Surgery – Instruments – Orthopedic instrumentation
Reexamination Certificate
1999-08-06
2001-05-15
Mancene, Gene (Department: 3732)
Surgery
Instruments
Orthopedic instrumentation
C606S060000, C606S064000
Reexamination Certificate
active
06231576
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an intramedullary nail having the shape of a helically twisted blade and at least one hole at one of its free ends. The intramedullary nail is especially suited for the humerus and for other long bones where the insertion plane is different from the locking plane. It is also suited for long bones of young children where the growth plate should not be disturbed.
BACKGROUND OF THE INVENTION
In recent years, the intramedullary nail has gradually gained importance and is becoming a standard method of internal fixation. Since the development of this method by Küntscher, the intramedullary nail has broadened its range of indications (e.g. locking nail, Gamma nail, reconstruction nail).
The basic concept behind nailing has remained the same, namely the introduction of a tube or full cylinder into the medullary cavity of a tubular bone. Initially, nailing was used for the femur, but now, its application has been extended to include all large tubular bones. Until now, the cross-sections of intramedullary nails have remained more or less the same. Even the cloverleaf cross-section developed by Küntscher some 50 years ago is still being used today, even though it has been proven that this shape brings no special advantages. Additionally, changing the cross-section from the circular or almost circular cross-section was never considered necessary before, since the medullary cavity of the various tubular bones was usually opened using a rotating or circular cutting device.
Today, more attention is being paid to the rate of fracture healing, and it is believed that healing occurs faster if less damage is done to the bone during placement of an implant in the body. By choosing an appropriate cross-sectional shape for an intramedullary nail which corresponds to the anatomy of the medullary canal, it is possible to eliminate unnecessary removal of bone and tissue from the medullary canal, thus promoting healing by minimizing damage to the medullary canal and its vascularization.
The choice of nail cross-section is particularly relevant for the humerus bone, in which the medullary canal is not round along its entire length but is in fact flat and thin in the distal part. For this bone, a nail with the typical circular or almost circular cross-section would not be appropriate. A nail with a flattened cross-section would be more suited for the distal humerus.
SUMMARY OF THE INVENTION
The opening of the medullary canal for insertion of the nail can also cause unnecessary damage to the bone, as well as to the surrounding soft tissue and joint. Intramedullary nails are typically stiff and are traditionally inserted through an insertion site which is in line with the longitudinal axis of the bone.
For the case of the humerus bone, antegrade insertion (i.e. insertion from the shoulder end of the humerus) typically goes directly through the rotator cuff. This can cause irreparable damage to the joint capsule and instability of the shoulder joint, and can result in post-operative pain and/or loss of range of motion of the limb.
For the case of growing young bones, insertion sites in line with the longitudinal axis of the bone tend to interfere with the growth plate, which can affect the normal development and growth of the bone. An “ideal” insertion site avoids both the joint for long bones, as well as the growth plate in young patients. For the humerus, an ideal antegrade insertion site is located slightly distal to the greater trochanter on the lateral aspect, thus completely avoiding the joint capsule and rotator cuff at the shoulder. This ideal insertion site can not be used with traditionally stiff nails, and could only be used with nails which would be flexible along their length.
The invention as claimed aims at solving the above described problems by providing an intramedullary nail whose flexibility is achieved by relying on the natural mechanical properties (i.e, modulus of elasticity) of the nail material.
The nail is made long and thin with the smallest dimension of the cross-section being much smaller than the largest dimension, so that it is flexible along the longitudinal axis of the nail.
If such a thin, flat nail is inserted into the humerus using the ideal, antegrade, lateral insertion site described above, the distal part of the flat nail will be oriented 90° to the flat distal humerus. In order to correct this orientation problem of the flat distal nail in the flat distal humerus, the nail is twisted 90°, or any multiple of 90°, along its length. This does not significantly affect the local bending properties of the nail (i.e. short length along the axis), since bending can still occur at any one longitudinal section. Retrograde insertion (i.e. insertion from the elbow end of the humerus) of this flat nail into the humerus is not affected by this twist, since the proximal humerus is relatively large, has a round cross-section, and allows the distal part of the nail to be oriented in any way inside the proximal humerus. Therefore, this flat, twisted nail is suitable for both antegrade and retrograde insertion into the humerus.
For long, thin, flat nails, the 90° twist of the nail has an additional advantage: to bring added bending stability to the longitudinal device. A flat longitudinal device, after implantation, may bend in the medullary canal under a bending load. With an added longitudinal twist, however, the device is more stable longitudinally and is, thus, less likely to bend in the canal during normal limb activity.
For thin, flat nails, the 90° twist of the nail has yet another advantage, related to locking for both an antegradely- or retrogradely-inserted nail. For a locking hole to be designed through a nail, the cross-sectional dimension orthogonal to the hole axis must be greater than the diameter of the locking hole. Depending of course on the diameter of the locking bolt, this may not be possible for the thin, flat nail design if the locking hole goes through the largest dimension of the cross-section, i.e. through the thickest wall. From an anatomical point of view, the presence of nerves and other important tissues can also limit the possibility of locking directions. In the proximal humerus, the brachial plexus nerve group located antero-medially should be avoided during locking. Thus, for a retrograde nail, the safest locking in the proximal humerus is in the lateral-medial direction. In contrast, for an antegrade nail, locking in the flat distal humerus is most commonly done in the posterior-anterior direction, i.e. through the thinnest wall of the humerus.
Therefore, to satisfy anatomical and mechanical requirements for locking, the proximal and distal locking holes and screws should be oriented 90° (or a multiple of 90°) to each other.
For a thin, flat cross-section such as for the invention described here, this can only be achieved with a nail which is twisted 90°, or multiple of 90°, along its length.
For the purpose of closing a fracture gap, a wire or cable loop, as described in U.S. Pat. No. 5,034,012, can be added to the nail according to the invention.
The wire/cable can be looped around the entire length of the nail, preferably inset in slots, grooves or other shape of opening on the narrow sides of the flat cross-section of the nail. The closed end of the wire or cable (i.e. the looped part) at the distal tip of the nail is protected from being damaged or deformed during insertion by a metal U-shaped part surrounding it, which is part of the nail. The two ends of the wire/cable at the open end of the loop are fixed inside a nut, which is inside the proximal part of the nail. To reduce a fracture gap using this wire/cable loop design in the flat, thin nail design, the locking bolt is first placed at the far end of the nail through the locking hole and the loose wire/cable loop. The inner nut is then rotated counter-clockwise with a screwdriver, causing the nut to move upwards and the wire/cable to be pulled upwards. As this is done, the wire/cable loop around the locking bolt at the distal part of the nail
Däscher Peter
Filoso Silvana
Frigg Robert
Mancene Gene
Pennie & Edmonds LLP
Priddy Michael B.
Synthes (U.S.A.)
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