (with acetal bush assembled) is then located over the taper and the implant is firmly impacted until good stability is achieved. The shoulder of the implant should seat approximately 3 mm proud of the final osteotomy plane after calcar milling (Figs 17.20A and B). The definitive femoral head implant is impacted on the trunion and hip reduced (Figs 17.21A and B).
Tips and Tricks
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Valgus position of the implant can be ensured by using goniometer as well as by measuring the distance of the line drawn along the proposed position of the implant in the femoral neck and the base of the greater trochanter during preoperative planning
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Increasing valgus accommodates a longer implant in the neck.
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While preparing the cavity in the femoral neck, one should stop as soon as any of the cortices is reached, most commonly anterior, to avoid compromising the fixation of the implant in the neck.
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End loading of the implant as well as the varus positioning must be avoided at all times.
Results
In a prospective, non-randomized, pilot clinical study of the Silent hip to assess the in vivo stability using radio-stereometric analysis (RSA) and the clinical outcomes, Sullivan et al15 reported good clinical results in 41 patients at two years follow-up. The assessment of the
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Figures 17.16A to E: The trial implant is inserted into the prepared femoral cavity using the T-handle. Note that the trial implant sits recessed into the initial resection because of the conservative neck cut. Trial reduction is done
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Silent Hip
Figure 17.17: The calcar mill used to remove the excess bone
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Figure 17.18: After the excess bone has been removed, the bone level is now flush with the face of trial
Figure 17.19: The final prepared femoral neck for the Silent implant after removal of trial implant
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Figures 17.20A and B: The Silent implant has been implanted. The shoulder of the implant should seat approximately 3 mm proud of the final osteotomy plane
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Figures 17.21A and B: A ceramic head positioned and final reduction done
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radiographs showed increased bone density in the region of the calcar. Two year RSA data showed an acceptable level of femoral stem migration (<1 mm in any of the 3 axes).
In the second phase of the study at a follow up of 5 years,16 the RSA findings showed no
evidence of any patterns of continuing migration that would be a predictor for early to mid-term loosening.
Silent Hip
Illustrative Case
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A 25-year-old female presented with a diagnosis of secondary osteoarthritis following Tuberculosis hip (Fig. 17.22A). On Clinical examination, she had a true shortening of 1 cm. Her hip was fixed in 20° Abduction and rest of movements were jog. A total hip replacement was done using the SilentTM Stem and Pinnacle cup with a ceramic on ceramic articulation (Fig. 17.22B).
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Figure 17.22A: Preoperative X-ray showing secondary osteoarthritis of left hip following tuberculosis
Figure 17.22B: Postoperative X-ray after implantation of Silent TM Stem and PinnacleTM cup
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References
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McAuley JP, Culpepper WJ, Engh CA. Total hip arthroplasty: concerns with extensively porous coated femoral components. Clin Orthop 1998;355:182-8.
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Claus AM, Hopper RH Jr, Engh CA. Fractures of the greater trochanter induced by osteolysis with the anatomic medullary locking prosthesis. J Arthroplasty 2002;17:706-12.
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Engh CA Jr, Young AM, Engh CA Sr, Hopper RH Jr. Clinical consequences of stress shielding after porous-coated total hip arthroplasty. Clin Orthop Rel Research 2003;417:157-63.
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Mishra AK, Skinner HB, Davidson JA. Stem loosening and thigh pain in THA: are they related to the prosthesis stiff ness? Orthopaedics 1997;20:58-61.
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Fumero S, Dettoni A, Gallinaro M, Crova M. Thigh pain in cementless hip replacement. Clinical and radiographic correlations. Ital J Orthop Traumatol 1992;18:167-72.
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Kim YH, Kim JS, Cho SH. Primary total hip arthroplasty with a cementless porous-coated anatomic total hip prosthesis: 10- to 12- year results of prospective and consecutive series. J Arthroplasty 1999;14:538-48.
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Renkawitz T, Santori FS, Grifka J, Valverde C, Morlock MM, Learmonth ID. A new short uncemented proximally fixed anatomic femoral implant with a prominent lateral flare: design rationals and study design of an international clinical trial. BMC Musculoskeletal Disorders 2008;9:147.
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Tai CL, Shih CH, Chen WP, Lee SS, Liu YL, Hsieh PH, et al. Finite element analysis of the cervico-trochanteric stemless femoral prosthesis. Clin Biomech (Bristol, Avon) 2003;18:53-8.
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Niinimaki T, Junila J, Jalovaara P. A proximal fixed anatomic femoral stem reduces stress shielding. Int Orthop 2001;25:85-8.
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Kulkarni M, Wylde V, Aspros D, Learmonth ID. Early clinical experience with a metaphyseal loading implant: Why have a stem? Hip International 2006;16(1)(Suppl 3):S3-S8.
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Walker PS, Culligan S, Hua J, Muirhead-Allwood SK, Bentley G. The effect of a lateral flare feature on uncemented hip stems. Hip Int 1999;9:71-80.
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Jasty M, Krushell R, Zalenski E, O’ Connor D, Sedlacek R, Harris W. The contribution of the nonporous distal stem to the stability of proximally porous coated canine femoral components. J Arthroplasty 1993;8:33-41.
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Malchau H, Wang YX, Karrholm J, Herberts P. Scandinavian multicenter porous coated anatomic total hip arthroplasty study. Clinical and radiographic results with 7 to 10 years follow-up evaluation. J Arthroplasty 1997;12:133-48.
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Smith-Petersen MN. Approach to and exposure of the hip joint for mold arthroplasty. J Bone Joint Surg Am 1949;31:40-6.
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James Sullivan, Matthias Honl. Stability, clinical and radiographic results of the Silent hip, a new, neck preserving femoral prosthesis in a prospective study including radiostereometric analysis. EFFORT 2009 Poster.
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Marcellino Maheson, Matthias Honl, James Sullivan, Andreas Kurth, Nicola Pace, Philippe Piriou, Richard Verheul, Craig Waller. The SilentTM Hip—A new solution in primary total hip artroplasty. Short- to mid-t