Rasp Test at DUKE

3. Scientific Comparis

. EFFECT ON INTRAMEDULLAR FEMORAL PRESSURE BY A NEW HOLLOW RASP SYSTEM VS. STANDARD RASPS

Burgkart RH*, Glisson R**, Vail T**, Gradinger R*

* Clinic of Orthopaedics and Sport-Orthopaedics, Technische Universität München, Ismaningerstr. 22, 81675 München , Germany

** Division of Orthopaedic Surgery, Duke University Medical Center , Durham , NC , USA

Introduction:

Fat embolic syndrome is a well known life threatening risk in total hip arthroplasty. One of the most important pathogenetic factors for the bone marrow intravasation are any manipulations, which lead to increased intramedullar pressure. So fat embolic phenomena documented by transoesophageal echocardiography occur - beside stem insertion - also during the preparation of the proximal femur with awls and rasps (Hofmann 1999, Schmidt 2000). Therefore the reduction of intramedullar pressure for this preparation step is an essential aim to minimize the fat embolic risks.

The objective of the present study was to evaluate the effect of a new developed hollow rasp system on the intramedullar femoral pressure in comparison to the conventional rasp system with the identical geometry.

Materials and Methods:

12 fresh frozen human femora ( 6 pairs) with a mean age of 55.8 years (23-69 yrs) were used for this study. The macroscopic and radiographic examination revealed no pathologic changes. Using an oscillating saw the proximal osteotomy was performed. For an identical starting position of the rasps a central 9 mm hole was drilled in the cancellous bone along the axis of the proximal femur with a depth of 40 mm. According to the x-rays the final rasp size and there distal tip position was determined. 1 cm and 11 cm below this assessed distal tip position two 5 mm holes where drilled through the anterior cortex of the femora. Two pressure transducers were threaded through the predrilled holes to reach into the medullary canal proximally (P1) and distally (P2) (Fig.9). A special testing device was constructed to hold the femur and the rasp in a standardized position (Fig.10).

With a precision gliding mechanism a modified mallet ( 1.2 kg, similar to mallet weights used in surgery ) hit the rasp always from an equidistant altitude in respect to the actual rasp position (acceleration distance 1.05 m ) to guarantee identical impact forces (Fig.11).
Before testing low viscosity mineral oil , similar to the properties of the fatty liquid phase of the bone marrow (e.g. released after drilling the cortical holes), was filled in the femoral cavity since all air was removed. After calibration of the pressure transducers the insertion of the smallest rasp started and the data were continuos digital documented in 0.2 s intervals. Before the insertion of the next rasp size always a recalibration was performed.

For statistical analysis of the paired data the Wilcoxon Signed Rank Test was applied.

Results:

From each rasping procedure of a single rasp size the maximum intramedullar pressure was determined.

In the six femora prepared with the standard rasps the maximum pressure values ranged for the proximal location (P1) from 11.3 to 1035.3 mmHg with a mean of 208.6 mmHg (SD + 256.6). At the distal measurement point (P2) the values were significantly smaller (p<0.005) compared to proximally with a range from 4.5 to 527.4 mmHg and a mean of 111.0 mmHg (SD + 111.0).

In the six opposite femora prepared with a new developed hollow rasp system the maximum intramedullar pressures for P1 ranged from 3.0 to 152.3 mmHg with a mean of 40.5 mmHg (SD + 33.8). For the hollow rasps no statistical difference was found for the proximal values compared to the distal ones, for which a range of 4.5 to 45.0 mmHg with a mean of 29.3 mmHg (SD + 13.5) was found.

Table 1: Comparison of intramedullar pressures caused by the standard solid rasps vs. the new developed hollow rasp system of identical geometry. The intramedullar pressures caused by the standard rasps are significantly higher vs. the hollow rasp system.

p < 0.0001; ** p < 0.005; n. s. = not significant

The comparison of the pressure data between the standard rasp vs. the hollow rasp group demonstrated a high statistical difference. So proximally as well as distally the intramedullar pressures in the femora prepared by the standard solid rasps were significantly higher than in the hollow rasp group (p<0.0001 proximal and distal) (Tab. 1, Fig.12).

Discussion:

We constructed a special testing device to hold the femur and the rasp in a standardized position and with a precision gliding mechanism a modified mallet hit the rasp always from an equidistant altitude in respect to the actual rasp position to guarantee identical impact forces. With high sensitive transducers the intramedullar pressure was measured 1 and 11 cm below the final position of the distal tip of the rasp. Under these standardized conditions 12 paired femora were analyzed.

The data of this study demonstrate that the used standard solid rasps caused around the distal tip of the rasp mean intramedullar pressures of 209 mmHg with a maximum of 1035 mmHg. These values are obvious above the physiological systolic pressure of a human being. And even the distal measurements with a mean of 111 mmHg are higher than the normal diastolic blood pressure. This data suggest a high potential risk for bone marrow intravasation and consecutive embolic phenomena.

In contrast the new developed hollow rasp system of identical geometry caused proximally as well as distally highly significant lower intramedullar pressures than the standard solid rasps. The mean values in both measured locations are smaller than 45 mmHg with only a single value above 90 mmHg. The macroscopic evaluation of the rasps, removed after the rasping procedure, already indicate one of the most important factors for the pressure differences. Whereas the ”teeth” of the solid rasps were almost completely filled with compacted bony debris (Fig.13+14) the hollow rasps always had remaining open areas to the hollow center of the tool (Fig.15).

Therefore – especially in the smallest rasp size – a lot of bone marrow debris was found in the ”core” of the hollow rasps (Fig.16, at the left the smallest rasp, Fig.17).

The present study proves the significant effect of different rasp designs on the intramedullar femoral pressure. On the other hand in vivo studies demonstrated that the rise of the intramedullar pressure in the femur is the most important pathogenetic factor of pulmonary embolism during total hip arthroplasty (Pitto 1998).

Therefore a rasp design as the new developed hollow rasp system with a significant reduction of intramedullar pressure is of very high clinical relevance.

So in addition to other important rules as careful preparation techniques (Hofmann 1999, Wenda 1993) the tested hollow rasp design can get a simple, but important tool to minimize the risk of bone marrow intravasation and to subsequently prevent fat embolism with cardiorespiratory deterioration.

References:

Hofmann S et al (1999) Clin Orthop 360:136-46

Pitto RP et al (1998) Clin Orthop 355:23-34

Schmidt J et al (2000) Arch Orthop Trauma Surg 120:100-2

Wenda K et al (1993) Arch Orthop Trauma Surg 112:260-65

Standard solid rasp developed intramedular presure versus Hollowrasp



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