WO2010023478A1

WO2010023478A1 - Patella position measuring device

Info

Publication number: WO2010023478A1
Application number: PCT/GB2009/051075
Authority: WO
Inventors: Kevin Daniel Campbell-Karn
Original assignee: London Metropolitan University
Priority date: 2008-09-01
Filing date: 2009-08-27
Publication date: 2010-03-04
Also published as: GB2463061A; GB0815862D0; GB2463061B; EP2344037A1

Abstract

A device for measuring the lateral position of the patella relative to the femur of an individual comprising a patella calliper with a first pair of jaws comprising a first jaw with a first jaw face and a second jaw with a second jaw face, said first and second jaws being capable of movement towards and away from each other, to allow said jaw faces to contact, in use, the outer edges of a patella a femur calliper with a second pair of jaws, comprising a first jaw with a first jaw face and a second jaw with a second jaw face, said first and second jaws being capable of movement towards and away from each other, to allow said jaw faces to contact, in use, the outer edges of a femur, adjacent the patella, the first pair of jaws being movably attached to the second pair of jaws and measuring means to measure the lateral displacement of the first pair of jaws relative to the second pair of jaws. The device is best illustrated by figure 1 of the drawings. Also, a kit comprising the device and a further calibration device comprising two forms, one of a greater diameter than the other, collocated in such a way that the forms share a common axis.

Description

PATELLA POSITION MEASURING DEVICE

Field of the Invention

The invention relates to patella position measuring devices. More specifically, the invention relates to patella position measuring callipers. Most specifically, the invention relates to callipers used to measure the mal-tracking of a patella in an individual, relative to that individual's femur.

Review of the art known to the Applicant and problems to be solved

Patello femoral pain syndrome ("PFPS") is a debilitating condition and constitutes a high percentage of all knee complaints. PFPS has long been recognised as a disruption between the kinematic qualities of the PFJ and amongst other problems is associated with incorrect tracking of the patella in the trochlear groove.

Lateral mal-tracking is thought to be the main contributing factor to PFPS. Patellar mal- tracking includes movement which causes the patella to have an abnormal position in the trochlear groove through flexion and extension movements of the knee. This leads to increased stress on a portion of the retropatellar surface which in turn causes the hyaline cartilage lining to become damaged and to irritate pain sensitive structures. Therefore, PFPS is due to increased contact pressure which irritates the retropatellar structures, causing a cartilage lesion.

PFPS is a complex condition that is not easily diagnosed. It is still more difficult to accurately identify the causative structures that are producing the problem in a given case. Common causes of PFPS which have been identified include a combination of: increased tension of the lateral structures; weakness of the medial structures; and poor neuromuscular mechanics. The range in which PFPS is found to be most prominent has been identified as being during the last 30° of extension, which is thought to be due to the patella dropping deeper into the trochlea as the knee flexes past 30°.

Mal-tracking can be influenced by many intrinsic and extrinsic factors. Examples include: patella retinaculum tension, patellar femoral ligament tension, rotation of the femur, incorrect neuromechanics of the quadriceps, weakness of the vastus medialis, and tightness of the vastus lateralis and the iliotibial band. One or more of these factors may cause the patella to track laterally, often in combination with tilt. The main cause of PFPS is due to these influences on Patellar positioning.

To ensure that PFPS is correctly treated, the diagnosis is critical. Current methodologies include: clinical assessment using the McConnell technique; Q-angle measurement; functional testing; and scanning techniques including x-ray, MRI, and CT. Issues that arise with the current methodologies include lack of objectivity in the case of the McConnell technique, cost in the case of the MRI scanner and invasiveness in the case of fluoroscopy. Furthermore, there have been questions raised about the reliability and validity of clinical assessment with contrasting results being obtained in the surrounding literature. (Compare Wilson T. "The Measurement of Patellar Alignment in Latello femoral Pain Syndrome: are we confusing assumptions with evidence?" (Journal of orthopaedic and sports physical therapy 2007; 37(6):330-341) with Keller JM, Levine WN. "Evaluation and Imaging of the Patello femoral joint" (Operative techniques in Orthopaedics 2207; 17 :204-210) as well as with McE wan I, Herrington L, Thorn J. "The validity of clinical measures of patella position" (Manual therapy 2007; 12:226-230) and with Herrington LC. "The inter-tester reliability of a clinical measurement used to determine the medial/lateral orientation of the patella" {Manual Therapy 2000:7(3): 163- 167.))

This profusion of views may lead to an incorrect or contested diagnosis, due perhaps to confusion as to the correct measurements to be sought, or to the correct means with which to seek them. Utilisation of the wrong method brings with it a risk of recovery time being compromised. Further, if the appropriate method is not immediately accessible, it may lead to the application of a symptom-based treatment regime that is at risk of being ineffective and potentially harmful. If the treatment therapist wants to identify an objective measure aside from the observation of pain to examine the progress of the patient it is necessary for the therapist to have the correct apparatus.

Clearly then, clinical methods of assessing the displacement of the patello-femoral joint involve subjective observations. These are performed by the examination of the surrounding structures that may predispose a person to PFPS. However, these can be easily criticised for their poor reliability. There are no quantifiable outcome measures for observations to truly ascertain if any displacement exists. The Q-angle and other lower limb alignment measures have been associated with PFPS but not as a direct measure of specific patellar mal-tracking. It is, therefore, essential that the clinical environment has a reliable, valid, cheap, and accessible assessment method for identifying displacement of the patella.

Magnetic resonance imaging ("MRI") is considered to be the "gold standard" for clinical assessment of PFPS and is the method of choice for accurate assessment of patellar mal- tracking. Although MRI is under constant development and is becoming more accessible by the clinician with easier access to imaging methods for wider imaging parameters, there are currently many limitations to the application of MRI. These, principally, are the restrictions of traditional MRI scanners; they only allow for a supine assessment of the patient and thus scans can only present a non- weight bearing limb. First, this means that the stresses put on the knee in use cannot be measured using this technique. Second, the MRI scanner is unable to image the entire flexion range. Such limitations reduce the external validity due to the scanner not permitting dynamic weight bearing activities, the parameters within which PFPS is encountered. They are also unable to track the patella through the complete range of movement unless individual images of angles are taken which will take a considerable amount of time to ensure a good image quality. This would lead to further problems because the participant would need to maintain a contracted state for the images to represent weight-bearing activity.

For a clinical setting to assess and manage a PFPS subject a device needs to be cheap and easy to use with good reliability and validity.

It is an object of the invention to attempt to provide a solution to these and other problems.

Summary of the invention

In a first broad, independent aspect, the invention comprises a device for measuring the lateral position of the patella relative to the femur of an individual comprising a patella calliper with a first pair of jaws comprising a first jaw with a first jaw face and a second jaw with a second jaw face, said first and second jaws being capable of movement towards and away from each other, to allow said jaw faces to contact, in use, the outer edges of a patella, a femur calliper with a second pair of jaws, comprising a first jaw with a first jaw face and a second jaw with a second jaw face, said first and second jaws being capable of movement towards and away from each other, to allow said jaw faces to contact, in use, the outer edges of a femur, adjacent the patella, the first pair of jaws being movably attached to the second pair of jaws and measuring means to measure the lateral displacement of the first pair of jaws relative to the second pair of jaws.

The clear advantage of this device is that, in combining two callipers, it dispenses with the need to use two separate callipers at the same time. This allows the measurement of a specific lateral position of a patella to be done by a lone individual with relative ease.

This device also meets the need for the provision of a relatively simple, sturdy and cheap- to-produce device. It may be used to produce measurable results, and may further be produced in a lightweight, portable form. A handheld version is envisaged as a popular embodiment, and this, of course, would allow such devices to be brought to the patient, for example if the patient was immobile. As such, the invention provides a useful and flexible diagnostic tool.

In a first subsidiary aspect, the invention comprises a device wherein the distance between the first and second jaw faces of the femur calliper at full extension is greater than the distance between the first and second jaw faces of the patella calliper at full extension.

In a second subsidiary aspect, the invention comprises a device wherein the measuring means is attached to the device.

This attachment allows for accuracy and consistency of measurement. Also, this clearly stops the measuring means from getting mislaid, and simplifies use, as an attached measuring device dispenses with the need to hold said measuring device.

In a third subsidiary aspect, the invention comprises a device wherein the jaws of at least one of the patella calliper and the femur calliper are configured such that the first jaw face maintains its orientation in relation to the second jaw face as the jaw faces move.

This simplifies the measuring process, because one does not have to create a measuring scale which takes account of the orientation of the jaws of the callipers. It also allows for a consistency of grip to be maintained on the subject patella or femur during flexion of the limb.

In a fourth subsidiary aspect, the invention comprises a device wherein jaws are configured such that movement of one jaw relative to the other calliper produces a corresponding movement of the other jaw in an opposite direction and of equal distance.

This is particularly important in relation to function of the patella callipers, in particular embodiments of the invention, where lateral mal-tracking of the knee is measured by taking a note of the position of the central point of the callipers in relation to a measure. By designing the paired calliper arms to move dependently, it enables there to be a central position which can be extrapolated from each calliper.

In a fifth subsidiary aspect, the invention comprises a device further comprising an actuator for actuating the movement of at least one pair of jaws.

The use of an actuator may allow for more precise adjustments of the jaws, and may further allow for the automatic adjustment of the jaws.

In a sixth subsidiary aspect, the invention comprises a device wherein both pairs of jaws have an actuator and said actuators share controlling means.

This is advantageous because it allows for a more compact device.

In a seventh subsidiary aspect, the invention comprises a device wherein the jaws of the patella calliper further comprise inwardly-projecting lugs, at least one of which being moveable, arranged such that the lugs may be positioned on an anterior face of a patella, the relative position of the lugs thereby giving an indication of the degree of tilt of the patella.

Thus, an extra dimension of the mal-tracking can be measured and understood.

In an eighth subsidiary aspect, the invention comprises a device wherein the jaw faces of the patella calliper lie in substantially the same plane as the jaw faces of the femur calliper.

This is particularly advantageous, especially when opposed to the use of two separate callipers, because it would clearly be difficult, particularly for the individual, to gain a measure of both femur and patella in the same plain, at the same degree of flexion.

In a ninth subsidiary aspect, the invention comprises a device wherein the jaws of at least one of the callipers is releaseably lockable. Locking allows is advantageous in two ways. First, locking allows measurements to be taken at relative leisure. Second, the possibility of locking a pair of jaws (most appropriately the jaws of the femur calliper) would allow for firmness of position while the other calliper was moved into place.

In a tenth subsidiary aspect, the invention comprises a device wherein at least one actuator comprises a rack and pinion mechanism.

The rack and pinion mechanism is a simple means of allowing for precise adjustments, of a regular distance. As such, it is appropriate for a measurement device.

In an eleventh subsidiary aspect, the invention comprises a device further comprising a tilt indicator.

The tilt indicator allows for fine adjustment of the angle of the device. It is particularly important, because mal-tracking will tend to be measured at a typical angle of flexion of the knee. Consistency of angling therefore begets accuracy of measurement.

In a twelfth subsidiary aspect, the invention comprises a device wherein the Patella calliper has a maximum separation of up to 80mm.

In a thirteenth subsidiary aspect, the invention comprises a device wherein the Femur calliper has a maximum separation of up to 200mm.

Clearly, although these are the preferred operational ranges, being felt by the inventor to constitute the ranges of human Patellar and femoral widths respectively, a device with a somewhat wider or narrower reach should also be considered to be covered by this patent, not least because a scale version of this device could be used in veterinary medicine.

In a fourteenth subsidiary aspect, the invention comprises a device wherein at least one of the jaws has a bevelled edge. Bevelling the jaws makes the device less likely to harm the patient, who is most likely quite sensitive to pain due during the taking of measurements, due to his or her knee injury.

The invention also comprises a device substantially as described herein with reference to and as illustrated in any appropriate combination of the accompanying drawings.

In a third broad, independent aspect, the invention comprises a kit comprising a device according to any of the preceding claims and a calibration device comprising two forms, one of a greater diameter than the other, collocated in such a way that the forms share a common axis.

Preferably, at least one of the two forms is cylindrical.

The calibration device is a quick and simple way of ensuring that the two devices share a centre point, before they are used. The simplicity of the calibration device makes it robust; still more so the cylindrical design.

The invention also comprises a kit substantially as described herein with reference to and as illustrated in any appropriate combination of the accompanying drawings.

Brief description of the figures

The invention will be more fully understood in the light of the following description of several specific embodiments. The description is made with reference to the accompanying drawings of which:

Figure 1 is an elevation of a patella calliper; Figure 2 is an elevation of a femur calliper; Figure 3 is an alternative embodiment of a patella calliper; Figure 4 is an alternative embodiment of a femur calliper; Figure 5 is a diagrammatic side elevation view of a jaw arrangement of a patella calliper; Figure 6 is a diagrammatic schematic plan view of a jaw arrangement of a patella calliper; Figure 7 is a diagrammatic side elevation view of a jaw arrangement; Figure 8 is a diagrammatic schematic plan view of a jaw arrangement; Figure 9 is a diagrammatic elevation view of a further embodiment of the invention; Figure 10 is a diagrammatic elevation view of a further embodiment of the invention, illustrating the use of a tilt measurement device; Figure 11 is a diagrammatic elevation view of an embodiment of a calibration device.

Description of Preferred Embodiments

Figure 1 is an elevational view of a patella calliper 16 forming part of a measuring device 10 of the present invention. The calliper comprises a first jaw 20 with a jaw face 22, facing towards the jaw face 26 of a second jaw 24. The jaws 20 24 are slideably mounted onto a plate 100. Each of the jaws is provided with a rack 64 engaged with a common pinion 66, forming a double rack and pinion mechanism. Rotation of the pinion 66 thus causes the jaws to move apart, or to move together, each jaw moving at an equal rate with respect to the plate 100, but in an opposite direction. A scale 102 is marked along an edge of the plate 100. A slot 104 is provided in the back plate for slideably mounting the patella calliper to the femur calliper 32 illustrated in Figure 2.

Figure 2 illustrates a femur calliper 32 forming part of a measuring device 10 of the present invention. The calliper comprises a first jaw 36 with a jaw face 38, facing towards the jaw face 42 of a second jaw 40. The jaws 36, 40 are slideably mounted onto a plate 106. In a similar fashion to the patella calliper of Figure 1, the jaws 36, 40 are each provided with a rack 64, engaged with a common pinion 66, so forming another double rack and pinion mechanism, operating in a fashion analogous to that described for the patella calliper. The plate 106 is provided with a slot to allow the femur calliper 32 to be slideably joined to the patella calliper 16 of Figure 1. An indicium in then form of a line, or pointer, 110 is provided on the plate 106 such that, when the two callipers 16, 32 are slideably connected, the pointer 110 visually interacts with the scale 102 of the patella calliper. The plate 106 of the femur calliper 32 is also provided with a tilt indicator 68 comprising two orthogonally-mounted spirit levels 112, 114, to allow an operator to ensure that the spatial position of the device can be reproducibly fixed between measurements. When the two callipers are joined, the jaws 36, 40 of the femur calliper extend beyond the jaws 20, 24 of the patella calliper.

Figure 3 illustrates an alternative embodiment of a patella calliper 16 forming part of a measuring device 10 of the present invention. The calliper 16 has two jaws 20, 24, each with a respective jaw face 22, 26 facing each other. In this embodiment, the jaws 20, 24 are mounted for rotation on a plate 100. The proximal ends of the jaws are provided with outwardly- facing teeth 116, the teeth of the first jaw 20 meshing with those of the second jaw 24. In this way, when a first jaw 20 is rotated with respect to the plate 100, a corresponding counter-rotation is produced in the second jaw 24. The plate 100 is also provided with a slot 104 to allow it to be slideably-mounted to a femur calliper such as the one illustrated in Figure 2, or Figure 4. The plate 100 is provided with a scale 102.

Figure 4 illustrates an alternative embodiment of a femur calliper forming part of a device 10 of the present invention. The calliper has two jaws, 36, 40, each with a jaw face 38, 40; the faces arranged to face each other. In an analogous fashion to the patella calliper of Figure 3, the femur calliper 32 has two jaws 36, 40, having jaw faces 38, 42, facing each other. The jaw faces of this calliper are elongate, allowing for adjustment of the position of the device 10 on the knee area of an individual such that the jaws faces 22, 26 of the patella calliper 16 mat be position on the outside edges of an individual's patella, and the jaw faces 38, 42 of the femur calliper 32 positioned on the epicondyles of the femur. Again, the jaws 36, 40 are mounted for rotation on a plate 106, and the edges of the jaws 36, 40 are provided with intermeshing teeth 116 such that rotation of one jaw produces a corresponding counter-rotation of the other jaw. An indicium in the form of a line, or pointer 110 is also provided on the plate 106 of the calliper 32. When the two callipers 16, 32 are slideably mounted together, the interaction of the indicium 110 with the scale 102 gives a measure of the lateral alignment of the patella and the femur when the jaw faces of each pair of jaws are placed into abutment with the bilateral borders of the patella and epicondyles of the femur. Figure 5 illustrates, in side elevation, a first jaw 20 of a patella calliper 16, and a first jaw 36 of a femur calliper 32, as part of an embodiment of a device 10 of the present invention. In this embodiment, the jaws 20, 24 (not illustrated in Figure 5) of the patella calliper 16 are disposed adjacent jaws 36, 40 (not illustrated in Figure 5) of the femur calliper 32. The jaws 36, 40 of the femur calliper 32 extend beyond those of the patella calliper 16, to allow them to be brought into abutment with the outside edges of a femur 14 of an individual.

Figure 6 illustrates, in schematic plan view, the jaw arrangement Figure 5. It can be seen that the two jaw faces 22, 26 of the patella calliper's jaws 20, 24 lie in a plane adjacent a plane in which the two jaw faces 38, 42 of the femur calliper's jaws 36, 40 are located. As a result, the jaw faces 38, 42 of the patella calliper 16 abut the edges of the patella 12 in a slightly different plane to the abutment points of the jaw faces 38, 42 of the femur calliper 32 on the femur 14.

Figure 7 and Figure 8 illustrate, respectively a side elevation and a schematic plan view of an alternative, and preferred embodiment of jaw arrangements of a device according to the present invention. Figure 7 illustrates that the end of the first jaw 36 of the femur calliper 32 projects to one side such that the jaw faces of all four jaws (20, 24, 36, 40) lie in substantially the same plane. Alternative means of achieving the co-planar location of all four jaw faces will be evident to the skilled addressee, e.g. by passing the jaws of the patella calliper through the jaws of the femur calliper, or by re-aligning the jaw faces of the patellar calliper.

Figure 9 illustrates, in elevation view, a yet further embodiment of the invention in which the jaws 20, 24 of the patella calliper 16 are equipped with a second measuring device 60 to measure the tilt of a patella 12 relative to the jaws. The measuring device comprises a pair of inwardly-projecting lugs 62, arranged to abut a top surface of the patella 12. The lugs are moveably mounted (e.g. slideably mounted) on the patella jaws 20, 24. Once the jaw faces 22, 26 have been moved into position to abut the edges 28, 30 of the patella 12, the lugs 62 may be moved towards the patella, to abut its top surface. In other embodiments, one such lug may be in a fixed position, and the other moveable; the fixed lug may be positioned on the top of the patella, and the moveable lug moved until it abuts the top surface of the patella. An indicium, such as a line, 200, is affixed to the lug to interact with a scale, 202, on the jaw 20. Once the lugs 62 are abutting the top surface of the patella, a reading from the scale (or scales) indicates the degree of tilt of the patella 12 relative to the jaws. As well as a scale and indicium, the position of the lug or lugs, relative to the jaw may also be determined by use of other measuring means such as electrical or optical sensors.

Figure 10 illustrates the patella tilt measuring device 60 in abutment with a tilted patella 12. The difference in position of the two lugs 62 may be read by means of the scales 202 and indicia 200.

In the embodiment illustrated in Figures 9 and 10, the patella tilt measurement device 60 further comprises biasing means 204, such as a compression spring, to bias the lugs towards the distal end of the jaws 20, 24, and thereby towards the patella 12, in use. In this way, as the jaws 20, 24 of the patella calliper are moved towards the patella, the lugs 60, automatically track the position of either edge 28, 30 of the patella 12.

The tilt of the patella 12 relative to the femur 14 may thereby be determined by a number of means. For example, the leg of an individual may be held in a given position (e.g. by means of a bracing member), and the measuring device equipped with a levelling device such as a spirit level, preferably with two such spirit levels mounted orthogonally to each other, such that the calliper is held in a pre-defined orientation relative to the leg of the individual each time it is used. Alternatively, the jaws of the femur calliper may be furnished with similar inwardly-projecting locating lugs, to position the jaws of the femur calliper on the femur in a reproducible manner.

Figure 11 illustrates a kit comprising a measuring device of the invention 10 and a calibration device 72. In this embodiment, the calibration device 72 is an offset twin circular calibration device. It has been created by machining two cylinders 74, 76 in a piece of aluminium to produce two centrally spaced measures. In other embodiments, different shapes other than cylinders can be used to substitute one or both of the cylinders and materials familiar to the skilled man may be used. In use, the jaw faces 22, 26 of either the patella calliper may be aligned with the smaller cylinder 76 or the jaw faces 38, 42 of the femur calliper with the larger cylinder 74, or both. The cylinders 74, 76 are so arranged that when both sets of faces 22, 26, 38, 42 are calibrated using said cylinders, the faces are aligned to share a common central point. Once the callipers are aligned by this calibration a marker can be set on one calliper and a measuring device can be zeroed on the other to give positive and negative measurements from this central position.

Accuracy

Confidential trials were carried out to determine the accuracy of the measurements produced by the calliper. The accuracy of the results was judged by comparing them with those which were produced by an MRI scanner. The Bland and Altman method for testing the accuracy of two clinical measures was used to identify the accuracy of the patello femoral tracking calliper compared to the MRI scanner. The results show that the patello femoral tracking callipers had a mean measurement of 0.5mm lateral in the prediction of patella position with an accuracy of +/- 4mm when compared with the results obtained via MRI. (See table 1 for details)

Table 1. Bland and Altman outcome results for validity and coefficient of variation outcomes for reliability.

The intra-tester reliability was assessed and the results indicate that there was a strong correlation coefficient with an R- Value of 0.99 (p<0.01). A coefficient of variation was also analysed and gave a score of 3.86. (see table 1)

Claims

1 A device for measuring the lateral position of the patella relative to the femur of an individual comprising:

a patella calliper with a first pair of jaws comprising a first jaw with a first jaw face and a second jaw with a second jaw face, said first and second jaws being capable of movement towards and away from each other, to allow said jaw faces to contact, in use, the outer edges of a patella;

a femur calliper with a second pair of jaws, comprising a first jaw with a first jaw face and a second jaw with a second jaw face, said first and second jaws being capable of movement towards and away from each other, to allow said jaw faces to contact, in use, the outer edges of a femur, adjacent the patella;

the first pair of jaws being movably attached to the second pair of jaws;

and measuring means to measure the lateral displacement of the first pair of jaws relative to the second pair of jaws.

2 A device according to claim 1 wherein the distance between the first and second jaw faces of the femur calliper at full extension is greater than the distance between the first and second jaw faces of the patella calliper at full extension.

3 A device according to either of claims 1 and 2 wherein the measuring means is attached to the device.

4 A device according to any of the preceding claims wherein the jaws of at least one of the patella calliper and the femur calliper are configured such that the first jaw face maintains its orientation in relation to the second jaw face as the jaw faces move. 5 A device according to any of the preceding claims wherein jaws are configured such that movement of one jaw relative to the other calliper produces a corresponding movement of the other jaw in an opposite direction and of equal distance.

6 A device according to any of the preceding claims, further comprising an actuator for actuating the movement of at least one pair of jaws.

7 A device according to any of the preceding claims, wherein both pairs of jaws have an actuator and said actuators share controlling means.

8 A device according to any of the preceding claims wherein the jaws of the patella calliper further comprise inwardly-projecting lugs, at least one of the lugs being mo veable, arranged such that the lugs may be positioned on an anterior face of a patella, the relative position of the lugs thereby giving an indication of the degree of tilt of the patella.

9 A device according to any of the preceding claims wherein the jaw faces of the patella calliper lie in substantially the same plane as the jaw faces of the femur calliper.

10 A device according to any of the preceding claims wherein the jaws of at least one of the callipers is releaseably lockable.

11 A device according to any of claims 7-11 wherein at least one actuator comprises a rack and pinion mechanism.

12 A device according to any of the preceding claims further comprising a tilt indicator.

13 A device according to any of the preceding claims, wherein the Patella calliper has a maximum separation of up to 80mm. 14 A device according to any of the preceding claims, wherein the Femur calliper has a maximum separation of up to 200mm.

15 A device according to any of the preceding claims, wherein at least one of the jaws has a bevelled edge.

16 A device substantially as described herein with reference to and as illustrated in any appropriate combination of the accompanying drawings.

17 A kit comprising a device according to any of the preceding claims and a calibration device comprising two forms, one of a greater diameter than the other, collocated in such a way that the forms share a common axis.

18 A kit according to claim 18 wherein at least one of the two forms is cylindrical.

19 A kit substantially as described herein with reference to and as illustrated in any appropriate combination of the accompanying drawings.