The invention relates to a medical instrument for cutting tissue, comprising a tubular outer shaft, which has at least one window in the region of its distal end, further comprising a tubular inner shaft which is rotatable about a longitudinal axis and housed in the outer shaft and which has at its distal end a window arranged in the region of the at least one window of the outer shaft and provided with at least one cutting element.
Such instruments, also known as rotational cutting instruments or shavers, are used for example in minimally invasive surgery for separating tissue in a human or animal body.
A medical instrument known from U.S. Pat. No. 5,492,527 has a handgrip, connected on the distal side torque-proof to a tubular outer shaft. The outer shaft has a window at its distal end. Housed in the outer shaft is a tubular inner shaft, which likewise has a window at its distal end, in the region of the window of the outer shaft, with at least one cutting element. On its proximal side the inner shaft can be connected to drive means, for example an internal or external electric motor, for driving the inner shaft in rotation relative to the outer shaft about a longitudinal axis.
To remove tissue, the cutting element cooperates with the window of the outer shaft as the inner shaft revolves, such that the cutting element passes by the window and thus separates any tissue located to the side of the outer shaft. The separated tissue is suctioned in a proximal direction through the window via the inner shaft, to which a suction line is attached.
The initial position of the outer and inner shaft can be determined by the surgical procedure to be carried out, the preset position of both components being defined for example by the windows of the outer and inner shaft being arranged in a specific manner relative to one another. It may be preferred that the outer shaft and the inner shaft assume such a rotational positioning relative to one another that the window of the outer shaft is occluded by the distal end of the inner shaft, causing the inner shaft to be separated from the external environment. This is required in particular in procedures when the distal end of the outer shaft is inserted into the operating site with a suction line already attached and activated, to prevent sucking in uninvolved tissue or fluid, at the same time preventing both damage to the uninvolved tissue and also blockage of the distal end of the shaft. In other cases it can be desirable for the inner shaft to assume a rotational positioning relative to the outer shaft where the window of the outer shaft is opened to its maximum. Such a position of the inner shaft relative to the outer shaft is advantageous for example for interventions in the field of ear, nose and throat surgery, where the distal end of the outer shaft is not inserted deeply into the operating site and tissue or fluid is already being suctioned upon insertion to achieve better visual control for the surgeon.
The medical instrument known from the abovementioned U.S. Pat. No. 5,492,527 further has a holder provided between the outer shaft and the handgrip, with which holder the window of the outer shaft can be arranged torque-proof in a defined position relative to the handgrip. The holder is configured as two inter-engaging toothed rings with flat teeth, which can be pushed together by axial movement of a sleeve taking up the toothed rings. The outer shaft can be rotated about the longitudinal axis of the medical instrument relative to the handgrip in 15° increments, while the inner shaft remains in its starting position. When the medical instrument is in the operating state, the inner shaft rotates about the longitudinal axis, while the outer shaft is held in the previously set position.
A drawback to this medical instrument is that although the holder positions the outer shaft relative to the handgrip in a desired position, it does not hold the outer shaft and the inner shaft relative to one another in a preset rotational position. The inner shaft is borne to freely rotate in the outer shaft, resulting in the initial rotational positioning of both components, in particular the relative position of both windows to one another, being changed due to unwanted rotation of the inner shaft. This easily changed relative arrangement of the inner and outer shaft can lead to the abovedescribed unwanted suctioning of uninvolved tissue or fluid, in particular when the instrument is inserted into the body with activated suction line.
It is also disadvantageous that the outer shaft can describe an unwanted rotational movement, since the toothed rings are designed flat such that they can also be displaced relative to one another easily without axial shifting of the sleeve. The position of the outer shaft relative to the inner shaft can as a result be altered through slight inattention by the surgeon, for example by unwanted, manually forced rotating of the outer shaft, such that the outer shaft rotates relative to the handgrip and also relative to the inner shaft and the preset position of the respective components changes. Operating the medical instrument thus requires a high degree of attentiveness and caution on the part of the surgeon.
An added disadvantage is that the holder is structurally very complicated and results in high manufacturing costs for the medical instrument.