THE OMNIFIT SPECIALTY FEMORAL SYSTEM

DESIGN RATIONALE

The Omnifit Specialty Femoral System is designed to ad- dress the problems of revision surgery where extensive fixation, both distally and proximally, is required for sup- port of the prosthesis. The stern is fabricated of cobalt- chromium alloy with a coarse cobalt-chromium plasma spray surface placed over virtually the entire length of the stem.

The Specialty Stem is designed with a large proximal cross section to provide for improved load distribution over a broad area. Distally, the design incorporates a cylindrical section to more effectively use the available bone of the diaphysis. A comprehensive size range mates eight proximal cross sections with 10 distal cylin- drical diameters to provide for patient-matched sizing in revision circumstances.

Instrumentation used with the Specialty Stem facili- tates stem placement and fit. The tapered axial reamers are capable of machining cortical bone to achieve exacting fit to the stem in the proximal axisymmetric region. Fully fluted cylindrical reamers in 1/2-ram increments prepare the distal canal to achieve circumferential contact with the distal cylindrical geometry of the stem.

The broach/trial contours the proximal femur to the asymmetric geometry of the stem to provide a comple- mentary fit of the stem in bone. The broaches are de- signed as single-pass instruments, with a series of ribs that are intended to seat against highly densified cancel- lous or cortical bone. The rib pattern allows for an accu- rate gauge of the level of stem fit, while grooves leave behind adequate cancellous bone for an interference press fit.

SURGICAL TECHNIQUE

Because the distal and proximal portion of the implant are sized and prepared independently, initial canal prepara- tion begins with the cylindrical axial reamers (Fig 1). Once the distal size has been determined, preparation with tapered reaming and broaching is performed to fit the most appropriate proximal size for that patient. The chart (Fig 2) provides distal/proximal sizing variation in- formation. The size of the cylindrical portion of the stern is determined from preoperative templating and is con- firmed by intraoperative feel or by intraoperative x-ray.

Positioning of the cylindrical reamers is critical with a straight stem design (Fig 3). It is important to position the cylindrical reamer laterally so that it is not pushed into varus by an overhanging trochanter. It is also un- desirable to position the reamer too far anteriorly or pos-

Reprinted from the Omnifit Specialty Femoral System (technical monograph no. LSP-29) by permission of Osteonics, Allendale, N J.

1048-6666/92/0204-0004505.00/0

teriorly. Use of a gouge or similar instrument to remove bone from this area may facilitate true axial alignment of the reamer.

In a revision situation, if there is any femoral neck left, the position of the pilot hole generally corresponds to the original insertion of the piriformis tendon.

Using a hand tool, serial cylindrical reaming proceeds in 1-mm increments until endosteal contact is felt. Cy- lindrical reamers are rigid and fully fluted and are cali- brated for length of the stem in 10-ram increments. Depth of reaming will depend on the size of the stem. For example, if preoperative templating indicates a no. 9 stem with a 13-mm distal diameter, depth of insertion of the cylindrical reamer is 175 mm. (The length designa- tion of the reamer provides the necessary reaming depth to insert a stem of that length and is greater than the stem length. Therefore, it cannot be used as a measuring de- vice.) The chart indicates depth of insertion of reamers for all stem sizes (Table 1). The cylindrical reamers are inserted so that the marking for stem length intersects the most proximal level of the final neck resection (Fig 4). Reamers have been designed to extend 6 mm (1/4 in) be- yond the intended final stem seating level. This design feature allows for slight variations in stern seating level based on patient bone quality and other intraoperative variables. Failure to insert the reamers to their appropri- ate depth may result in femoral fracture during stem in- sertion.

Beginning with the cylindrical reamer, which is 2 mm smaller than the intended implant size, reaming pro- gresses sequentially in 1/2-mm increments and can be used with a power source. An intraoperative x-ray is helpful in determining alignment and fit of the cylindrical reamers. Irrigation~of the canal may be helpful during reaming.

The stem diameter has a 0.5-mm oversize built into it. A choice can be made intraoperatively to underream by 0.5 mm or to ream line-to-line. For example, for a no. 9 Stem with a 13-mm diameter, the true outside diameter is 13.5 mm. A choice may be made intraoperatively to con- clude final cylindrical reaming at 13 mm or 13.5 mm. This depends on the surgeon's preference and on bone quality. Underreaming over the entire stem length in good quality bone may result in significant difficulty dur- ing stem insertion.

Preoperative templating will assist in determining the approximate cylindrical diameter, which can be con- firmed intraoperatively. At this point, an initial stem size confirmation may be made. For example, if preoperative templating indicates a no. 9 stem with a 13-mm diameter, cylindrical reaming progresses to 13-mm or 13.5-mm (de- pending on surgeon preference and quality of bone), and a determlnahon is made on the amount of endosteal con- tact. If adequate circumferential contact is made distally, the proximal canal is then initially prepared with tapered reamers and broaches. A final stem size determination

Operative Techniques in Orthopaedics, Vol 2, No 4 (October), 1992: pp 225-230 225

A

t

prepared by ' ~ it-: Cylindrical Reamer , B

Fig 1. (A) Fu l l y fluted cylindrical reamer. (B) The canal segment prepared by the cy l in - d r i c a l reamer is lo - ca ted in the isthmus as demonstrated b y the box . ( C o u r t e s y of Osteonlcs.)

will be made following final tapered reaming and broach- ing, which may require further cylindrical reaming for diameter and depth.

In cases of revision surgery, the neck resection level and/or angle may need to be adjusted to conform with the neck angle of the Specialty Stem and to optimize stem fit.

The level of neck resection can be set by making a mea- surement from the lesser trochanter to the resection guide based on the measurement obtained in the preop- erative analysis. Adding 1 to 2 mm to this measurement will leave adequate bone to allow for accurate calcar plan- ing, which will be performed following final broaching (Fig 5).

Once the distal femoral canal has been machined, the proximal canal is prepared using tapered axial reamers (Fig 6). Reaming is initiated with the tapered axial reamer that is two sizes smaller than the templated stem size and progresses sequentially upward until the antic- ipated final size is reached. For example, for a no. 9 Stem, tapered reaming begins with a press-fit no. 7 and proceeds to a press-fit no. 9. However, final size deter- mination will be determined following broaching. At this point, it is critical to keep in mind the last size of the tapered reamer used in case further tapered reaming be- comes necessary.

7 8 9 I0 !1 12 13 14

II 12 13 14 15 16 17 18 19 20

PROXIMAL

DISTAL (MM)

Fig 2. Permutations of distal and proximal sizing variations available with the Specialty system. (Courtesy of Osteon ics )

A

. !

\

B

t Fig 3. A x i a l alignment must be maintained dur- ing reaming. The bone of the greater trochanter must not push the rigid reamer i n to a varus posi- tion. (Courtesy of Os- teon ics . )

Accurate use of the tapered axial reamer requires the passage of the reamer into the canal so that the most proximal level of the cutting flutes intersects the most proximal aspect of the level of the final neck resection (Fig 7).

The femoral broach/trial is used to contour the proximal. femur. Unlike the reamers, which are "designed to cut

TABLE 1. Algorithm for Stem Size Distal Diameter, Final Cylindrical Reamer Size, and Depth of Cylindrical Reamer Size

Stem Distal Final Cylindrical Cylindrical Reamer Size Diameter (mm)* Reamer (mm)t Depth of Insertion

7 11 11 or 11.5 165 7 13 13 or 13.5 8 12 12 or 12.5 165 8 14 14 or 14.5 9 13 13 or 13,5 175 9 15 15 or 15.5

10 14 14 or 14.5 175. 10 16 16 or 16.5 11 15 15 or 15.5 175 11 17 17 or 17.5 12 16 16 or 16.5 185 12 18 18 or 18.5 13 17 17 or 17.5 185 13 19 19 or 19.5 14 18 18 or 18.5 185 14 - 20 20 or 20.5

/

NOTE. The length designation on the reamer provides the neces- sary reaming depth to insert a stem of that length and is greater than the stem length. Therefore, it cannot be used as a measuring device.

* 0.5-mm oversize built into diameter. t Reaming to the nominal diameter may create excessive interfer-

ence and may increase diff iculty of stem insertion.

226

B

Fig 4. The fully fluted cy- lindrical reamer is placed in the canal so the mark- ing for the stem length in- tersects the most proxi- mal level of the final neck resection. (Courtesy of Osteonics.)

I 7 can~ sagmat

Fig 6. Tapered reaming prepares that part of the canal proximal to the isthmus but distal to the lesser trochanter. (Cour- tesy of Osteonics.)

Tapered Axial ReameJ

A t r S

Fig 5. Using the femoral neck resection guide, a methy- lene blue mark would be made to indicate the level of resection. The initial neck resection is made 1 to 2 mm higher, as shown, to allow for calcar planing. Calcar planing would remove an additional 1 to 2 mm of bone. (Courtesy of Osteonics.)

OMNIFIT SPECIALTY FEMORAL SYSTEM '227

A Fig 7. (A) Axial alignment must be maintained during ream- ing, (B) Misalignment will result in an improperly prepared canal. (Courtesy of Osteonics.)

C-Taper Stern Trial Head

h,,. ~ . C.T~er Trial Ne~

Fig 8. Femoral broach trial with associated trial distal tip and head neck trials. (Courtesy of Os- teonics.)

cortical bone, broaches are primarily crushing and scrap- ing instruments used to shape cancellous bone (Fig 8).

The broach is designed with a series of ribs that are intended to seat against highly densified cancellous or

cortical bone when the final broach is used. The outside dimension of the ribs is equivalent in size to the corre- sponding stem size, thus providing close approximation to the final seating level of the stem. Grooves in the broach leave behind ribs of cancellous bone to provide for an interference fit of stem to bone (Fig 9). However, the broach does not reflect the true length of the implant.

Thus, the modular broach allows for (1) an accurate confirmation of the ability to fit a given stem size; (2) ribbed areas of cancellous bone for interference fit of the stem; (3) an approximation of the seating level relative to the initial resection level; and (4) ability for calcar planing and trial reduction with the seated broach.

During sequential broaching, accurate use of the broach requires passage of the broach into the canal so that the most proximal level of the cutting teeth intersects the most proximal aspect of the final neck resection (Fig 10). This may mean that the broach is countersunk below the preliminary resection to allow for subsequent calcar planing. Broaching progresses sequentially upward in size to the broach numbered the same as the stem and application.

The final stem size determination is made with the ta- pered reamers and broach, based on the amount of achievable canal fill and stability of the broach in the ca- nal. Once that determination is made, if the final broach size differs from the size initially prepared for with cylin- drical and tapered reamers, it is necessary to go back through these steps to finalize the preparation. Once the appropriate size broach is fully seated in the canal, the handle is detached to allow for calcar planing and trial reduction.

The calcar planer is driven by hand or by means of a

~ m k d .

111 earn" almmlt~ ~ k m .Ir

II m

m

/

Fig 9. Cross-sectional analysis of femoral canal during var- ious stages of canal preparation. (Courtesy of Osteonics.)

228

Br0adVTrlal

Fig 10. Broaching of the ,f~ femoral canal must be performed in the proper longitudinal orientation, / in the same fashion that the reaming was per- T~.~r,-~.~,~ formed. (Courtesy of Os- teonics.)

F~aJ Br0~.h/Tna~ in B0~e

Fig 12. Broach/trial as- sembly. (Courtesy of Os- teonics.)

Fig 11. The calcar planer is used with the seated fi- na l b r o a c h / t r i a l to achieve the final neck re- section level. (Courtesy of Osteonics.)

power source to achieve a final neck resection level and angle to optimize stem fit and collar/calcar contact (Fig 11). Sequential upward planing may be performed when using a large stem or when excessive bone must be re- moved.

The C-Taper trial neck is chosen, which has the same base neck length as the intended implant and is placed over the post on the seated broach (Fig 12).

The plastic Omnifit C-Taper Stem Trial Heads may be used with these trial necks, which correspond to the neck length capabilities for the femoral component . The broach/trial assembly allows thorough evaluation of hip mechanics during trial reduction. The assembled trial provides an approximation of leg length, stability, and offset.

Once the final stem size determination has been made, it is critical to ensure that the cylindrical reaming has been made to a length sufficient for the chosen stem size. The final cylindrical reamer used is reinserted into the canal to

Fig 13. As a final depth check, the fully fluted cylindrical reamer is placed in the canal so the marking for the stem length intersects the most proximal level of the final neck resec- tion. (Courtesy of Osteonics.)

the appropriate stem length marking as a final depth check (Fig 13). For example, if the final stem size is no. 9 with a 13-mm diameter, the 13- or 13.5-mm cylindrical reamer is inserted to the level indicating 175, which is the stem length. If the reamer cannot be inserted to the ap- propriate stem length level, further cylindrical reaming to achieve appropriate depth may be required (see Table 1).

It is important to match depth of cylindrical reamers to the intended stem size and length. Failure to insert the reamers to their appropriate depth may result in femoral fracture during stem insertion.

Implantation of the stem is performed following trial reduction. Because the Omnifit Specialty Stem engages

OMNIFIT SPECIALTY FEMORAL SYSTEM 229

the reamed diaphysis distally, it is critical that the stem version be correct and maintained in the appropriate ori- entation throughout the impaction process. Impaction of the implant can result in a fracture of either the anterior or posterior metaphysis if the axial alignment is not correct.

The stem is carefully placed in the intramedullary ca- nal, making certain that rotational alignment is appropri- ate. The assistant manages the version control arm, and impaction begins while maintaining the proper amount of version. The Version Control Arm should be torqued

slightly with each impaction blow. Anteversion can be checked by placing a reference mark on the medial cortex and aligning it with the medial aspect of the stem. The stem should advance slightly with each impaction blow.

When the proximal segment of the stem begins to en- gage, and stem rotation is determined, the Version Con- trol Arm is removed. Final seating of the stem is per- formed with no further adjustments to rotation. The stem seating level may vary due to variables in prepara- tion and implant seating.

230